[LB policy API] pass address lists down via an iterator interface (#34753)

This avoids storing unnecessary copies of the address list in each node of the LB policy tree.

Closes #34753

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/34753 from markdroth:lb_address_list_iterator 1d39465fbc
PiperOrigin-RevId: 582891475

BUILD

@@ -2941,6 +2941,7 @@ grpc_cc_library(
         "//src/core:lib/resolver/endpoint_addresses.h",
     ],
     external_deps = [
+        "absl/functional:function_ref",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",

src/core/BUILD

@@ -4140,6 +4140,7 @@ grpc_cc_library(
     external_deps = [
         "absl/base:core_headers",
         "absl/container:inlined_vector",
+        "absl/functional:function_ref",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",
@@ -4667,6 +4668,7 @@ grpc_cc_library(
         "ext/filters/client_channel/lb_policy/xds/xds_cluster_resolver.cc",
     ],
     external_deps = [
+        "absl/functional:function_ref",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",
@@ -4689,6 +4691,7 @@ grpc_cc_library(
         "no_destruct",
         "pollset_set",
         "ref_counted_string",
+        "resolved_address",
         "validation_errors",
         "//:channel_arg_names",
         "//:config",
@@ -4837,6 +4840,7 @@ grpc_cc_library(
         "ext/filters/client_channel/lb_policy/address_filtering.h",
     ],
     external_deps = [
+        "absl/functional:function_ref",
         "absl/status:statusor",
         "absl/strings",
     ],
@@ -4845,6 +4849,7 @@ grpc_cc_library(
         "channel_args",
         "ref_counted",
         "ref_counted_string",
+        "resolved_address",
         "//:endpoint_addresses",
         "//:gpr_platform",
         "//:ref_counted_ptr",
@@ -4914,6 +4919,7 @@ grpc_cc_library(
         "lb_policy",
         "subchannel_interface",
         "//:debug_location",
+        "//:endpoint_addresses",
         "//:gpr",
         "//:grpc_base",
         "//:ref_counted_ptr",
@@ -4931,7 +4937,7 @@ grpc_cc_library(
         "ext/filters/client_channel/lb_policy/endpoint_list.h",
     ],
     external_deps = [
-        "absl/functional:any_invocable",
+        "absl/functional:function_ref",
         "absl/status",
         "absl/status:statusor",
         "absl/types:optional",
@@ -5325,6 +5331,7 @@ grpc_cc_library(
     ],
     external_deps = [
         "absl/base:core_headers",
+        "absl/functional:function_ref",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",

src/core/ext/filters/client_channel/client_channel.cc

@@ -1599,7 +1599,12 @@ absl::Status ClientChannel::CreateOrUpdateLbPolicyLocked(
     Resolver::Result result) {
   // Construct update.
   LoadBalancingPolicy::UpdateArgs update_args;
-  update_args.addresses = std::move(result.addresses);
+  if (!result.addresses.ok()) {
+    update_args.addresses = result.addresses.status();
+  } else {
+    update_args.addresses = std::make_shared<EndpointAddressesListIterator>(
+        std::move(*result.addresses));
+  }
   update_args.config = std::move(lb_policy_config);
   update_args.resolution_note = std::move(result.resolution_note);
   // Remove the config selector from channel args so that we're not holding

src/core/ext/filters/client_channel/lb_policy/address_filtering.cc

@@ -20,11 +20,13 @@
 
 #include <stddef.h>
 
-#include <algorithm>
 #include <utility>
 
+#include "absl/functional/function_ref.h"
+
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
+#include "src/core/lib/iomgr/resolved_address.h"
 
 namespace grpc_core {
 
@@ -43,32 +45,63 @@ int HierarchicalPathArg::ChannelArgsCompare(const HierarchicalPathArg* a,
   return 0;
 }
 
+namespace {
+
+class HierarchicalAddressIterator : public EndpointAddressesIterator {
+ public:
+  HierarchicalAddressIterator(
+      std::shared_ptr<EndpointAddressesIterator> parent_it,
+      RefCountedStringValue child_name)
+      : parent_it_(std::move(parent_it)), child_name_(std::move(child_name)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    RefCountedPtr<HierarchicalPathArg> remaining_path_attr;
+    parent_it_->ForEach([&](const EndpointAddresses& endpoint) {
+      const auto* path_arg = endpoint.args().GetObject<HierarchicalPathArg>();
+      if (path_arg == nullptr) return;
+      const std::vector<RefCountedStringValue>& path = path_arg->path();
+      auto it = path.begin();
+      if (it == path.end()) return;
+      if (*it != child_name_) return;
+      ChannelArgs args = endpoint.args();
+      ++it;
+      if (it != path.end()) {
+        std::vector<RefCountedStringValue> remaining_path(it, path.end());
+        if (remaining_path_attr == nullptr ||
+            remaining_path_attr->path() != remaining_path) {
+          remaining_path_attr =
+              MakeRefCounted<HierarchicalPathArg>(std::move(remaining_path));
+        }
+        args = args.SetObject(remaining_path_attr);
+      }
+      callback(EndpointAddresses(endpoint.addresses(), args));
+    });
+  }
+
+ private:
+  std::shared_ptr<EndpointAddressesIterator> parent_it_;
+  RefCountedStringValue child_name_;
+};
+
+}  // namespace
+
 absl::StatusOr<HierarchicalAddressMap> MakeHierarchicalAddressMap(
-    const absl::StatusOr<EndpointAddressesList>& addresses) {
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses) {
   if (!addresses.ok()) return addresses.status();
   HierarchicalAddressMap result;
-  RefCountedPtr<HierarchicalPathArg> remaining_path_attr;
-  for (const EndpointAddresses& endpoint_addresses : *addresses) {
-    const auto* path_arg =
-        endpoint_addresses.args().GetObject<HierarchicalPathArg>();
-    if (path_arg == nullptr) continue;
+  (*addresses)->ForEach([&](const EndpointAddresses& endpoint) {
+    const auto* path_arg = endpoint.args().GetObject<HierarchicalPathArg>();
+    if (path_arg == nullptr) return;
     const std::vector<RefCountedStringValue>& path = path_arg->path();
     auto it = path.begin();
-    if (it == path.end()) continue;
-    EndpointAddressesList& target_list = result[*it];
-    ChannelArgs args = endpoint_addresses.args();
-    ++it;
-    if (it != path.end()) {
-      std::vector<RefCountedStringValue> remaining_path(it, path.end());
-      if (remaining_path_attr == nullptr ||
-          remaining_path_attr->path() != remaining_path) {
-        remaining_path_attr =
-            MakeRefCounted<HierarchicalPathArg>(std::move(remaining_path));
-      }
-      args = args.SetObject(remaining_path_attr);
+    if (it == path.end()) return;
+    auto& target_list = result[*it];
+    if (target_list == nullptr) {
+      target_list =
+          std::make_shared<HierarchicalAddressIterator>(*addresses, *it);
     }
-    target_list.emplace_back(endpoint_addresses.addresses(), args);
-  }
+  });
   return result;
 }
 

src/core/ext/filters/client_channel/lb_policy/address_filtering.h

@@ -20,6 +20,7 @@
 #include <grpc/support/port_platform.h>
 
 #include <map>
+#include <memory>
 #include <utility>
 #include <vector>
 
@@ -105,12 +106,12 @@ class HierarchicalPathArg : public RefCounted<HierarchicalPathArg> {
 // A map from the next path element to the endpoint addresses that fall
 // under that path element.
 using HierarchicalAddressMap =
-    std::map<RefCountedStringValue, EndpointAddressesList,
+    std::map<RefCountedStringValue, std::shared_ptr<EndpointAddressesIterator>,
              RefCountedStringValueLessThan>;
 
 // Splits up the addresses into a separate list for each child.
 absl::StatusOr<HierarchicalAddressMap> MakeHierarchicalAddressMap(
-    const absl::StatusOr<EndpointAddressesList>& addresses);
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses);
 
 }  // namespace grpc_core
 

src/core/ext/filters/client_channel/lb_policy/endpoint_list.cc

@@ -118,7 +118,7 @@ void EndpointList::Endpoint::Init(
   GPR_ASSERT(config.ok());
   // Update child policy.
   LoadBalancingPolicy::UpdateArgs update_args;
-  update_args.addresses.emplace().emplace_back(addresses);
+  update_args.addresses = std::make_shared<SingleEndpointIterator>(addresses);
   update_args.args = child_args;
   update_args.config = std::move(*config);
   // TODO(roth): If the child reports a non-OK status with the update,
@@ -163,15 +163,16 @@ RefCountedPtr<SubchannelInterface> EndpointList::Endpoint::CreateSubchannel(
 //
 
 void EndpointList::Init(
-    const EndpointAddressesList& endpoints, const ChannelArgs& args,
-    absl::AnyInvocable<OrphanablePtr<Endpoint>(RefCountedPtr<EndpointList>,
-                                               const EndpointAddresses&,
-                                               const ChannelArgs&)>
+    EndpointAddressesIterator* endpoints, const ChannelArgs& args,
+    absl::FunctionRef<OrphanablePtr<Endpoint>(RefCountedPtr<EndpointList>,
+                                              const EndpointAddresses&,
+                                              const ChannelArgs&)>
         create_endpoint) {
-  for (const EndpointAddresses& addresses : endpoints) {
+  if (endpoints == nullptr) return;
+  endpoints->ForEach([&](const EndpointAddresses& endpoint) {
     endpoints_.push_back(
-        create_endpoint(Ref(DEBUG_LOCATION, "Endpoint"), addresses, args));
-  }
+        create_endpoint(Ref(DEBUG_LOCATION, "Endpoint"), endpoint, args));
+  });
 }
 
 void EndpointList::ResetBackoffLocked() {

src/core/ext/filters/client_channel/lb_policy/endpoint_list.h

@@ -25,7 +25,7 @@
 #include <utility>
 #include <vector>
 
-#include "absl/functional/any_invocable.h"
+#include "absl/functional/function_ref.h"
 #include "absl/status/status.h"
 #include "absl/types/optional.h"
 
@@ -53,7 +53,7 @@ namespace grpc_core {
 class MyEndpointList : public EndpointList {
  public:
   MyEndpointList(RefCountedPtr<MyLbPolicy> lb_policy,
-                 const EndpointAddressesList& endpoints,
+                 EndpointAddressesIterator* endpoints,
                  const ChannelArgs& args)
       : EndpointList(std::move(lb_policy),
                      GRPC_TRACE_FLAG_ENABLED(grpc_my_tracer)
@@ -184,8 +184,8 @@ class EndpointList : public InternallyRefCounted<EndpointList> {
   EndpointList(RefCountedPtr<LoadBalancingPolicy> policy, const char* tracer)
       : policy_(std::move(policy)), tracer_(tracer) {}
 
