From 9e841d1aff6b33408dc9ee9480927a1ef69df8ec Mon Sep 17 00:00:00 2001
From: "Mark D. Roth" <roth@google.com>
Date: Mon, 1 Apr 2019 14:55:29 -0700
Subject: [PATCH] Backport pick_first fix to v1.20.x

---
 .../lb_policy/pick_first/pick_first.cc        | 26 ++++++++++++++-----
 test/cpp/end2end/client_lb_end2end_test.cc    | 26 +++++++++++++++++++
 2 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
index 86c6b25ac63..3da0b59f279 100644
--- a/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc
@@ -188,8 +188,14 @@ void PickFirst::ShutdownLocked() {
 void PickFirst::ExitIdleLocked() {
   if (idle_) {
     idle_ = false;
-    if (subchannel_list_ != nullptr &&
-        subchannel_list_->num_subchannels() > 0) {
+    if (subchannel_list_ == nullptr ||
+        subchannel_list_->num_subchannels() == 0) {
+      grpc_error* error =
+          GRPC_ERROR_CREATE_FROM_STATIC_STRING("No addresses to connect to");
+      channel_control_helper()->UpdateState(
+          GRPC_CHANNEL_TRANSIENT_FAILURE, GRPC_ERROR_REF(error),
+          UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(error)));
+    } else {
       subchannel_list_->subchannel(0)
           ->CheckConnectivityStateAndStartWatchingLocked();
     }
@@ -253,13 +259,19 @@ void PickFirst::UpdateLocked(UpdateArgs args) {
   grpc_channel_args_destroy(new_args);
   if (subchannel_list->num_subchannels() == 0) {
     // Empty update or no valid subchannels. Unsubscribe from all current
-    // subchannels and put the channel in TRANSIENT_FAILURE.
+    // subchannels.
     subchannel_list_ = std::move(subchannel_list);  // Empty list.
     selected_ = nullptr;
-    grpc_error* error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Empty update");
-    channel_control_helper()->UpdateState(
-        GRPC_CHANNEL_TRANSIENT_FAILURE, GRPC_ERROR_REF(error),
-        UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(error)));
+    // If not idle, put the channel in TRANSIENT_FAILURE.
+    // (If we are idle, then this will happen in ExitIdleLocked() if we
+    // haven't gotten a non-empty update by the time the application tries
+    // to start a new call.)
+    if (!idle_) {
+      grpc_error* error = GRPC_ERROR_CREATE_FROM_STATIC_STRING("Empty update");
+      channel_control_helper()->UpdateState(
+          GRPC_CHANNEL_TRANSIENT_FAILURE, GRPC_ERROR_REF(error),
+          UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(error)));
+    }
     return;
   }
   // If one of the subchannels in the new list is already in state
diff --git a/test/cpp/end2end/client_lb_end2end_test.cc b/test/cpp/end2end/client_lb_end2end_test.cc
index 4244690d76b..77f9db94acc 100644
--- a/test/cpp/end2end/client_lb_end2end_test.cc
+++ b/test/cpp/end2end/client_lb_end2end_test.cc
@@ -940,6 +940,32 @@ TEST_F(ClientLbEnd2endTest, PickFirstPendingUpdateAndSelectedSubchannelFails) {
   WaitForServer(stub, 1, DEBUG_LOCATION, true /* ignore_failure */);
 }
 
+TEST_F(ClientLbEnd2endTest, PickFirstStaysIdleUponEmptyUpdate) {
+  // Start server, send RPC, and make sure channel is READY.
+  const int kNumServers = 1;
+  StartServers(kNumServers);
+  auto channel = BuildChannel("");  // pick_first is the default.
+  auto stub = BuildStub(channel);
+  SetNextResolution(GetServersPorts());
+  CheckRpcSendOk(stub, DEBUG_LOCATION);
+  EXPECT_EQ(channel->GetState(false), GRPC_CHANNEL_READY);
+  // Stop server.  Channel should go into state IDLE.
+  servers_[0]->Shutdown();
+  EXPECT_TRUE(WaitForChannelNotReady(channel.get()));
+  EXPECT_EQ(channel->GetState(false), GRPC_CHANNEL_IDLE);
+  // Now send resolver update that includes no addresses.  Channel
+  // should stay in state IDLE.
+  SetNextResolution({});
+  EXPECT_FALSE(channel->WaitForStateChange(
+      GRPC_CHANNEL_IDLE, grpc_timeout_seconds_to_deadline(3)));
+  // Now bring the backend back up and send a non-empty resolver update,
+  // and then try to send an RPC.  Channel should go back into state READY.
+  StartServer(0);
+  SetNextResolution(GetServersPorts());
+  CheckRpcSendOk(stub, DEBUG_LOCATION);
+  EXPECT_EQ(channel->GetState(false), GRPC_CHANNEL_READY);
+}
+
 TEST_F(ClientLbEnd2endTest, RoundRobin) {
   // Start servers and send one RPC per server.
   const int kNumServers = 3;