Store address hierarchy information in attributes instead of channel args.

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

@@ -18,64 +18,77 @@
 
 #include "src/core/ext/filters/client_channel/lb_policy/address_filtering.h"
 
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_join.h"
+
 #include "src/core/lib/channel/channel_args.h"
 
 #define GRPC_ARG_HIERARCHICAL_PATH "grpc.internal.address.hierarchical_path"
 
 namespace grpc_core {
 
+const char* kHierarchicalPathAttributeKey = "hierarchical_path";
+
 namespace {
 
-void* HierarchicalPathCopy(void* p) {
-  std::vector<std::string>* path = static_cast<std::vector<std::string>*>(p);
-  return static_cast<void*>(new std::vector<std::string>(*path));
-}
+class HierarchicalPathAttribute : public ServerAddress::AttributeInterface {
+ public:
+  explicit HierarchicalPathAttribute(std::vector<std::string> path)
+      : path_(std::move(path)) {}
 
-void HierarchicalPathDestroy(void* p) {
-  std::vector<std::string>* path = static_cast<std::vector<std::string>*>(p);
-  delete path;
-}
+  std::unique_ptr<AttributeInterface> Copy() const override {
+    return absl::make_unique<HierarchicalPathAttribute>(path_);
+  }
 
-int HierarchicalPathCompare(void* p1, void* p2) {
-  std::vector<std::string>* path1 = static_cast<std::vector<std::string>*>(p1);
-  std::vector<std::string>* path2 = static_cast<std::vector<std::string>*>(p2);
-  for (size_t i = 0; i < path1->size(); ++i) {
-    if (path2->size() == i) return 1;
-    int r = (*path1)[i].compare((*path2)[i]);
-    if (r != 0) return r;
+  int Cmp(const AttributeInterface* other) const override {
+    const std::vector<std::string>& other_path =
+        static_cast<const HierarchicalPathAttribute*>(other)->path_;
+    for (size_t i = 0; i < path_.size(); ++i) {
+      if (other_path.size() == i) return 1;
+      int r = path_[i].compare(other_path[i]);
+      if (r != 0) return r;
+    }
+    if (other_path.size() > path_.size()) return -1;
+    return 0;
   }
-  if (path2->size() > path1->size()) return -1;
-  return 0;
-}
 
-const grpc_arg_pointer_vtable hierarchical_path_arg_vtable = {
-    HierarchicalPathCopy, HierarchicalPathDestroy, HierarchicalPathCompare};
+  std::string ToString() const override {
+    return absl::StrCat("[", absl::StrJoin(path_, ", "), "]");
+  }
+
+  const std::vector<std::string>& path() const { return path_; }
+
+ private:
+  std::vector<std::string> path_;
+};
 
 }  // namespace
 
-grpc_arg MakeHierarchicalPathArg(const std::vector<std::string>& path) {
-  return grpc_channel_arg_pointer_create(
-      const_cast<char*>(GRPC_ARG_HIERARCHICAL_PATH),
-      const_cast<std::vector<std::string>*>(&path),
-      &hierarchical_path_arg_vtable);
+std::unique_ptr<ServerAddress::AttributeInterface>
+MakeHierarchicalPathAttribute(std::vector<std::string> path) {
+  return absl::make_unique<HierarchicalPathAttribute>(std::move(path));
 }
 
 HierarchicalAddressMap MakeHierarchicalAddressMap(
     const ServerAddressList& addresses) {
   HierarchicalAddressMap result;
   for (const ServerAddress& address : addresses) {
-    auto* path = grpc_channel_args_find_pointer<std::vector<std::string>>(
-        address.args(), GRPC_ARG_HIERARCHICAL_PATH);
-    if (path == nullptr || path->empty()) continue;
-    auto it = path->begin();
+    const HierarchicalPathAttribute* path_attribute =
+        static_cast<const HierarchicalPathAttribute*>(
+            address.GetAttribute(kHierarchicalPathAttributeKey));
+    if (path_attribute == nullptr) continue;
+    const std::vector<std::string>& path = path_attribute->path();
+    auto it = path.begin();
     ServerAddressList& target_list = result[*it];
+    std::unique_ptr<HierarchicalPathAttribute> new_attribute;
     ++it;
-    std::vector<std::string> remaining_path(it, path->end());
-    const char* name_to_remove = GRPC_ARG_HIERARCHICAL_PATH;
-    grpc_arg new_arg = MakeHierarchicalPathArg(remaining_path);
-    grpc_channel_args* new_args = grpc_channel_args_copy_and_add_and_remove(
-        address.args(), &name_to_remove, 1, &new_arg, 1);
-    target_list.emplace_back(address.address(), new_args);
+    if (it != path.end()) {
+      std::vector<std::string> remaining_path(it, path.end());
+      new_attribute = absl::make_unique<HierarchicalPathAttribute>(
+          std::move(remaining_path));
+    }
+    target_list.emplace_back(address.WithAttribute(
+        kHierarchicalPathAttributeKey, std::move(new_attribute)));
   }
   return result;
 }