-  void Init(const EndpointAddressesList& endpoints, const ChannelArgs& args,
-            absl::AnyInvocable<OrphanablePtr<Endpoint>(
+  void Init(EndpointAddressesIterator* endpoints, const ChannelArgs& args,
+            absl::FunctionRef<OrphanablePtr<Endpoint>(
                 RefCountedPtr<EndpointList>, const EndpointAddresses&,
                 const ChannelArgs&)>
                 create_endpoint);

src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc

@@ -72,6 +72,7 @@
 #include <vector>
 
 #include "absl/container/inlined_vector.h"
+#include "absl/functional/function_ref.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_cat.h"
@@ -384,9 +385,9 @@ class GrpcLb : public LoadBalancingPolicy {
     // Returns a text representation suitable for logging.
     std::string AsText() const;
 
-    // Extracts all non-drop entries into an EndpointAddressesList.
-    EndpointAddressesList GetServerAddressList(
-        GrpcLbClientStats* client_stats) const;
+    // Extracts all non-drop entries into an EndpointAddressesIterator.
+    std::shared_ptr<EndpointAddressesIterator> GetServerAddressList(
+        GrpcLbClientStats* client_stats);
 
     // Returns true if the serverlist contains at least one drop entry and
     // no backend address entries.
@@ -400,6 +401,8 @@ class GrpcLb : public LoadBalancingPolicy {
     const char* ShouldDrop();
 
    private:
+    class AddressIterator;
+
     std::vector<GrpcLbServer> serverlist_;
 
     // Accessed from the picker, so needs synchronization.
@@ -504,6 +507,8 @@ class GrpcLb : public LoadBalancingPolicy {
     RefCountedPtr<GrpcLb> parent_;
   };
 
+  class NullLbTokenEndpointIterator;
+
   void ShutdownLocked() override;
 
   // Helper functions used in UpdateLocked().
@@ -569,7 +574,8 @@ class GrpcLb : public LoadBalancingPolicy {
   // Whether we're in fallback mode.
   bool fallback_mode_ = false;
   // The backend addresses from the resolver.
-  absl::StatusOr<EndpointAddressesList> fallback_backend_addresses_;
+  absl::StatusOr<std::shared_ptr<NullLbTokenEndpointIterator>>
+      fallback_backend_addresses_;
   // The last resolution note from our parent.
   // To be passed to child policy when fallback_backend_addresses_ is empty.
   std::string resolution_note_;
@@ -594,11 +600,30 @@ class GrpcLb : public LoadBalancingPolicy {
 };
 
 //
-// GrpcLb::Serverlist
+// GrpcLb::Serverlist::AddressIterator
 //
 
-bool GrpcLb::Serverlist::operator==(const Serverlist& other) const {
-  return serverlist_ == other.serverlist_;
+bool IsServerValid(const GrpcLbServer& server, size_t idx, bool log) {
+  if (server.drop) return false;
+  if (GPR_UNLIKELY(server.port >> 16 != 0)) {
+    if (log) {
+      gpr_log(GPR_ERROR,
+              "Invalid port '%d' at index %" PRIuPTR
+              " of serverlist. Ignoring.",
+              server.port, idx);
+    }
+    return false;
+  }
+  if (GPR_UNLIKELY(server.ip_size != 4 && server.ip_size != 16)) {
+    if (log) {
+      gpr_log(GPR_ERROR,
+              "Expected IP to be 4 or 16 bytes, got %d at index %" PRIuPTR
+              " of serverlist. Ignoring",
+              server.ip_size, idx);
+    }
+    return false;
+  }
+  return true;
 }
 
 void ParseServer(const GrpcLbServer& server, grpc_resolved_address* addr) {
@@ -623,6 +648,53 @@ void ParseServer(const GrpcLbServer& server, grpc_resolved_address* addr) {
   }
 }
 
+class GrpcLb::Serverlist::AddressIterator : public EndpointAddressesIterator {
+ public:
+  AddressIterator(RefCountedPtr<Serverlist> serverlist,
+                  RefCountedPtr<GrpcLbClientStats> client_stats)
+      : serverlist_(std::move(serverlist)),
+        client_stats_(std::move(client_stats)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    for (size_t i = 0; i < serverlist_->serverlist_.size(); ++i) {
+      const GrpcLbServer& server = serverlist_->serverlist_[i];
+      if (!IsServerValid(server, i, false)) continue;
+      // Address processing.
+      grpc_resolved_address addr;
+      ParseServer(server, &addr);
+      // LB token processing.
+      const size_t lb_token_length = strnlen(
+          server.load_balance_token, GPR_ARRAY_SIZE(server.load_balance_token));
+      std::string lb_token(server.load_balance_token, lb_token_length);
+      if (lb_token.empty()) {
+        auto addr_uri = grpc_sockaddr_to_uri(&addr);
+        gpr_log(GPR_INFO,
+                "Missing LB token for backend address '%s'. The empty token "
+                "will be used instead",
+                addr_uri.ok() ? addr_uri->c_str()
+                              : addr_uri.status().ToString().c_str());
+      }
+      // Return address with a channel arg containing LB token and stats object.
+      callback(EndpointAddresses(
+          addr, ChannelArgs().SetObject(MakeRefCounted<TokenAndClientStatsArg>(
+                    std::move(lb_token), client_stats_))));
+    }
+  }
+
+ private:
+  RefCountedPtr<Serverlist> serverlist_;
+  RefCountedPtr<GrpcLbClientStats> client_stats_;
+};
+
+//
+// GrpcLb::Serverlist
+//
+
+bool GrpcLb::Serverlist::operator==(const Serverlist& other) const {
+  return serverlist_ == other.serverlist_;
+}
+
 std::string GrpcLb::Serverlist::AsText() const {
   std::vector<std::string> entries;
   for (size_t i = 0; i < serverlist_.size(); ++i) {
@@ -642,59 +714,12 @@ std::string GrpcLb::Serverlist::AsText() const {
   return absl::StrJoin(entries, "");
 }
 
-bool IsServerValid(const GrpcLbServer& server, size_t idx, bool log) {
-  if (server.drop) return false;
-  if (GPR_UNLIKELY(server.port >> 16 != 0)) {
-    if (log) {
-      gpr_log(GPR_ERROR,
-              "Invalid port '%d' at index %" PRIuPTR
-              " of serverlist. Ignoring.",
-              server.port, idx);
-    }
-    return false;
-  }
-  if (GPR_UNLIKELY(server.ip_size != 4 && server.ip_size != 16)) {
-    if (log) {
-      gpr_log(GPR_ERROR,
-              "Expected IP to be 4 or 16 bytes, got %d at index %" PRIuPTR
-              " of serverlist. Ignoring",
-              server.ip_size, idx);
-    }
-    return false;
-  }
-  return true;
-}
-
 // Returns addresses extracted from the serverlist.
-EndpointAddressesList GrpcLb::Serverlist::GetServerAddressList(
-    GrpcLbClientStats* client_stats) const {
+std::shared_ptr<EndpointAddressesIterator>
+GrpcLb::Serverlist::GetServerAddressList(GrpcLbClientStats* client_stats) {
   RefCountedPtr<GrpcLbClientStats> stats;
   if (client_stats != nullptr) stats = client_stats->Ref();
-  EndpointAddressesList endpoints;
-  for (size_t i = 0; i < serverlist_.size(); ++i) {
-    const GrpcLbServer& server = serverlist_[i];
-    if (!IsServerValid(server, i, false)) continue;
-    // Address processing.
-    grpc_resolved_address addr;
-    ParseServer(server, &addr);
-    // LB token processing.
-    const size_t lb_token_length = strnlen(
-        server.load_balance_token, GPR_ARRAY_SIZE(server.load_balance_token));
-    std::string lb_token(server.load_balance_token, lb_token_length);
-    if (lb_token.empty()) {
-      auto addr_uri = grpc_sockaddr_to_uri(&addr);
-      gpr_log(GPR_INFO,
-              "Missing LB token for backend address '%s'. The empty token will "
-              "be used instead",
-              addr_uri.ok() ? addr_uri->c_str()
-                            : addr_uri.status().ToString().c_str());
-    }
-    // Add address with a channel arg containing LB token and stats object.
-    endpoints.emplace_back(
-        addr, ChannelArgs().SetObject(MakeRefCounted<TokenAndClientStatsArg>(
-                  std::move(lb_token), stats)));
-  }
-  return endpoints;
+  return std::make_shared<AddressIterator>(Ref(), std::move(stats));
 }
 
 bool GrpcLb::Serverlist::ContainsAllDropEntries() const {
@@ -1503,6 +1528,31 @@ void GrpcLb::ResetBackoffLocked() {
   }
 }
 
+// Endpoint iterator wrapper to add null LB token attribute.
+class GrpcLb::NullLbTokenEndpointIterator : public EndpointAddressesIterator {
+ public:
+  explicit NullLbTokenEndpointIterator(
+      std::shared_ptr<EndpointAddressesIterator> parent_it)
+      : parent_it_(std::move(parent_it)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    parent_it_->ForEach([&](const EndpointAddresses& endpoint) {
+      if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_glb_trace)) {
+        gpr_log(GPR_INFO, "[grpclb %p] fallback address: %s", this,
+                endpoint.ToString().c_str());
+      }
+      callback(EndpointAddresses(endpoint.addresses(),
+                                 endpoint.args().SetObject(empty_token_)));
+    });
+  }
+
+ private:
+  std::shared_ptr<EndpointAddressesIterator> parent_it_;
+  RefCountedPtr<TokenAndClientStatsArg> empty_token_ =
+      MakeRefCounted<TokenAndClientStatsArg>("", nullptr);
+};
+
 absl::Status GrpcLb::UpdateLocked(UpdateArgs args) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_glb_trace)) {
     gpr_log(GPR_INFO, "[grpclb %p] received update", this);
@@ -1512,19 +1562,11 @@ absl::Status GrpcLb::UpdateLocked(UpdateArgs args) {
   GPR_ASSERT(config_ != nullptr);
   args_ = std::move(args.args);
   // Update fallback address list.
-  fallback_backend_addresses_ = std::move(args.addresses);
-  if (fallback_backend_addresses_.ok()) {
-    // Add null LB token attributes.
-    for (EndpointAddresses& endpoint : *fallback_backend_addresses_) {
-      endpoint = EndpointAddresses(
-          endpoint.addresses(),
-          endpoint.args().SetObject(
-              MakeRefCounted<TokenAndClientStatsArg>("", nullptr)));
-      if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_glb_trace)) {
-        gpr_log(GPR_INFO, "[grpclb %p] fallback address: %s", this,
-                endpoint.ToString().c_str());
-      }
-    }
+  if (!args.addresses.ok()) {
+    fallback_backend_addresses_ = args.addresses.status();
+  } else {
+    fallback_backend_addresses_ = std::make_shared<NullLbTokenEndpointIterator>(
+        std::move(*args.addresses));
   }
   resolution_note_ = std::move(args.resolution_note);
   // Update balancer channel.
@@ -1756,6 +1798,12 @@ OrphanablePtr<LoadBalancingPolicy> GrpcLb::CreateChildPolicyLocked(
   return lb_policy;
 }
 
