Fix RR concurrent updates

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

@@ -195,11 +195,28 @@ static void rr_subchannel_list_unref(grpc_exec_ctx *exec_ctx,
 static void rr_subchannel_list_shutdown(grpc_exec_ctx *exec_ctx,
                                         rr_subchannel_list *subchannel_list,
                                         const char *reason) {
+  if (subchannel_list->shutting_down) {
+    if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+      gpr_log(GPR_DEBUG, "Subchannel list %p already shutting down",
+              (void *)subchannel_list);
+    }
+    return;
+  };
+  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+    gpr_log(GPR_DEBUG, "Shutting down subchannel_list %p",
+            (void *)subchannel_list);
+  }
   GPR_ASSERT(!subchannel_list->shutting_down);
   subchannel_list->shutting_down = true;
   for (size_t i = 0; i < subchannel_list->num_subchannels; i++) {
     subchannel_data *sd = &subchannel_list->subchannels[i];
     if (sd->subchannel != NULL) {  // if subchannel isn't shutdown, unsubscribe.
+      if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+        gpr_log(GPR_DEBUG,
+                "Unsubscribing from subchannel %p as part of shutting down "
+                "subchannel_list %p",
+                (void *)sd->subchannel, (void *)subchannel_list);
+      }
       grpc_subchannel_notify_on_state_change(exec_ctx, sd->subchannel, NULL,
                                              NULL,
                                              &sd->connectivity_changed_closure);
@@ -228,13 +245,14 @@ static size_t get_next_ready_subchannel_index_locked(
     const size_t index = (i + p->last_ready_subchannel_index + 1) %
                          p->subchannel_list->num_subchannels;
     if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
-      gpr_log(GPR_DEBUG,
+      gpr_log(
+          GPR_DEBUG,
           "[RR %p] checking subchannel %p, subchannel_list %p, index %lu: "
-              "state=%d",
-              (void *)p,
-              (void *)p->subchannel_list->subchannels[index].subchannel,
+          "state=%s",
+          (void *)p, (void *)p->subchannel_list->subchannels[index].subchannel,
           (void *)p->subchannel_list, (unsigned long)index,
-              p->subchannel_list->subchannels[index].curr_connectivity_state);
+          grpc_connectivity_state_name(
+              p->subchannel_list->subchannels[index].curr_connectivity_state));
     }
     if (p->subchannel_list->subchannels[index].curr_connectivity_state ==
         GRPC_CHANNEL_READY) {
@@ -511,16 +529,27 @@ static void rr_connectivity_changed_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                            grpc_error *error) {
   subchannel_data *sd = arg;
   round_robin_lb_policy *p = sd->subchannel_list->policy;
+  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+    gpr_log(
+        GPR_DEBUG,
+        "[RR %p] connectivity changed for subchannel %p, subchannel_list %p: "
+        "prev_state=%s new_state=%s p->shutdown=%d "
+        "sd->subchannel_list->shutting_down=%d error=%s",
+        (void *)p, (void *)sd->subchannel, (void *)sd->subchannel_list,
+        grpc_connectivity_state_name(sd->prev_connectivity_state),
+        grpc_connectivity_state_name(sd->pending_connectivity_state_unsafe),
+        p->shutdown, sd->subchannel_list->shutting_down,
+        grpc_error_string(error));
+  }
   // If the policy is shutting down, unref and return.
   if (p->shutdown) {
     rr_subchannel_list_unref(exec_ctx, sd->subchannel_list, "pol_shutdown");
     GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, &p->base, "pol_shutdown");
     return;
   }
-  if (sd->subchannel_list->shutting_down) {
+  if (sd->subchannel_list->shutting_down && error == GRPC_ERROR_CANCELLED) {
     // the subchannel list associated with sd has been discarded. This callback
     // corresponds to the unsubscription.
-    GPR_ASSERT(error == GRPC_ERROR_CANCELLED);
     rr_subchannel_list_unref(exec_ctx, sd->subchannel_list, "sl_shutdown");
     GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, &p->base, "sl_shutdown");
     return;
@@ -536,13 +565,6 @@ static void rr_connectivity_changed_locked(grpc_exec_ctx *exec_ctx, void *arg,
   // state (which was set by the connectivity state watcher) to
   // curr_connectivity_state, which is what we use inside of the combiner.
   sd->curr_connectivity_state = sd->pending_connectivity_state_unsafe;
-  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
-    gpr_log(GPR_DEBUG,
-            "[RR %p] connectivity changed for subchannel %p: "
-            "prev_state=%d new_state=%d",
-            (void *)p, (void *)sd->subchannel, sd->prev_connectivity_state,
-            sd->curr_connectivity_state);
-  }
   // Update state counters and determine new overall state.
   update_state_counters_locked(sd);
   sd->prev_connectivity_state = sd->curr_connectivity_state;

test/cpp/end2end/client_lb_end2end_test.cc

@@ -463,6 +463,11 @@ TEST_F(ClientLbEnd2endTest, RoundRobinManyUpdates) {
   EXPECT_EQ("round_robin", channel_->GetLoadBalancingPolicyName());
 }
 
+TEST_F(ClientLbEnd2endTest, RoundRobinConcurrentUpdates) {
+  // TODO(dgq): replicate the way internal testing exercises the concurrent
+  // update provisions of RR.
+}
+
 TEST_F(ClientLbEnd2endTest, RoundRobinReconnect) {
   // Start servers and send one RPC per server.
   const int kNumServers = 1;