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

@@ -23,8 +23,6 @@
 #include <string>
 #include <vector>
 
-#include "absl/strings/string_view.h"
-
 #include "src/core/ext/filters/client_channel/server_address.h"
 
 // The resolver returns a flat list of addresses.  When a hierarchy of
@@ -81,9 +79,13 @@
 
 namespace grpc_core {
 
-// Constructs a channel arg containing the hierarchical path
-// to be associated with an address.
-grpc_arg MakeHierarchicalPathArg(const std::vector<std::string>& path);
+// The attribute key to be used for hierarchical paths in ServerAddress.
+extern const char* kHierarchicalPathAttributeKey;
+
+// Constructs an address attribute containing the hierarchical path
+// to be associated with the address.
+std::unique_ptr<ServerAddress::AttributeInterface>
+MakeHierarchicalPathAttribute(std::vector<std::string> path);
 
 // A map from the next path element to the addresses that fall under
 // that path element.

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

@@ -591,10 +591,9 @@ ServerAddressList EdsLb::CreateChildPolicyAddressesLocked() {
       std::vector<std::string> hierarchical_path = {
           priority_child_name, locality_name->AsHumanReadableString()};
       for (const auto& endpoint : locality.endpoints) {
-        grpc_arg new_arg = MakeHierarchicalPathArg(hierarchical_path);
-        grpc_channel_args* args =
-            grpc_channel_args_copy_and_add(endpoint.args(), &new_arg, 1);
-        addresses.emplace_back(endpoint.address(), args);
+        addresses.emplace_back(endpoint.WithAttribute(
+            kHierarchicalPathAttributeKey,
+            MakeHierarchicalPathAttribute(hierarchical_path)));
       }
     }
   }

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

@@ -20,6 +20,15 @@
 
 #include "src/core/ext/filters/client_channel/server_address.h"
 
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_join.h"
+
+#include "src/core/lib/iomgr/sockaddr_utils.h"
+
 namespace grpc_core {
 
 //
@@ -39,6 +48,38 @@ ServerAddress::ServerAddress(
   address_.len = static_cast<socklen_t>(address_len);
 }
 
+ServerAddress::ServerAddress(const ServerAddress& other)
+    : address_(other.address_), args_(grpc_channel_args_copy(other.args_)) {
+  for (const auto& p : other.attributes_) {
+    attributes_[p.first] = p.second->Copy();
+  }
+}
+ServerAddress& ServerAddress::operator=(const ServerAddress& other) {
+  address_ = other.address_;
+  grpc_channel_args_destroy(args_);
+  args_ = grpc_channel_args_copy(other.args_);
+  attributes_.clear();
+  for (const auto& p : other.attributes_) {
+    attributes_[p.first] = p.second->Copy();
+  }
+  return *this;
+}
+
+ServerAddress::ServerAddress(ServerAddress&& other) noexcept
+    : address_(other.address_),
+      args_(other.args_),
+      attributes_(std::move(other.attributes_)) {
+  other.args_ = nullptr;
+}
+ServerAddress& ServerAddress::operator=(ServerAddress&& other) noexcept {
+  address_ = other.address_;
+  grpc_channel_args_destroy(args_);
+  args_ = other.args_;
+  other.args_ = nullptr;
+  attributes_ = std::move(other.attributes_);
+  return *this;
+}
+
 namespace {
 
 int CompareAttributes(
@@ -78,4 +119,43 @@ int ServerAddress::Cmp(const ServerAddress& other) const {
   return CompareAttributes(attributes_, other.attributes_);
 }
 
+const ServerAddress::AttributeInterface* ServerAddress::GetAttribute(
+    const char* key) const {
+  auto it = attributes_.find(key);
+  if (it == attributes_.end()) return nullptr;
+  return it->second.get();
+}
+
+// Returns a copy of the address with a modified attribute.
+// If the new value is null, the attribute is removed.
+ServerAddress ServerAddress::WithAttribute(
+    const char* key, std::unique_ptr<AttributeInterface> value) const {
+  ServerAddress address = *this;
+  if (value == nullptr) {
+    address.attributes_.erase(key);
+  } else {
+    address.attributes_[key] = std::move(value);
+  }
+  return address;
+}
+
+std::string ServerAddress::ToString() const {
+  std::vector<std::string> parts = {
+      grpc_sockaddr_to_string(&address_, false),
+  };
+  if (args_ != nullptr) {
+    parts.emplace_back(
+        absl::StrCat("args={", grpc_channel_args_string(args_), "}"));
+  }
+  if (!attributes_.empty()) {
+    std::vector<std::string> attrs;
+    for (const auto& p : attributes_) {
+      attrs.emplace_back(absl::StrCat(p.first, "=", p.second->ToString()));
+    }
+    parts.emplace_back(
+        absl::StrCat("attributes={", absl::StrJoin(attrs, ", "), "}"));
+  }
+  return absl::StrJoin(parts, " ");
+}
+
 }  // namespace grpc_core

src/core/ext/filters/client_channel/server_address.h

@@ -22,6 +22,7 @@
 #include <grpc/support/port_platform.h>
 
 #include <map>
+#include <memory>
 
 #include "absl/container/inlined_vector.h"
 