+bool EndpointIteratorIsEmpty(const EndpointAddressesIterator& endpoints) {
+  bool empty = true;
+  endpoints.ForEach([&](const EndpointAddresses&) { empty = false; });
+  return empty;
+}
+
 void GrpcLb::CreateOrUpdateChildPolicyLocked() {
   if (shutting_down_) return;
   // Construct update args.
@@ -1769,16 +1817,17 @@ void GrpcLb::CreateOrUpdateChildPolicyLocked() {
     // picks.
     update_args.addresses = fallback_backend_addresses_;
     if (fallback_backend_addresses_.ok() &&
-        fallback_backend_addresses_->empty()) {
+        EndpointIteratorIsEmpty(**fallback_backend_addresses_)) {
       update_args.resolution_note = absl::StrCat(
-          "grpclb in fallback mode without any balancer addresses: ",
+          "grpclb in fallback mode without any fallback addresses: ",
           resolution_note_);
     }
   } else {
     update_args.addresses = serverlist_->GetServerAddressList(
         lb_calld_ == nullptr ? nullptr : lb_calld_->client_stats());
     is_backend_from_grpclb_load_balancer = true;
-    if (update_args.addresses.ok() && update_args.addresses->empty()) {
+    if (update_args.addresses.ok() &&
+        EndpointIteratorIsEmpty(**update_args.addresses)) {
       update_args.resolution_note = "empty serverlist from grpclb balancer";
     }
   }

src/core/ext/filters/client_channel/lb_policy/outlier_detection/outlier_detection.cc

@@ -661,7 +661,7 @@ absl::Status OutlierDetectionLb::UpdateLocked(UpdateArgs args) {
   if (args.addresses.ok()) {
     std::set<EndpointAddressSet> current_endpoints;
     std::set<grpc_resolved_address, ResolvedAddressLessThan> current_addresses;
-    for (const EndpointAddresses& endpoint : *args.addresses) {
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
       EndpointAddressSet key(endpoint.addresses());
       current_endpoints.emplace(key);
       for (const grpc_resolved_address& address : endpoint.addresses()) {
@@ -708,7 +708,7 @@ absl::Status OutlierDetectionLb::UpdateLocked(UpdateArgs args) {
         }
         it->second->DisableEjection();
       }
-    }
+    });
     // Remove any entries we no longer need in the subchannel map.
     for (auto it = subchannel_state_map_.begin();
          it != subchannel_state_map_.end();) {
@@ -753,7 +753,6 @@ absl::Status OutlierDetectionLb::UpdateLocked(UpdateArgs args) {
   update_args.addresses = std::move(args.addresses);
   update_args.resolution_note = std::move(args.resolution_note);
   update_args.config = config_->child_policy();
-  // Update the policy.
   update_args.args = std::move(args.args);
   if (GRPC_TRACE_FLAG_ENABLED(grpc_outlier_detection_lb_trace)) {
     gpr_log(GPR_INFO,

src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc

@@ -21,7 +21,6 @@
 #include <inttypes.h>
 #include <string.h>
 
-#include <algorithm>
 #include <memory>
 #include <set>
 #include <string>
@@ -114,7 +113,7 @@ class PickFirst : public LoadBalancingPolicy {
    public:
     class SubchannelData {
      public:
-      SubchannelData(SubchannelList* subchannel_list,
+      SubchannelData(SubchannelList* subchannel_list, size_t index,
                      RefCountedPtr<SubchannelInterface> subchannel);
 
       SubchannelInterface* subchannel() const { return subchannel_.get(); }
@@ -125,12 +124,6 @@ class PickFirst : public LoadBalancingPolicy {
         return connectivity_status_;
       }
 
-      // Returns the index into the subchannel list of this object.
-      size_t Index() const {
-        return static_cast<size_t>(this -
-                                   &subchannel_list_->subchannels_.front());
-      }
-
       // Resets the connection backoff.
       void ResetBackoffLocked() {
         if (subchannel_ != nullptr) subchannel_->ResetBackoff();
@@ -153,10 +146,8 @@ class PickFirst : public LoadBalancingPolicy {
       class Watcher
           : public SubchannelInterface::ConnectivityStateWatcherInterface {
        public:
-        Watcher(SubchannelData* subchannel_data,
-                RefCountedPtr<SubchannelList> subchannel_list)
-            : subchannel_data_(subchannel_data),
-              subchannel_list_(std::move(subchannel_list)) {}
+        Watcher(RefCountedPtr<SubchannelList> subchannel_list, size_t index)
+            : subchannel_list_(std::move(subchannel_list)), index_(index) {}
 
         ~Watcher() override {
           subchannel_list_.reset(DEBUG_LOCATION, "Watcher dtor");
@@ -164,8 +155,8 @@ class PickFirst : public LoadBalancingPolicy {
 
         void OnConnectivityStateChange(grpc_connectivity_state new_state,
                                        absl::Status status) override {
-          subchannel_data_->OnConnectivityStateChange(new_state,
-                                                      std::move(status));
+          subchannel_list_->subchannels_[index_].OnConnectivityStateChange(
+              new_state, std::move(status));
         }
 
         grpc_pollset_set* interested_parties() override {
@@ -173,8 +164,8 @@ class PickFirst : public LoadBalancingPolicy {
         }
 
        private:
-        SubchannelData* subchannel_data_;
         RefCountedPtr<SubchannelList> subchannel_list_;
+        const size_t index_;
       };
 
       // This method will be invoked once soon after instantiation to report
@@ -193,6 +184,7 @@ class PickFirst : public LoadBalancingPolicy {
 
       // Backpointer to owning subchannel list.  Not owned.
       SubchannelList* subchannel_list_;
+      const size_t index_;
       // The subchannel.
       RefCountedPtr<SubchannelInterface> subchannel_;
       // Will be non-null when the subchannel's state is being watched.
@@ -205,7 +197,8 @@ class PickFirst : public LoadBalancingPolicy {
     };
 
     SubchannelList(RefCountedPtr<PickFirst> policy,
-                   EndpointAddressesList addresses, const ChannelArgs& args);
+                   EndpointAddressesIterator* addresses,
+                   const ChannelArgs& args);
 
     ~SubchannelList() override;
 
@@ -413,9 +406,9 @@ void PickFirst::ResetBackoffLocked() {
 
 void PickFirst::AttemptToConnectUsingLatestUpdateArgsLocked() {
   // Create a subchannel list from latest_update_args_.
-  EndpointAddressesList addresses;
+  EndpointAddressesIterator* addresses = nullptr;
   if (latest_update_args_.addresses.ok()) {
-    addresses = *latest_update_args_.addresses;
+    addresses = latest_update_args_.addresses->get();
   }
   // Replace latest_pending_subchannel_list_.
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace) &&
@@ -425,7 +418,7 @@ void PickFirst::AttemptToConnectUsingLatestUpdateArgsLocked() {
             latest_pending_subchannel_list_.get());
   }
   latest_pending_subchannel_list_ = MakeOrphanable<SubchannelList>(
-      Ref(), std::move(addresses), latest_update_args_.args);
+      Ref(), addresses, latest_update_args_.args);
   // Empty update or no valid subchannels.  Put the channel in
   // TRANSIENT_FAILURE and request re-resolution.
   if (latest_pending_subchannel_list_->size() == 0) {
@@ -483,9 +476,7 @@ class AddressFamilyIterator {
 absl::Status PickFirst::UpdateLocked(UpdateArgs args) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace)) {
     if (args.addresses.ok()) {
-      gpr_log(GPR_INFO,
-              "Pick First %p received update with %" PRIuPTR " addresses", this,
-              args.addresses->size());
+      gpr_log(GPR_INFO, "Pick First %p received update", this);
     } else {
       gpr_log(GPR_INFO, "Pick First %p received update with address error: %s",
               this, args.addresses.status().ToString().c_str());
@@ -495,51 +486,59 @@ absl::Status PickFirst::UpdateLocked(UpdateArgs args) {
   absl::Status status;
   if (!args.addresses.ok()) {
     status = args.addresses.status();
-  } else if (args.addresses->empty()) {
-    status = absl::UnavailableError("address list must not be empty");
   } else {
-    // Shuffle the list if needed.
-    auto config = static_cast<PickFirstConfig*>(args.config.get());
-    if (config->shuffle_addresses()) {
-      absl::c_shuffle(*args.addresses, bit_gen_);
-    }
-    // Flatten the list so that we have one address per endpoint.
-    // While we're iterating, also determine the desired address family
-    // order and the index of the first element of each family, for use in
-    // the interleaving below.
-    std::set<absl::string_view> address_families;
-    std::vector<AddressFamilyIterator> address_family_order;
     EndpointAddressesList endpoints;
-    for (const auto& endpoint : *args.addresses) {
-      for (const auto& address : endpoint.addresses()) {
-        endpoints.emplace_back(address, endpoint.args());
-        if (IsPickFirstHappyEyeballsEnabled()) {
-          absl::string_view scheme = GetAddressFamily(address);
-          bool inserted = address_families.insert(scheme).second;
-          if (inserted) {
-            address_family_order.emplace_back(scheme, endpoints.size() - 1);
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
+      endpoints.push_back(endpoint);
+    });
+    if (endpoints.empty()) {
+      status = absl::UnavailableError("address list must not be empty");
+    } else {
+      // Shuffle the list if needed.
+      auto config = static_cast<PickFirstConfig*>(args.config.get());
+      if (config->shuffle_addresses()) {
+        absl::c_shuffle(endpoints, bit_gen_);
+      }
+      // Flatten the list so that we have one address per endpoint.
+      // While we're iterating, also determine the desired address family
+      // order and the index of the first element of each family, for use in
+      // the interleaving below.
+      std::set<absl::string_view> address_families;
+      std::vector<AddressFamilyIterator> address_family_order;
+      EndpointAddressesList flattened_endpoints;
+      for (const auto& endpoint : endpoints) {
+        for (const auto& address : endpoint.addresses()) {
+          flattened_endpoints.emplace_back(address, endpoint.args());
+          if (IsPickFirstHappyEyeballsEnabled()) {
+            absl::string_view scheme = GetAddressFamily(address);
+            bool inserted = address_families.insert(scheme).second;
+            if (inserted) {
+              address_family_order.emplace_back(scheme,
+                                                flattened_endpoints.size() - 1);
+            }
           }
         }
       }
-    }
-    // Interleave addresses as per RFC-8305 section 4.
-    if (IsPickFirstHappyEyeballsEnabled()) {
-      EndpointAddressesList interleaved_endpoints;
-      interleaved_endpoints.reserve(endpoints.size());
-      std::vector<bool> endpoints_moved(endpoints.size());
-      size_t scheme_index = 0;
-      for (size_t i = 0; i < endpoints.size(); ++i) {
-        EndpointAddresses* endpoint;
-        do {
-          auto& iterator = address_family_order[scheme_index++ %
-                                                address_family_order.size()];
-          endpoint = iterator.Next(endpoints, &endpoints_moved);
-        } while (endpoint == nullptr);
-        interleaved_endpoints.emplace_back(std::move(*endpoint));
+      endpoints = std::move(flattened_endpoints);
+      // Interleave addresses as per RFC-8305 section 4.
+      if (IsPickFirstHappyEyeballsEnabled()) {
+        EndpointAddressesList interleaved_endpoints;
+        interleaved_endpoints.reserve(endpoints.size());
+        std::vector<bool> endpoints_moved(endpoints.size());
+        size_t scheme_index = 0;
+        for (size_t i = 0; i < endpoints.size(); ++i) {
+          EndpointAddresses* endpoint;
+          do {
+            auto& iterator = address_family_order[scheme_index++ %
+                                                  address_family_order.size()];
+            endpoint = iterator.Next(endpoints, &endpoints_moved);
+          } while (endpoint == nullptr);
+          interleaved_endpoints.emplace_back(std::move(*endpoint));
+        }
+        endpoints = std::move(interleaved_endpoints);
       }
-      args.addresses = std::move(interleaved_endpoints);
-    } else {
-      args.addresses = std::move(endpoints);
+      args.addresses =
+          std::make_shared<EndpointAddressesListIterator>(std::move(endpoints));
     }
   }
   // If the update contains a resolver error and we have a previous update
@@ -617,18 +616,20 @@ void PickFirst::HealthWatcher::OnConnectivityStateChange(
 //
 
