Potential fix for race condition exposed by Node

src/core/channel/subchannel_call_holder.c

@@ -168,21 +168,24 @@ retry:
 
 static void subchannel_ready(grpc_exec_ctx *exec_ctx, void *arg, bool success) {
   grpc_subchannel_call_holder *holder = arg;
-  grpc_subchannel_call *call;
   gpr_mu_lock(&holder->mu);
   GPR_ASSERT(holder->creation_phase ==
              GRPC_SUBCHANNEL_CALL_HOLDER_PICKING_SUBCHANNEL);
-  call = GET_CALL(holder);
-  GPR_ASSERT(call == NULL || call == CANCELLED_CALL);
   holder->creation_phase = GRPC_SUBCHANNEL_CALL_HOLDER_NOT_CREATING;
   if (holder->connected_subchannel == NULL) {
     fail_locked(exec_ctx, holder);
   } else {
-    gpr_atm_rel_store(
+    if (!gpr_atm_rel_cas(
         &holder->subchannel_call,
+        0,
         (gpr_atm)(uintptr_t)grpc_connected_subchannel_create_call(
-            exec_ctx, holder->connected_subchannel, holder->pollset));
-    retry_waiting_locked(exec_ctx, holder);
+            exec_ctx, holder->connected_subchannel, holder->pollset))) {
+      GPR_ASSERT(gpr_atm_acq_load(&holder->subchannel_call) == 1);
+      /* if this cas fails, the call was cancelled before the pick completed */
+      fail_locked(exec_ctx, holder);
+    } else {
+      retry_waiting_locked(exec_ctx, holder);
+    }
   }
   gpr_mu_unlock(&holder->mu);
   GRPC_CALL_STACK_UNREF(exec_ctx, holder->owning_call, "pick_subchannel");