@@ -42,15 +43,22 @@ class ServerAddress {
   // Base class for resolver-supplied attributes.
   // Unlike channel args, these attributes don't affect subchannel
   // uniqueness or behavior.  They are for use by LB policies only.
+  //
+  // Attributes are keyed by a C string that is unique by address, not
+  // by value.  All attributes added with the same key must be of the
+  // same type.
   class AttributeInterface {
    public:
-    virtual ~AttributeInterface();
+    virtual ~AttributeInterface() = default;
 
     // Creates a copy of the attribute.
     virtual std::unique_ptr<AttributeInterface> Copy() const = 0;
 
     // Compares this attribute with another.
     virtual int Cmp(const AttributeInterface* other) const = 0;
+
+    // Returns a human-readable representation of the attribute.
+    virtual std::string ToString() const = 0;
   };
 
   // Takes ownership of args.
@@ -65,38 +73,12 @@ class ServerAddress {
   ~ServerAddress() { grpc_channel_args_destroy(args_); }
 
   // Copyable.
-  ServerAddress(const ServerAddress& other)
-      : address_(other.address_), args_(grpc_channel_args_copy(other.args_)) {
-    for (const auto& p : other.attributes_) {
-      attributes_[p.first] = p.second->Copy();
-    }
-  }
-  ServerAddress& operator=(const ServerAddress& other) {
-    address_ = other.address_;
-    grpc_channel_args_destroy(args_);
-    args_ = grpc_channel_args_copy(other.args_);
-    attributes_.clear();
-    for (const auto& p : other.attributes_) {
-      attributes_[p.first] = p.second->Copy();
-    }
-    return *this;
-  }
+  ServerAddress(const ServerAddress& other);
+  ServerAddress& operator=(const ServerAddress& other);
 
   // Movable.
-  ServerAddress(ServerAddress&& other)
-      : address_(other.address_),
-        args_(other.args_),
-        attributes_(std::move(other.attributes_)) {
-    other.args_ = nullptr;
-  }
-  ServerAddress& operator=(ServerAddress&& other) {
-    address_ = other.address_;
-    grpc_channel_args_destroy(args_);
-    args_ = other.args_;
-    other.args_ = nullptr;
-    attributes_ = std::move(other.attributes_);
-    return *this;
-  }
+  ServerAddress(ServerAddress&& other) noexcept;
+  ServerAddress& operator=(ServerAddress&& other) noexcept;
 
   bool operator==(const ServerAddress& other) const { return Cmp(other) == 0; }
 
@@ -105,11 +87,14 @@ class ServerAddress {
   const grpc_resolved_address& address() const { return address_; }
   const grpc_channel_args* args() const { return args_; }
 
-  const AttributeInterface* GetAttribute(const char* key) const {
-    auto it = attributes_.find(key);
-    if (it == attributes_.end()) return nullptr;
-    return it->second.get();
-  }
+  const AttributeInterface* GetAttribute(const char* key) const;
+
+  // Returns a copy of the address with a modified attribute.
+  // If the new value is null, the attribute is removed.
+  ServerAddress WithAttribute(const char* key,
+                              std::unique_ptr<AttributeInterface> value) const;
+
+  std::string ToString() const;
 
  private:
   grpc_resolved_address address_;

src/core/ext/xds/xds_api.cc

@@ -381,9 +381,7 @@ XdsApi::RdsUpdate::VirtualHost* XdsApi::RdsUpdate::FindVirtualHostForDomain(
 std::string XdsApi::EdsUpdate::Priority::Locality::ToString() const {
   std::vector<std::string> endpoint_strings;
   for (const ServerAddress& endpoint : endpoints) {
-    endpoint_strings.emplace_back(
-        absl::StrCat(grpc_sockaddr_to_string(&endpoint.address(), false), "{",
-                     grpc_channel_args_string(endpoint.args()), "}"));
+    endpoint_strings.emplace_back(endpoint.ToString());
   }
   return absl::StrCat("{name=", name->AsHumanReadableString(),
                       ", lb_weight=", lb_weight, ", endpoints=[",