 PickFirst::SubchannelList::SubchannelData::SubchannelData(
-    SubchannelList* subchannel_list,
+    SubchannelList* subchannel_list, size_t index,
     RefCountedPtr<SubchannelInterface> subchannel)
-    : subchannel_list_(subchannel_list), subchannel_(std::move(subchannel)) {
+    : subchannel_list_(subchannel_list),
+      index_(index),
+      subchannel_(std::move(subchannel)) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace)) {
     gpr_log(GPR_INFO,
             "[PF %p] subchannel list %p index %" PRIuPTR
             " (subchannel %p): starting watch",
-            subchannel_list_->policy_.get(), subchannel_list_,
-            subchannel_list_->size(), subchannel_.get());
+            subchannel_list_->policy_.get(), subchannel_list_, index_,
+            subchannel_.get());
   }
   auto watcher = std::make_unique<Watcher>(
-      this, subchannel_list_->Ref(DEBUG_LOCATION, "Watcher"));
+      subchannel_list_->Ref(DEBUG_LOCATION, "Watcher"), index_);
   pending_watcher_ = watcher.get();
   subchannel_->WatchConnectivityState(std::move(watcher));
 }
@@ -639,7 +640,7 @@ void PickFirst::SubchannelList::SubchannelData::ShutdownLocked() {
       gpr_log(GPR_INFO,
               "[PF %p] subchannel list %p index %" PRIuPTR " of %" PRIuPTR
               " (subchannel %p): cancelling watch and unreffing subchannel",
-              subchannel_list_->policy_.get(), subchannel_list_, Index(),
+              subchannel_list_->policy_.get(), subchannel_list_, index_,
               subchannel_list_->size(), subchannel_.get());
     }
     subchannel_->CancelConnectivityStateWatch(pending_watcher_);
@@ -659,7 +660,7 @@ void PickFirst::SubchannelList::SubchannelData::OnConnectivityStateChange(
         "status=%s, shutting_down=%d, pending_watcher=%p, "
         "seen_transient_failure=%d, p->selected_=%p, "
         "p->subchannel_list_=%p, p->latest_pending_subchannel_list_=%p",
-        p, subchannel_list_, Index(), subchannel_list_->size(),
+        p, subchannel_list_, index_, subchannel_list_->size(),
         subchannel_.get(),
         (connectivity_state_.has_value()
              ? ConnectivityStateName(*connectivity_state_)
@@ -771,7 +772,7 @@ void PickFirst::SubchannelList::SubchannelData::OnConnectivityStateChange(
   if (!IsPickFirstHappyEyeballsEnabled()) {
     // Ignore any other updates for subchannels we're not currently trying to
     // connect to.
-    if (Index() != subchannel_list_->attempting_index_) return;
+    if (index_ != subchannel_list_->attempting_index_) return;
     // React to the connectivity state.
     ReactToConnectivityStateLocked();
     return;
@@ -784,7 +785,7 @@ void PickFirst::SubchannelList::SubchannelData::OnConnectivityStateChange(
       if (!prev_seen_transient_failure && seen_transient_failure_) {
         // If a connection attempt fails before the timer fires, then
         // cancel the timer and start connecting on the next subchannel.
-        if (Index() == subchannel_list_->attempting_index_) {
+        if (index_ == subchannel_list_->attempting_index_) {
           if (subchannel_list_->timer_handle_.has_value()) {
             p->channel_control_helper()->GetEventEngine()->Cancel(
                 *subchannel_list_->timer_handle_);
@@ -858,7 +859,7 @@ void PickFirst::SubchannelList::SubchannelData::
       // We skip subchannels in state TRANSIENT_FAILURE to avoid a
       // large recursion that could overflow the stack.
       SubchannelData* found_subchannel = nullptr;
-      for (size_t next_index = Index() + 1;
+      for (size_t next_index = index_ + 1;
            next_index < subchannel_list_->size(); ++next_index) {
         SubchannelData* sc = &subchannel_list_->subchannels_[next_index];
         GPR_ASSERT(sc->connectivity_state_.has_value());
@@ -946,14 +947,14 @@ void PickFirst::SubchannelList::SubchannelData::RequestConnectionWithTimer() {
     GPR_ASSERT(connectivity_state_ == GRPC_CHANNEL_CONNECTING);
   }
   // If this is not the last subchannel in the list, start the timer.
-  if (Index() != subchannel_list_->size() - 1) {
+  if (index_ != subchannel_list_->size() - 1) {
     PickFirst* p = subchannel_list_->policy_.get();
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace)) {
       gpr_log(GPR_INFO,
               "Pick First %p subchannel list %p: starting Connection "
               "Attempt Delay timer for %" PRId64 "ms for index %" PRIuPTR,
               p, subchannel_list_, p->connection_attempt_delay_.millis(),
-              Index());
+              index_);
     }
     subchannel_list_->timer_handle_ =
         p->channel_control_helper()->GetEventEngine()->RunAfter(
@@ -1041,7 +1042,7 @@ void PickFirst::SubchannelList::SubchannelData::ProcessUnselectedReadyLocked() {
   }
   // Unref all other subchannels in the list.
   for (size_t i = 0; i < subchannel_list_->size(); ++i) {
-    if (i != Index()) {
+    if (i != index_) {
       subchannel_list_->subchannels_[i].ShutdownLocked();
     }
   }
@@ -1052,7 +1053,7 @@ void PickFirst::SubchannelList::SubchannelData::ProcessUnselectedReadyLocked() {
 //
 
 PickFirst::SubchannelList::SubchannelList(RefCountedPtr<PickFirst> policy,
-                                          EndpointAddressesList addresses,
+                                          EndpointAddressesIterator* addresses,
                                           const ChannelArgs& args)
     : InternallyRefCounted<SubchannelList>(
           GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace) ? "SubchannelList"
@@ -1062,14 +1063,12 @@ PickFirst::SubchannelList::SubchannelList(RefCountedPtr<PickFirst> policy,
                 .Remove(
                     GRPC_ARG_INTERNAL_PICK_FIRST_OMIT_STATUS_MESSAGE_PREFIX)) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace)) {
-    gpr_log(GPR_INFO,
-            "[PF %p] Creating subchannel list %p for %" PRIuPTR
-            " subchannels - channel args: %s",
-            policy_.get(), this, addresses.size(), args_.ToString().c_str());
+    gpr_log(GPR_INFO, "[PF %p] Creating subchannel list %p - channel args: %s",
+            policy_.get(), this, args_.ToString().c_str());
   }
-  subchannels_.reserve(addresses.size());
+  if (addresses == nullptr) return;
   // Create a subchannel for each address.
-  for (const EndpointAddresses& address : addresses) {
+  addresses->ForEach([&](const EndpointAddresses& address) {
     GPR_ASSERT(address.addresses().size() == 1);
     RefCountedPtr<SubchannelInterface> subchannel =
         policy_->channel_control_helper()->CreateSubchannel(
@@ -1081,7 +1080,7 @@ PickFirst::SubchannelList::SubchannelList(RefCountedPtr<PickFirst> policy,
                 "[PF %p] could not create subchannel for address %s, ignoring",
                 policy_.get(), address.ToString().c_str());
       }
-      continue;
+      return;
     }
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_pick_first_trace)) {
       gpr_log(GPR_INFO,
@@ -1090,8 +1089,8 @@ PickFirst::SubchannelList::SubchannelList(RefCountedPtr<PickFirst> policy,
               policy_.get(), this, subchannels_.size(), subchannel.get(),
               address.ToString().c_str());
     }
-    subchannels_.emplace_back(this, std::move(subchannel));
-  }
+    subchannels_.emplace_back(this, subchannels_.size(), std::move(subchannel));
+  });
 }
 
