Fixed ordering in adding pending picks to PF

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

@@ -279,11 +279,11 @@ bool PickFirst::PickLocked(PickState* pick, grpc_error** error) {
         "No pick result available but synchronous result required.");
     return true;
   }
+  pick->next = pending_picks_;
+  pending_picks_ = pick;
   if (!started_picking_) {
     StartPickingLocked();
   }
-  pick->next = pending_picks_;
-  pending_picks_ = pick;
   return false;
 }
 

test/cpp/end2end/client_lb_end2end_test.cc

@@ -353,6 +353,23 @@ TEST_F(ClientLbEnd2endTest, PickFirst) {
   EXPECT_EQ("pick_first", channel->GetLoadBalancingPolicyName());
 }
 
+TEST_F(ClientLbEnd2endTest, PickFirstProcessPending) {
+  StartServers(1);                  // Single server
+  auto channel = BuildChannel("");  // test that pick first is the default.
+  auto stub = BuildStub(channel);
+  SetNextResolution({servers_[0]->port_});
+  WaitForServer(stub, 0, DEBUG_LOCATION);
+  // Create a new channel and its corresponding PF LB policy, which will pick
+  // the subchannels in READY state from the previous RPC against the same
+  // target (even if it happened over a different channel, because subchannels
+  // are globally reused). Progress should happen without any transition from
+  // this READY state.
+  auto second_channel = BuildChannel("");
+  auto second_stub = BuildStub(second_channel);
+  SetNextResolution({servers_[0]->port_});
+  CheckRpcSendOk(second_stub, DEBUG_LOCATION);
+}
+
 TEST_F(ClientLbEnd2endTest, PickFirstBackOffInitialReconnect) {
   ChannelArguments args;
   constexpr int kInitialBackOffMs = 100;