Deadlock with TransportSet
Hello,

I was testing Grpc with RoundRobinLB and a custom NameResolver when this deadlock happened:

> Found one Java-level deadlock:
> 
> "grpc-timer-0":
>   waiting to lock monitor 0x00007fa1b00062c8 (object 0x00000007397d7f88, a java.lang.Object),
>   which is held by "main"
> "main":
>   waiting to lock monitor 0x00007fa1800087f8 (object 0x00000007397d7e00, a java.lang.Object),
>   which is held by "grpc-timer-0"
> 
> "grpc-timer-0":
>   at io.grpc.internal.DelayedClientTransport.hasPendingStreams(DelayedClientTransport.java:284)
> - waiting to lock <0x00000007397d7f88> (a java.lang.Object)
>   at io.grpc.internal.TransportSet$1EndOfCurrentBackoff.run(TransportSet.java:246)
> - locked <0x00000007397d7e00> (a java.lang.Object)
>   at io.grpc.internal.LogExceptionRunnable.run(LogExceptionRunnable.java:56)
>   at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
>   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
>   at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
>   at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
>   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
>   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
>   at java.lang.Thread.run(Thread.java:745)
> 
> "main":
>   at io.grpc.internal.InUseStateAggregator.updateObjectInUse(InUseStateAggregator.java:50)
> - waiting to lock <0x00000007397d7e00> (a java.lang.Object)
>   at io.grpc.internal.TransportSet$BaseTransportListener.transportInUse(TransportSet.java:357)
>   at io.grpc.internal.DelayedClientTransport.newStream(DelayedClientTransport.java:128)
> - locked <0x00000007397d7f88> (a java.lang.Object)
>   at io.grpc.internal.ClientCallImpl.start(ClientCallImpl.java:214)
>   at io.grpc.stub.ClientCalls.startCall(ClientCalls.java:273)
>   at io.grpc.stub.ClientCalls.asyncUnaryRequestCall(ClientCalls.java:252)
>   at io.grpc.stub.ClientCalls.futureUnaryCall(ClientCalls.java:189)
>   at io.grpc.stub.ClientCalls.blockingUnaryCall(ClientCalls.java:135)
>   at [...]remote.TestGrpc$TestBlockingStub.sayHello(TestGrpc.java:156)

I don't know if it may relate to my own code or if the issue is on grpc side.

diff --git a/core/src/main/java/io/grpc/internal/TransportSet.java b/core/src/main/java/io/grpc/internal/TransportSet.java
index 025b83276..d89a8ac49 100644
--- a/core/src/main/java/io/grpc/internal/TransportSet.java
+++ b/core/src/main/java/io/grpc/internal/TransportSet.java
@@ -249,9 +249,14 @@ final class TransportSet implements WithLogId {
           delayedTransport.endBackoff();
           boolean shutdownDelayedTransport = false;
           Runnable runnable = null;
+          // TransportSet as a channel layer class should not call into transport methods while
+          // holding the lock, thus we call hasPendingStreams() outside of the lock.  It will cause
+          // a _benign_ race where the TransportSet may transition to CONNECTING when there is not
+          // pending stream.
+          boolean hasPendingStreams = delayedTransport.hasPendingStreams();
           synchronized (lock) {
             reconnectTask = null;
-            if (delayedTransport.hasPendingStreams()) {
+            if (hasPendingStreams) {
               // Transition directly to CONNECTING
               runnable = startNewTransport(delayedTransport);
             } else {
['core/src/main/java/io/grpc/internal/TransportSet.java']