 PickFirst::SubchannelList::~SubchannelList() {

src/core/ext/filters/client_channel/lb_policy/priority/priority.cc

@@ -684,7 +684,8 @@ absl::Status PriorityLb::ChildPriority::UpdateLocked(
   if (priority_policy_->addresses_.ok()) {
     auto it = priority_policy_->addresses_->find(name_);
     if (it == priority_policy_->addresses_->end()) {
-      update_args.addresses.emplace();
+      update_args.addresses = std::make_shared<EndpointAddressesListIterator>(
+          EndpointAddressesList());
     } else {
       update_args.addresses = it->second;
     }

src/core/ext/filters/client_channel/lb_policy/ring_hash/ring_hash.cc

@@ -554,7 +554,8 @@ void RingHash::RingHashEndpoint::UpdateChildPolicyLocked() {
   GPR_ASSERT(config.ok());
   // Update child policy.
   LoadBalancingPolicy::UpdateArgs update_args;
-  update_args.addresses.emplace().emplace_back(ring_hash_->endpoints_[index_]);
+  update_args.addresses =
+      std::make_shared<SingleEndpointIterator>(ring_hash_->endpoints_[index_]);
   update_args.args = ring_hash_->args_;
   update_args.config = std::move(*config);
   // TODO(roth): If the child reports a non-OK status with the update,
@@ -622,18 +623,14 @@ absl::Status RingHash::UpdateLocked(UpdateArgs args) {
   // Check address list.
   if (args.addresses.ok()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_ring_hash_trace)) {
-      gpr_log(GPR_INFO, "[RH %p] received update with %" PRIuPTR " addresses",
-              this, args.addresses->size());
+      gpr_log(GPR_INFO, "[RH %p] received update", this);
     }
     // De-dup endpoints, taking weight into account.
     endpoints_.clear();
-    endpoints_.reserve(args.addresses->size());
     std::map<EndpointAddressSet, size_t> endpoint_indices;
-    size_t num_skipped = 0;
-    for (size_t i = 0; i < args.addresses->size(); ++i) {
-      EndpointAddresses& endpoint = (*args.addresses)[i];
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
       const EndpointAddressSet key(endpoint.addresses());
-      auto p = endpoint_indices.emplace(key, i - num_skipped);
+      auto p = endpoint_indices.emplace(key, endpoints_.size());
       if (!p.second) {
         // Duplicate endpoint.  Combine weights and skip the dup.
         EndpointAddresses& prev_endpoint = endpoints_[p.first->second];
@@ -651,11 +648,10 @@ absl::Status RingHash::UpdateLocked(UpdateArgs args) {
             prev_endpoint.addresses(),
             prev_endpoint.args().Set(GRPC_ARG_ADDRESS_WEIGHT,
                                      weight_arg + prev_weight_arg));
-        ++num_skipped;
       } else {
-        endpoints_.push_back(std::move(endpoint));
+        endpoints_.push_back(endpoint);
       }
-    }
+    });
   } else {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_ring_hash_trace)) {
       gpr_log(GPR_INFO, "[RH %p] received update with addresses error: %s",

src/core/ext/filters/client_channel/lb_policy/rls/rls.cc

@@ -707,7 +707,7 @@ class RlsLb : public LoadBalancingPolicy {
   OrphanablePtr<RlsChannel> rls_channel_ ABSL_GUARDED_BY(mu_);
 
   // Accessed only from within WorkSerializer.
-  absl::StatusOr<EndpointAddressesList> addresses_;
+  absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses_;
   ChannelArgs channel_args_;
   RefCountedPtr<RlsLbConfig> config_;
   RefCountedPtr<ChildPolicyWrapper> default_child_policy_;
@@ -1858,6 +1858,27 @@ RlsLb::RlsLb(Args args) : LoadBalancingPolicy(std::move(args)), cache_(this) {
   }
 }
 
+bool EndpointsEqual(
+    const absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> endpoints1,
+    const absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>>
+        endpoints2) {
+  if (endpoints1.status() != endpoints2.status()) return false;
+  if (endpoints1.ok()) {
+    std::vector<EndpointAddresses> e1_list;
+    (*endpoints1)->ForEach([&](const EndpointAddresses& endpoint) {
+      e1_list.push_back(endpoint);
+    });
+    size_t i = 0;
+    bool different = false;
+    (*endpoints2)->ForEach([&](const EndpointAddresses& endpoint) {
+      if (endpoint != e1_list[i++]) different = true;
+    });
+    if (different) return false;
+    if (i != e1_list.size()) return false;
+  }
+  return true;
+}
+
 absl::Status RlsLb::UpdateLocked(UpdateArgs args) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_rls_trace)) {
     gpr_log(GPR_INFO, "[rlslb %p] policy updated", this);
@@ -1875,7 +1896,7 @@ absl::Status RlsLb::UpdateLocked(UpdateArgs args) {
   // Swap out addresses.
   // If the new address list is an error and we have an existing address list,
   // stick with the existing addresses.
-  absl::StatusOr<EndpointAddressesList> old_addresses;
+  absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> old_addresses;
   if (args.addresses.ok()) {
     old_addresses = std::move(addresses_);
     addresses_ = std::move(args.addresses);
@@ -1888,7 +1909,7 @@ absl::Status RlsLb::UpdateLocked(UpdateArgs args) {
   bool update_child_policies =
       old_config == nullptr ||
       old_config->child_policy_config() != config_->child_policy_config() ||
-      old_addresses != addresses_ || args.args != channel_args_;
+      !EndpointsEqual(old_addresses, addresses_) || args.args != channel_args_;
   // If default target changes, swap out child policy.
   bool created_default_child = false;
   if (old_config == nullptr ||

src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.cc

@@ -125,14 +125,14 @@ class OldRoundRobin : public LoadBalancingPolicy {
       : public SubchannelList<RoundRobinSubchannelList,
                               RoundRobinSubchannelData> {
    public:
-    RoundRobinSubchannelList(OldRoundRobin* policy, ServerAddressList addresses,
+    RoundRobinSubchannelList(OldRoundRobin* policy,
+                             EndpointAddressesIterator* addresses,
                              const ChannelArgs& args)
         : SubchannelList(policy,
                          (GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)
                               ? "RoundRobinSubchannelList"
                               : nullptr),
-                         std::move(addresses), policy->channel_control_helper(),
-                         args) {
+                         addresses, policy->channel_control_helper(), args) {
       // Need to maintain a ref to the LB policy as long as we maintain
       // any references to subchannels, since the subchannels'
       // pollset_sets will include the LB policy's pollset_set.
@@ -277,13 +277,12 @@ void OldRoundRobin::ResetBackoffLocked() {
 }
 
 absl::Status OldRoundRobin::UpdateLocked(UpdateArgs args) {
-  ServerAddressList addresses;
+  EndpointAddressesIterator* addresses = nullptr;
   if (args.addresses.ok()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)) {
-      gpr_log(GPR_INFO, "[RR %p] received update with %" PRIuPTR " addresses",
-              this, args.addresses->size());
+      gpr_log(GPR_INFO, "[RR %p] received update", this);
     }
-    addresses = std::move(*args.addresses);
+    addresses = args.addresses->get();
   } else {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)) {
       gpr_log(GPR_INFO, "[RR %p] received update with address error: %s", this,
@@ -299,8 +298,8 @@ absl::Status OldRoundRobin::UpdateLocked(UpdateArgs args) {
     gpr_log(GPR_INFO, "[RR %p] replacing previous pending subchannel list %p",
             this, latest_pending_subchannel_list_.get());
   }
-  latest_pending_subchannel_list_ = MakeRefCounted<RoundRobinSubchannelList>(
-      this, std::move(addresses), args.args);
+  latest_pending_subchannel_list_ =
+      MakeRefCounted<RoundRobinSubchannelList>(this, addresses, args.args);
   latest_pending_subchannel_list_->StartWatchingLocked(args.args);
   // If the new list is empty, immediately promote it to
   // subchannel_list_ and report TRANSIENT_FAILURE.
@@ -524,7 +523,7 @@ class RoundRobin : public LoadBalancingPolicy {
   class RoundRobinEndpointList : public EndpointList {
    public:
     RoundRobinEndpointList(RefCountedPtr<RoundRobin> round_robin,
-                           const EndpointAddressesList& endpoints,
+                           EndpointAddressesIterator* endpoints,
                            const ChannelArgs& args)
         : EndpointList(std::move(round_robin),
                        GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)
@@ -687,13 +686,12 @@ void RoundRobin::ResetBackoffLocked() {
 }
 
 absl::Status RoundRobin::UpdateLocked(UpdateArgs args) {
-  EndpointAddressesList addresses;
+  EndpointAddressesIterator* addresses = nullptr;
   if (args.addresses.ok()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)) {
-      gpr_log(GPR_INFO, "[RR %p] received update with %" PRIuPTR " endpoints",
-              this, args.addresses->size());
+      gpr_log(GPR_INFO, "[RR %p] received update", this);
     }
-    addresses = std::move(*args.addresses);
+    addresses = args.addresses->get();
   } else {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_round_robin_trace)) {
       gpr_log(GPR_INFO, "[RR %p] received update with address error: %s", this,
@@ -710,8 +708,7 @@ absl::Status RoundRobin::UpdateLocked(UpdateArgs args) {
             latest_pending_endpoint_list_.get());
   }
   latest_pending_endpoint_list_ = MakeOrphanable<RoundRobinEndpointList>(
-      Ref(DEBUG_LOCATION, "RoundRobinEndpointList"), std::move(addresses),
-      args.args);
+      Ref(DEBUG_LOCATION, "RoundRobinEndpointList"), addresses, args.args);
   // If the new list is empty, immediately promote it to
   // endpoint_list_ and report TRANSIENT_FAILURE.
   if (latest_pending_endpoint_list_->size() == 0) {

src/core/ext/filters/client_channel/lb_policy/subchannel_list.h

@@ -42,6 +42,7 @@
 #include "src/core/lib/iomgr/iomgr_fwd.h"
 #include "src/core/lib/load_balancing/lb_policy.h"
 #include "src/core/lib/load_balancing/subchannel_interface.h"
+#include "src/core/lib/resolver/endpoint_addresses.h"
 #include "src/core/lib/resolver/server_address.h"
 #include "src/core/lib/transport/connectivity_state.h"
 
@@ -208,7 +209,7 @@ class SubchannelList : public DualRefCounted<SubchannelListType> {
 
  protected:
   SubchannelList(LoadBalancingPolicy* policy, const char* tracer,
-                 ServerAddressList addresses,
+                 EndpointAddressesIterator* addresses,
                  LoadBalancingPolicy::ChannelControlHelper* helper,
                  const ChannelArgs& args);
 
