[LB policies] avoid looping over all endpoints/addresses (#35445)

This should slightly increase per-channel memory but will eliminate some O(n^2) loops with large numbers of endpoints or addresses.

Closes #35445

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35445 from markdroth:lb_policies_remove_unnecessary_loop 94465f44ec
PiperOrigin-RevId: 596007480

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

@@ -69,6 +69,9 @@ class EndpointList::Endpoint::Helper
       grpc_connectivity_state state, const absl::Status& status,
       RefCountedPtr<LoadBalancingPolicy::SubchannelPicker> picker) override {
     auto old_state = std::exchange(endpoint_->connectivity_state_, state);
+    if (!old_state.has_value()) {
+      ++endpoint_->endpoint_list_->num_endpoints_seen_initial_state_;
+    }
     endpoint_->picker_ = std::move(picker);
     endpoint_->OnStateUpdate(old_state, state, status);
   }
@@ -181,11 +184,4 @@ void EndpointList::ResetBackoffLocked() {
   }
 }
 
-bool EndpointList::AllEndpointsSeenInitialState() const {
-  for (const auto& endpoint : endpoints_) {
-    if (!endpoint->connectivity_state().has_value()) return false;
-  }
-  return true;
-}
-
 }  // namespace grpc_core

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

@@ -199,7 +199,9 @@ class EndpointList : public InternallyRefCounted<EndpointList> {
 
   // Returns true if all endpoints have seen their initial connectivity
   // state notification.
-  bool AllEndpointsSeenInitialState() const;
+  bool AllEndpointsSeenInitialState() const {
+    return num_endpoints_seen_initial_state_ == size();
+  }
 
  private:
   // Returns the parent policy's helper.  Needed because the accessor
@@ -210,6 +212,7 @@ class EndpointList : public InternallyRefCounted<EndpointList> {
   RefCountedPtr<LoadBalancingPolicy> policy_;
   const char* tracer_;
   std::vector<OrphanablePtr<Endpoint>> endpoints_;
+  size_t num_endpoints_seen_initial_state_ = 0;
 };
 
 }  // namespace grpc_core

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

@@ -224,7 +224,9 @@ class PickFirst : public LoadBalancingPolicy {
    private:
     // Returns true if all subchannels have seen their initial
     // connectivity state notifications.
-    bool AllSubchannelsSeenInitialState();
+    bool AllSubchannelsSeenInitialState() const {
+      return num_subchannels_seen_initial_notification_ == size();
+    }
 
     // Looks through subchannels_ starting from attempting_index_ to
     // find the first one not currently in TRANSIENT_FAILURE, then
@@ -255,6 +257,8 @@ class PickFirst : public LoadBalancingPolicy {
     // TODO(roth): Remove this when we remove the Happy Eyeballs experiment.
     bool in_transient_failure_ = false;
 
+    size_t num_subchannels_seen_initial_notification_ = 0;
+
     // The index into subchannels_ to which we are currently attempting
     // to connect during the initial Happy Eyeballs pass.  Once the
     // initial pass is over, this will be equal to size().
@@ -754,6 +758,11 @@ void PickFirst::SubchannelList::SubchannelData::OnConnectivityStateChange(
     seen_transient_failure_ = true;
     subchannel_list_->last_failure_ = connectivity_status_;
   }
+  // If this is the initial connectivity state update for this subchannel,
+  // increment the counter in the subchannel list.
+  if (!old_state.has_value()) {
+    ++subchannel_list_->num_subchannels_seen_initial_notification_;
+  }
   // If we haven't yet seen the initial connectivity state notification
   // for all subchannels, do nothing.
   if (!subchannel_list_->AllSubchannelsSeenInitialState()) return;
@@ -1122,13 +1131,6 @@ void PickFirst::SubchannelList::ResetBackoffLocked() {
   }
 }
 
-bool PickFirst::SubchannelList::AllSubchannelsSeenInitialState() {
-  for (auto& sd : subchannels_) {
-    if (!sd.connectivity_state().has_value()) return false;
-  }
-  return true;
-}
-
 void PickFirst::SubchannelList::StartConnectingNextSubchannel() {
   // Find the next subchannel not in state TRANSIENT_FAILURE.
   // We skip subchannels in state TRANSIENT_FAILURE to avoid a