@@ -365,19 +366,18 @@ void SubchannelData<SubchannelListType, SubchannelDataType>::ShutdownLocked() {
 template <typename SubchannelListType, typename SubchannelDataType>
 SubchannelList<SubchannelListType, SubchannelDataType>::SubchannelList(
     LoadBalancingPolicy* policy, const char* tracer,
-    ServerAddressList addresses,
+    EndpointAddressesIterator* addresses,
     LoadBalancingPolicy::ChannelControlHelper* helper, const ChannelArgs& args)
     : DualRefCounted<SubchannelListType>(tracer),
       policy_(policy),
       tracer_(tracer) {
   if (GPR_UNLIKELY(tracer_ != nullptr)) {
-    gpr_log(GPR_INFO,
-            "[%s %p] Creating subchannel list %p for %" PRIuPTR " subchannels",
-            tracer_, policy, this, addresses.size());
+    gpr_log(GPR_INFO, "[%s %p] Creating subchannel list %p", tracer_, policy,
+            this);
   }
-  subchannels_.reserve(addresses.size());
+  if (addresses == nullptr) return;
   // Create a subchannel for each address.
-  for (ServerAddress address : addresses) {
+  addresses->ForEach([&](const EndpointAddresses& address) {
     RefCountedPtr<SubchannelInterface> subchannel =
         helper->CreateSubchannel(address.address(), address.args(), args);
     if (subchannel == nullptr) {
@@ -387,7 +387,7 @@ SubchannelList<SubchannelListType, SubchannelDataType>::SubchannelList(
                 "[%s %p] could not create subchannel for address %s, ignoring",
                 tracer_, policy_, address.ToString().c_str());
       }
-      continue;
+      return;
     }
     if (GPR_UNLIKELY(tracer_ != nullptr)) {
       gpr_log(GPR_INFO,
@@ -397,8 +397,8 @@ SubchannelList<SubchannelListType, SubchannelDataType>::SubchannelList(
               address.ToString().c_str());
     }
     subchannels_.emplace_back();
-    subchannels_.back().Init(this, std::move(address), std::move(subchannel));
-  }
+    subchannels_.back().Init(this, address, std::move(subchannel));
+  });
 }
 
 template <typename SubchannelListType, typename SubchannelDataType>

src/core/ext/filters/client_channel/lb_policy/weighted_round_robin/weighted_round_robin.cc

@@ -247,14 +247,13 @@ class OldWeightedRoundRobin : public LoadBalancingPolicy {
                               WeightedRoundRobinSubchannelData> {
    public:
     WeightedRoundRobinSubchannelList(OldWeightedRoundRobin* policy,
-                                     ServerAddressList addresses,
+                                     EndpointAddressesIterator* addresses,
                                      const ChannelArgs& args)
         : SubchannelList(policy,
                          (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)
                               ? "WeightedRoundRobinSubchannelList"
                               : nullptr),
-                         std::move(addresses), policy->channel_control_helper(),
-                         args) {
+                         addresses, policy->channel_control_helper(), args) {
       // Need to maintain a ref to the LB policy as long as we maintain
       // any references to subchannels, since the subchannels'
       // pollset_sets will include the LB policy's pollset_set.
@@ -675,11 +674,10 @@ void OldWeightedRoundRobin::ResetBackoffLocked() {
 absl::Status OldWeightedRoundRobin::UpdateLocked(UpdateArgs args) {
   global_stats().IncrementWrrUpdates();
   config_ = std::move(args.config);
-  ServerAddressList addresses;
+  std::shared_ptr<EndpointAddressesIterator> addresses;
   if (args.addresses.ok()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
-      gpr_log(GPR_INFO, "[WRR %p] received update with %" PRIuPTR " addresses",
-              this, args.addresses->size());
+      gpr_log(GPR_INFO, "[WRR %p] received update", this);
     }
     // Weed out duplicate addresses.  Also sort the addresses so that if
     // the set of the addresses don't change, their indexes in the
@@ -698,10 +696,12 @@ absl::Status OldWeightedRoundRobin::UpdateLocked(UpdateArgs args) {
         return memcmp(addr1.addr, addr2.addr, addr1.len) < 0;
       }
     };
-    std::set<ServerAddress, AddressLessThan> ordered_addresses(
-        args.addresses->begin(), args.addresses->end());
-    addresses =
-        ServerAddressList(ordered_addresses.begin(), ordered_addresses.end());
+    std::set<ServerAddress, AddressLessThan> ordered_addresses;
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
+      ordered_addresses.insert(endpoint);
+    });
+    addresses = std::make_shared<EndpointAddressesListIterator>(
+        ServerAddressList(ordered_addresses.begin(), ordered_addresses.end()));
   } else {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
       gpr_log(GPR_INFO, "[WRR %p] received update with address error: %s", this,
@@ -718,8 +718,8 @@ absl::Status OldWeightedRoundRobin::UpdateLocked(UpdateArgs args) {
             this, latest_pending_subchannel_list_.get());
   }
   latest_pending_subchannel_list_ =
-      MakeRefCounted<WeightedRoundRobinSubchannelList>(
-          this, std::move(addresses), args.args);
+      MakeRefCounted<WeightedRoundRobinSubchannelList>(this, addresses.get(),
+                                                       args.args);
   latest_pending_subchannel_list_->StartWatchingLocked(args.args);
   // If the new list is empty, immediately promote it to
   // subchannel_list_ and report TRANSIENT_FAILURE.
@@ -1079,7 +1079,7 @@ class WeightedRoundRobin : public LoadBalancingPolicy {
     };
 
     WrrEndpointList(RefCountedPtr<WeightedRoundRobin> wrr,
-                    const EndpointAddressesList& endpoints,
+                    EndpointAddressesIterator* endpoints,
                     const ChannelArgs& args)
         : EndpointList(std::move(wrr),
                        GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)
@@ -1516,11 +1516,10 @@ void WeightedRoundRobin::ResetBackoffLocked() {
 absl::Status WeightedRoundRobin::UpdateLocked(UpdateArgs args) {
   global_stats().IncrementWrrUpdates();
   config_ = std::move(args.config);
-  EndpointAddressesList addresses;
+  std::shared_ptr<EndpointAddressesIterator> addresses;
   if (args.addresses.ok()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
-      gpr_log(GPR_INFO, "[WRR %p] received update with %" PRIuPTR " addresses",
-              this, args.addresses->size());
+      gpr_log(GPR_INFO, "[WRR %p] received update", this);
     }
     // Weed out duplicate endpoints.  Also sort the endpoints so that if
     // the set of endpoints doesn't change, their indexes in the endpoint
@@ -1539,10 +1538,13 @@ absl::Status WeightedRoundRobin::UpdateLocked(UpdateArgs args) {
         return e1 < e2;
       }
     };
-    std::set<EndpointAddresses, EndpointAddressesLessThan> ordered_addresses(
-        args.addresses->begin(), args.addresses->end());
-    addresses = EndpointAddressesList(ordered_addresses.begin(),
-                                      ordered_addresses.end());
+    std::set<EndpointAddresses, EndpointAddressesLessThan> ordered_addresses;
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
+      ordered_addresses.insert(endpoint);
+    });
+    addresses =
+        std::make_shared<EndpointAddressesListIterator>(EndpointAddressesList(
+            ordered_addresses.begin(), ordered_addresses.end()));
   } else {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
       gpr_log(GPR_INFO, "[WRR %p] received update with address error: %s", this,
@@ -1559,7 +1561,7 @@ absl::Status WeightedRoundRobin::UpdateLocked(UpdateArgs args) {
             this, latest_pending_endpoint_list_.get());
   }
   latest_pending_endpoint_list_ =
-      MakeOrphanable<WrrEndpointList>(Ref(), std::move(addresses), args.args);
+      MakeOrphanable<WrrEndpointList>(Ref(), addresses.get(), args.args);
   // If the new list is empty, immediately promote it to
   // endpoint_list_ and report TRANSIENT_FAILURE.
   if (latest_pending_endpoint_list_->size() == 0) {

src/core/ext/filters/client_channel/lb_policy/weighted_target/weighted_target.cc

@@ -157,10 +157,10 @@ class WeightedTargetLb : public LoadBalancingPolicy {
 
     void Orphan() override;
 
-    absl::Status UpdateLocked(const WeightedTargetLbConfig::ChildConfig& config,
-                              absl::StatusOr<EndpointAddressesList> addresses,
-                              const std::string& resolution_note,
-                              const ChannelArgs& args);
+    absl::Status UpdateLocked(
+        const WeightedTargetLbConfig::ChildConfig& config,
+        absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses,
+        const std::string& resolution_note, const ChannelArgs& args);
     void ResetBackoffLocked();
     void DeactivateLocked();
 
@@ -338,11 +338,12 @@ absl::Status WeightedTargetLb::UpdateLocked(UpdateArgs args) {
       target = MakeOrphanable<WeightedChild>(
           Ref(DEBUG_LOCATION, "WeightedChild"), name);
     }
-    absl::StatusOr<EndpointAddressesList> addresses;
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses;
     if (address_map.ok()) {
       auto it = address_map->find(name);
       if (it == address_map->end()) {
-        addresses.emplace();
+        addresses = std::make_shared<EndpointAddressesListIterator>(
+            EndpointAddressesList());
       } else {
         addresses = std::move(it->second);
       }
@@ -589,7 +590,7 @@ WeightedTargetLb::WeightedChild::CreateChildPolicyLocked(
 
 absl::Status WeightedTargetLb::WeightedChild::UpdateLocked(
     const WeightedTargetLbConfig::ChildConfig& config,
-    absl::StatusOr<EndpointAddressesList> addresses,
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses,
     const std::string& resolution_note, const ChannelArgs& args) {
   if (weighted_target_policy_->shutting_down_) return absl::OkStatus();
   // Update child weight.

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_impl.cc

@@ -250,7 +250,7 @@ class XdsClusterImplLb : public LoadBalancingPolicy {
   OrphanablePtr<LoadBalancingPolicy> CreateChildPolicyLocked(
       const ChannelArgs& args);
   absl::Status UpdateChildPolicyLocked(
-      absl::StatusOr<EndpointAddressesList> addresses,
+      absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses,
       std::string resolution_note, const ChannelArgs& args);
 
   void MaybeUpdatePickerLocked();
@@ -569,7 +569,7 @@ OrphanablePtr<LoadBalancingPolicy> XdsClusterImplLb::CreateChildPolicyLocked(
 }
 
 absl::Status XdsClusterImplLb::UpdateChildPolicyLocked(
-    absl::StatusOr<EndpointAddressesList> addresses,
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses,
     std::string resolution_note, const ChannelArgs& args) {
   // Create policy if needed.
   if (child_policy_ == nullptr) {

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_manager.cc

@@ -149,7 +149,8 @@ class XdsClusterManagerLb : public LoadBalancingPolicy {
 
     absl::Status UpdateLocked(
         RefCountedPtr<LoadBalancingPolicy::Config> config,
-        const absl::StatusOr<EndpointAddressesList>& addresses,
+        const absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>>&
+            addresses,
         const ChannelArgs& args);
     void ExitIdleLocked();
     void ResetBackoffLocked();
@@ -482,7 +483,7 @@ XdsClusterManagerLb::ClusterChild::CreateChildPolicyLocked(
 
 absl::Status XdsClusterManagerLb::ClusterChild::UpdateLocked(
     RefCountedPtr<LoadBalancingPolicy::Config> config,
-    const absl::StatusOr<EndpointAddressesList>& addresses,
+    const absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>>& addresses,
     const ChannelArgs& args) {
   if (xds_cluster_manager_policy_->shutting_down_) return absl::OkStatus();
   // Update child weight.

src/core/ext/filters/client_channel/lb_policy/xds/xds_cluster_resolver.cc

@@ -28,6 +28,7 @@
 #include <utility>
 #include <vector>
 
+#include "absl/functional/function_ref.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_cat.h"
@@ -61,6 +62,7 @@
 #include "src/core/lib/gprpp/validation_errors.h"
 #include "src/core/lib/gprpp/work_serializer.h"
 #include "src/core/lib/iomgr/pollset_set.h"
+#include "src/core/lib/iomgr/resolved_address.h"
 #include "src/core/lib/json/json.h"
 #include "src/core/lib/json/json_args.h"
 #include "src/core/lib/json/json_object_loader.h"
@@ -390,7 +392,7 @@ class XdsClusterResolverLb : public LoadBalancingPolicy {
   absl::Status UpdateChildPolicyLocked();
   OrphanablePtr<LoadBalancingPolicy> CreateChildPolicyLocked(
       const ChannelArgs& args);
-  EndpointAddressesList CreateChildPolicyAddressesLocked();
+  std::shared_ptr<EndpointAddressesIterator> CreateChildPolicyAddressesLocked();
   std::string CreateChildPolicyResolutionNoteLocked();
   RefCountedPtr<Config> CreateChildPolicyConfigLocked();
   ChannelArgs CreateChildPolicyArgsLocked(const ChannelArgs& args_in);
@@ -529,10 +531,16 @@ XdsClusterResolverLb::DiscoveryMechanismEntry::config() const {
       ->config_->discovery_mechanisms()[discovery_mechanism->index()];
 }
 
+std::string MakeChildPolicyName(absl::string_view cluster_name,
+                                size_t child_number) {
+  return absl::StrCat("{cluster=", cluster_name,
+                      ", child_number=", child_number, "}");
+}
+
 std::string XdsClusterResolverLb::DiscoveryMechanismEntry::GetChildPolicyName(
     size_t priority) const {
-  return absl::StrCat("{cluster=", config().cluster_name,
-                      ", child_number=", priority_child_numbers[priority], "}");
+  return MakeChildPolicyName(config().cluster_name,
+                             priority_child_numbers[priority]);
 }
 
 //
@@ -768,39 +776,76 @@ void XdsClusterResolverLb::OnResourceDoesNotExist(size_t index,
 // child policy-related methods
 //
 
-EndpointAddressesList XdsClusterResolverLb::CreateChildPolicyAddressesLocked() {
-  EndpointAddressesList addresses;
-  for (const auto& discovery_entry : discovery_mechanisms_) {
-    const auto& priority_list =
-        GetUpdatePriorityList(*discovery_entry.latest_update);
-    for (size_t priority = 0; priority < priority_list.size(); ++priority) {
-      const auto& priority_entry = priority_list[priority];
-      std::string priority_child_name =
-          discovery_entry.GetChildPolicyName(priority);
-      for (const auto& p : priority_entry.localities) {
-        const auto& locality_name = p.first;
-        const auto& locality = p.second;
-        std::vector<RefCountedStringValue> hierarchical_path = {
-            RefCountedStringValue(priority_child_name),
-            RefCountedStringValue(locality_name->AsHumanReadableString())};
-        auto hierarchical_path_attr =
-            MakeRefCounted<HierarchicalPathArg>(std::move(hierarchical_path));
-        for (const auto& endpoint : locality.endpoints) {
-          uint32_t endpoint_weight =
-              locality.lb_weight *
-              endpoint.args().GetInt(GRPC_ARG_ADDRESS_WEIGHT).value_or(1);
-          addresses.emplace_back(
-              endpoint.addresses(),
-              endpoint.args()
-                  .SetObject(hierarchical_path_attr)
-                  .Set(GRPC_ARG_ADDRESS_WEIGHT, endpoint_weight)
-                  .SetObject(locality_name->Ref())
-                  .Set(GRPC_ARG_XDS_LOCALITY_WEIGHT, locality.lb_weight));
+class PriorityEndpointIterator : public EndpointAddressesIterator {
+ public:
+  struct DiscoveryMechanismResult {
+    std::shared_ptr<const XdsEndpointResource> update;
+    std::string cluster_name;
+    std::vector<size_t /*child_number*/> priority_child_numbers;
+
+    DiscoveryMechanismResult(
+        std::shared_ptr<const XdsEndpointResource> resource,
+        std::string cluster, std::vector<size_t> child_numbers)
+        : update(std::move(resource)),
+          cluster_name(std::move(cluster)),
+          priority_child_numbers(std::move(child_numbers)) {}
+
+    std::string GetChildPolicyName(size_t priority) const {
+      return MakeChildPolicyName(cluster_name,
+                                 priority_child_numbers[priority]);
+    }
+  };
+
+  explicit PriorityEndpointIterator(
+      std::vector<DiscoveryMechanismResult> results)
+      : results_(std::move(results)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    for (const auto& entry : results_) {
+      const auto& priority_list = GetUpdatePriorityList(*entry.update);
+      for (size_t priority = 0; priority < priority_list.size(); ++priority) {
+        const auto& priority_entry = priority_list[priority];
+        std::string priority_child_name = entry.GetChildPolicyName(priority);
+        for (const auto& p : priority_entry.localities) {
+          const auto& locality_name = p.first;
+          const auto& locality = p.second;
+          std::vector<RefCountedStringValue> hierarchical_path = {
+              RefCountedStringValue(priority_child_name),
+              RefCountedStringValue(locality_name->AsHumanReadableString())};
+          auto hierarchical_path_attr =
+              MakeRefCounted<HierarchicalPathArg>(std::move(hierarchical_path));
+          for (const auto& endpoint : locality.endpoints) {
+            uint32_t endpoint_weight =
+                locality.lb_weight *
+                endpoint.args().GetInt(GRPC_ARG_ADDRESS_WEIGHT).value_or(1);
+            callback(EndpointAddresses(
+                endpoint.addresses(),
+                endpoint.args()
+                    .SetObject(hierarchical_path_attr)
+                    .Set(GRPC_ARG_ADDRESS_WEIGHT, endpoint_weight)
+                    .SetObject(locality_name->Ref())
+                    .Set(GRPC_ARG_XDS_LOCALITY_WEIGHT, locality.lb_weight)));
+          }
         }
       }
     }
   }
-  return addresses;
+
+ private:
+  std::vector<DiscoveryMechanismResult> results_;
+};
+
+std::shared_ptr<EndpointAddressesIterator>
+XdsClusterResolverLb::CreateChildPolicyAddressesLocked() {
+  std::vector<PriorityEndpointIterator::DiscoveryMechanismResult> entries;
+  entries.reserve(discovery_mechanisms_.size());
+  for (const auto& discovery_entry : discovery_mechanisms_) {
+    entries.emplace_back(discovery_entry.latest_update,
+                         discovery_entry.config().cluster_name,
+                         discovery_entry.priority_child_numbers);
+  }
+  return std::make_shared<PriorityEndpointIterator>(std::move(entries));
 }
 
 std::string XdsClusterResolverLb::CreateChildPolicyResolutionNoteLocked() {

src/core/ext/filters/client_channel/lb_policy/xds/xds_override_host.cc

@@ -18,7 +18,6 @@
 
 #include "src/core/ext/filters/client_channel/lb_policy/xds/xds_override_host.h"
 
-#include <inttypes.h>
 #include <stddef.h>
 
 #include <algorithm>
@@ -34,6 +33,7 @@
 #include <vector>
 
 #include "absl/base/thread_annotations.h"
+#include "absl/functional/function_ref.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_cat.h"
@@ -300,8 +300,7 @@ class XdsOverrideHostLb : public LoadBalancingPolicy {
 
   void MaybeUpdatePickerLocked();
 
-  absl::StatusOr<EndpointAddressesList> UpdateAddressMap(
-      absl::StatusOr<EndpointAddressesList> endpoints);
+  void UpdateAddressMap(const EndpointAddressesIterator& endpoints);
 
   RefCountedPtr<SubchannelWrapper> AdoptSubchannel(
       const grpc_resolved_address& address,
@@ -508,12 +507,36 @@ void XdsOverrideHostLb::ResetBackoffLocked() {
   if (child_policy_ != nullptr) child_policy_->ResetBackoffLocked();
 }
 
+// Wraps the endpoint iterator and filters out endpoints in state DRAINING.
+class ChildEndpointIterator : public EndpointAddressesIterator {
+ public:
+  explicit ChildEndpointIterator(
+      std::shared_ptr<EndpointAddressesIterator> parent_it)
+      : parent_it_(std::move(parent_it)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    parent_it_->ForEach([&](const EndpointAddresses& endpoint) {
+      XdsHealthStatus status = GetEndpointHealthStatus(endpoint);
+      if (status.status() != XdsHealthStatus::kDraining) {
+        if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
+          gpr_log(GPR_INFO,
+                  "[xds_override_host_lb %p] endpoint %s: not draining, "
+                  "passing to child",
+                  this, endpoint.ToString().c_str());
+        }
+        callback(endpoint);
+      }
+    });
+  }
+
+ private:
+  std::shared_ptr<EndpointAddressesIterator> parent_it_;
+};
+
 absl::Status XdsOverrideHostLb::UpdateLocked(UpdateArgs args) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
-    gpr_log(GPR_INFO,
-            "[xds_override_host_lb %p] Received update with %" PRIuPTR
-            " addresses",
-            this, args.addresses.ok() ? args.addresses->size() : 0);
+    gpr_log(GPR_INFO, "[xds_override_host_lb %p] Received update", this);
   }
   auto old_config = std::move(config_);
   // Update config.
@@ -521,13 +544,24 @@ absl::Status XdsOverrideHostLb::UpdateLocked(UpdateArgs args) {
   if (config_ == nullptr) {
     return absl::InvalidArgumentError("Missing policy config");
   }
+  // Update address map and wrap endpoint iterator for child policy.
+  if (args.addresses.ok()) {
+    UpdateAddressMap(**args.addresses);
+    args.addresses =
+        std::make_shared<ChildEndpointIterator>(std::move(*args.addresses));
+  } else {
+    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
+      gpr_log(GPR_INFO, "[xds_override_host_lb %p] address error: %s", this,
+              args.addresses.status().ToString().c_str());
+    }
+  }
   // Create child policy if needed.
   if (child_policy_ == nullptr) {
     child_policy_ = CreateChildPolicyLocked(args.args);
   }
   // Update child policy.
   UpdateArgs update_args;
-  update_args.addresses = UpdateAddressMap(std::move(args.addresses));
+  update_args.addresses = std::move(args.addresses);
   update_args.resolution_note = std::move(args.resolution_note);
   update_args.config = config_->child_config();
   update_args.args = std::move(args.args);
@@ -578,18 +612,9 @@ OrphanablePtr<LoadBalancingPolicy> XdsOverrideHostLb::CreateChildPolicyLocked(
   return lb_policy;
 }
 
-absl::StatusOr<EndpointAddressesList> XdsOverrideHostLb::UpdateAddressMap(
-    absl::StatusOr<EndpointAddressesList> endpoints) {
-  if (!endpoints.ok()) {
-    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
-      gpr_log(GPR_INFO, "[xds_override_host_lb %p] address error: %s", this,
-              endpoints.status().ToString().c_str());
-    }
-    return endpoints;
-  }
-  // Construct the list of addresses to pass to the child policy and a
-  // map of address info from which to update subchannel_map_.
-  EndpointAddressesList child_addresses;
+void XdsOverrideHostLb::UpdateAddressMap(
+    const EndpointAddressesIterator& endpoints) {
+  // Construct a map of address info from which to update subchannel_map_.
   struct AddressInfo {
     XdsHealthStatus eds_health_status;
     RefCountedStringValue address_list;
@@ -597,25 +622,18 @@ absl::StatusOr<EndpointAddressesList> XdsOverrideHostLb::UpdateAddressMap(
         : eds_health_status(status), address_list(std::move(addresses)) {}
   };
   std::map<const std::string, AddressInfo> addresses_for_map;
-  for (const auto& endpoint : *endpoints) {
+  endpoints.ForEach([&](const EndpointAddresses& endpoint) {
     XdsHealthStatus status = GetEndpointHealthStatus(endpoint);
-    if (status.status() != XdsHealthStatus::kDraining) {
-      if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
-        gpr_log(GPR_INFO,
-                "[xds_override_host_lb %p] endpoint %s: not draining, "
-                "passing to child",
-                this, endpoint.ToString().c_str());
-      }
-      child_addresses.push_back(endpoint);
-    } else if (!config_->override_host_status_set().Contains(status)) {
-      // Skip draining hosts if not in the override status set.
+    // Skip draining hosts if not in the override status set.
+    if (status.status() == XdsHealthStatus::kDraining &&
+        !config_->override_host_status_set().Contains(status)) {
       if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_xds_override_host_trace)) {
         gpr_log(GPR_INFO,
                 "[xds_override_host_lb %p] endpoint %s: draining but not in "
                 "override_host_status set -- ignoring",
                 this, endpoint.ToString().c_str());
       }
-      continue;
+      return;
     }
     std::vector<std::string> addresses;
     addresses.reserve(endpoint.addresses().size());
@@ -641,7 +659,7 @@ absl::StatusOr<EndpointAddressesList> XdsOverrideHostLb::UpdateAddressMap(
           std::piecewise_construct, std::forward_as_tuple(addresses[i]),
           std::forward_as_tuple(status, std::move(address_list)));
     }
-  }
+  });
   // Now grab the lock and update subchannel_map_ from addresses_for_map.
   {
     MutexLock lock(&subchannel_map_mu_);
@@ -688,7 +706,6 @@ absl::StatusOr<EndpointAddressesList> XdsOverrideHostLb::UpdateAddressMap(
       it->second.set_address_list(std::move(address_info.address_list));
     }
   }
-  return child_addresses;
 }
 
 RefCountedPtr<XdsOverrideHostLb::SubchannelWrapper>

src/core/ext/filters/client_channel/lb_policy/xds/xds_wrr_locality.cc

@@ -21,7 +21,6 @@
 #include <memory>
 #include <string>
 #include <utility>
-#include <vector>
 
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
@@ -170,10 +169,10 @@ absl::Status XdsWrrLocalityLb::UpdateLocked(UpdateArgs args) {
   // Scan the addresses to find the weight for each locality.
   std::map<std::string, uint32_t> locality_weights;
   if (args.addresses.ok()) {
-    for (const auto& address : *args.addresses) {
-      auto* locality_name = address.args().GetObject<XdsLocalityName>();
+    (*args.addresses)->ForEach([&](const EndpointAddresses& endpoint) {
+      auto* locality_name = endpoint.args().GetObject<XdsLocalityName>();
       uint32_t weight =
-          address.args().GetInt(GRPC_ARG_XDS_LOCALITY_WEIGHT).value_or(0);
+          endpoint.args().GetInt(GRPC_ARG_XDS_LOCALITY_WEIGHT).value_or(0);
       if (locality_name != nullptr && weight > 0) {
         auto p = locality_weights.emplace(
             locality_name->AsHumanReadableString(), weight);
@@ -184,7 +183,7 @@ absl::Status XdsWrrLocalityLb::UpdateLocked(UpdateArgs args) {
                   p.first->first.c_str(), p.first->second, weight);
         }
       }
-    }
+    });
   }
   // Construct the config for the weighted_target policy.
   Json::Object weighted_targets;

src/core/lib/gprpp/ref_counted_string.h

@@ -104,6 +104,19 @@ inline bool operator==(const RefCountedStringValue& lhs,
   return lhs.as_string_view() == rhs.as_string_view();
 }
 
+inline bool operator!=(const RefCountedStringValue& lhs,
+                       absl::string_view rhs) {
+  return lhs.as_string_view() != rhs;
+}
+inline bool operator!=(absl::string_view lhs,
+                       const RefCountedStringValue& rhs) {
+  return lhs != rhs.as_string_view();
+}
+inline bool operator!=(const RefCountedStringValue& lhs,
+                       const RefCountedStringValue& rhs) {
+  return lhs.as_string_view() != rhs.as_string_view();
+}
+
 inline bool operator<(const RefCountedStringValue& lhs, absl::string_view rhs) {
   return lhs.as_string_view() < rhs;
 }

src/core/lib/load_balancing/lb_policy.h

@@ -346,7 +346,7 @@ class LoadBalancingPolicy : public InternallyRefCounted<LoadBalancingPolicy> {
   struct UpdateArgs {
     /// A list of endpoints, each with one or more address, or an error
     /// indicating a failure to obtain the list of addresses.
-    absl::StatusOr<EndpointAddressesList> addresses;
+    absl::StatusOr<std::shared_ptr<EndpointAddressesIterator>> addresses;
     /// The LB policy config.
     RefCountedPtr<Config> config;
     /// A human-readable note providing context about the name resolution that

src/core/lib/resolver/endpoint_addresses.cc

@@ -22,7 +22,6 @@
 
 #include <string.h>
 
-#include <algorithm>
 #include <string>
 #include <utility>
 #include <vector>

src/core/lib/resolver/endpoint_addresses.h

@@ -23,8 +23,11 @@
 
 #include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
+#include "absl/functional/function_ref.h"
+
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/iomgr/resolved_address.h"
 
@@ -64,6 +67,9 @@ class EndpointAddresses {
   bool operator==(const EndpointAddresses& other) const {
     return Cmp(other) == 0;
   }
+  bool operator!=(const EndpointAddresses& other) const {
+    return Cmp(other) != 0;
+  }
   bool operator<(const EndpointAddresses& other) const {
     return Cmp(other) < 0;
   }
@@ -111,6 +117,48 @@ class EndpointAddressSet {
   std::set<grpc_resolved_address, ResolvedAddressLessThan> addresses_;
 };
 
+// An iterator interface for endpoints.
+class EndpointAddressesIterator {
+ public:
+  virtual ~EndpointAddressesIterator() = default;
+
+  // Invokes callback once for each endpoint.
+  virtual void ForEach(
+      absl::FunctionRef<void(const EndpointAddresses&)> callback) const = 0;
+};
+
+// Iterator over a fixed list of endpoints.
+class EndpointAddressesListIterator : public EndpointAddressesIterator {
+ public:
+  explicit EndpointAddressesListIterator(EndpointAddressesList endpoints)
+      : endpoints_(std::move(endpoints)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    for (const auto& endpoint : endpoints_) {
+      callback(endpoint);
+    }
+  }
+
+ private:
+  EndpointAddressesList endpoints_;
+};
+
+// Iterator that returns only a single endpoint.
+class SingleEndpointIterator : public EndpointAddressesIterator {
+ public:
+  explicit SingleEndpointIterator(EndpointAddresses endpoint)
+      : endpoint_(std::move(endpoint)) {}
+
+  void ForEach(absl::FunctionRef<void(const EndpointAddresses&)> callback)
+      const override {
+    callback(endpoint_);
+  }
+
+ private:
+  EndpointAddresses endpoint_;
+};
+
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_LIB_RESOLVER_ENDPOINT_ADDRESSES_H

test/core/client_channel/lb_policy/lb_policy_test_lib.h

@@ -773,7 +773,8 @@ class LoadBalancingPolicyTest : public ::testing::Test {
       absl::Span<const EndpointAddresses> endpoints,
       RefCountedPtr<LoadBalancingPolicy::Config> config) {
     LoadBalancingPolicy::UpdateArgs update;
-    update.addresses.emplace(endpoints.begin(), endpoints.end());
+    update.addresses = std::make_shared<EndpointAddressesListIterator>(
+        EndpointAddressesList(endpoints.begin(), endpoints.end()));
     update.config = std::move(config);
     return update;
   }

test/core/client_channel/lb_policy/xds_override_host_test.cc

@@ -99,11 +99,13 @@ class XdsOverrideHostTest : public LoadBalancingPolicyTest {
                                                                   "HEALTHY"}) {
     LoadBalancingPolicy::UpdateArgs update;
     update.config = MakeXdsOverrideHostConfig(override_host_status);
-    update.addresses.emplace();
+    EndpointAddressesList endpoints;
     for (auto address_and_status : addresses_and_statuses) {
-      update.addresses->push_back(MakeAddressWithHealthStatus(
+      endpoints.push_back(MakeAddressWithHealthStatus(
           address_and_status.first, address_and_status.second));
     }
+    update.addresses =
+        std::make_shared<EndpointAddressesListIterator>(std::move(endpoints));
     EXPECT_EQ(ApplyUpdate(update, lb_policy()), absl::OkStatus());
   }
 

test/core/util/test_lb_policies.cc

@@ -414,17 +414,19 @@ class FixedAddressLoadBalancingPolicy : public ForwardingLoadBalancingPolicy {
             config->address().c_str());
     auto uri = URI::Parse(config->address());
     args.config.reset();
-    args.addresses = EndpointAddressesList();
+    EndpointAddressesList addresses;
     if (uri.ok()) {
       grpc_resolved_address address;
       GPR_ASSERT(grpc_parse_uri(*uri, &address));
-      args.addresses->emplace_back(address, ChannelArgs());
+      addresses.emplace_back(address, ChannelArgs());
     } else {
       gpr_log(GPR_ERROR,
               "%s: could not parse URI (%s), using empty address list",
               kFixedAddressLbPolicyName, uri.status().ToString().c_str());
       args.resolution_note = "no address in fixed_address_lb policy";
     }
+    args.addresses =
+        std::make_shared<EndpointAddressesListIterator>(std::move(addresses));
     return ForwardingLoadBalancingPolicy::UpdateLocked(std::move(args));
   }