fix

core/src/main/java/io/grpc/internal/RetriableStream.java

@@ -218,7 +218,7 @@ private void commitAndRun(Substream winningSubstream) {
     Runnable postCommitTask = commit(winningSubstream);
 
     if (postCommitTask != null) {
-      postCommitTask.run();
+      callExecutor.execute(postCommitTask);
     }
   }
 

core/src/test/java/io/grpc/internal/RetriableStreamTest.java

@@ -65,10 +65,12 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Random;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.ReentrantLock;
 import javax.annotation.Nullable;
 import org.junit.After;
 import org.junit.Test;
@@ -190,7 +192,7 @@ Status prestart() {
 
   private RetriableStream<String> retriableStream =
       newThrottledRetriableStream(null /* throttle */);
-  private final RetriableStream<String> hedgingStream =
+  private RetriableStream<String> hedgingStream =
       newThrottledHedgingStream(null /* throttle */);
 
   private ClientStreamTracer bufferSizeTracer;
@@ -206,9 +208,13 @@ method, new Metadata(), channelBufferUsed, PER_RPC_BUFFER_LIMIT, CHANNEL_BUFFER_
   }
 
   private RetriableStream<String> newThrottledHedgingStream(Throttle throttle) {
+    return newThrottledHedgingStream(throttle, MoreExecutors.directExecutor());
+  }
+
+  private RetriableStream<String> newThrottledHedgingStream(Throttle throttle, Executor executor) {
     return new RecordedRetriableStream(
         method, new Metadata(), channelBufferUsed, PER_RPC_BUFFER_LIMIT, CHANNEL_BUFFER_LIMIT,
-        MoreExecutors.directExecutor(), fakeClock.getScheduledExecutorService(),
+        executor, fakeClock.getScheduledExecutorService(),
         null, HEDGING_POLICY, throttle);
   }
 
@@ -2482,6 +2488,121 @@ public void hedging_pushback_negative() {
     inOrder.verifyNoMoreInteractions();
   }
 
+  // This is for hedging deadlock when multiple in-flight streams races when transports call back,
+  // particularly for OkHttp:
+  // e.g. stream1 subListener gets closed() and in turn creates another stream. This ends up with
+  // transport1 thread lock is held while waiting for transport2 lock for creating a new stream.
+  // Stream2 subListener gets headersRead() and then try to commit and cancel all other drained
+  // streams, including the ones that is on transport1. This causes transport2 thread lock held
+  // while waiting for transport1 (cancel stream requires lock). Thus deadlock.
+  // Deadlock could also happen when two streams both gets headerRead() at the same time.
+  // It is believed that retry does not have the issue because streams are created sequentially.
+  @Test(timeout = 15000)
+  public void hedging_deadlock() throws Exception {
+    ClientStream mockStream1 = mock(ClientStream.class); //on transport1
+    ClientStream mockStream2 = mock(ClientStream.class); //on transport2
+    ClientStream mockStream3 = mock(ClientStream.class); //on transport2
+    ClientStream mockStream4 = mock(ClientStream.class); //on transport1
+
+    ReentrantLock transport1Lock = new ReentrantLock();
+    ReentrantLock transport2Lock = new ReentrantLock();
+    InOrder inOrder = inOrder(
+        mockStream1, mockStream2, mockStream3,
+        retriableStreamRecorder, masterListener);
+    when(retriableStreamRecorder.newSubstream(anyInt()))
+        .thenReturn(mockStream1)
+        .thenReturn(mockStream2)
+        .thenReturn(mockStream3)
+        .thenAnswer(new Answer<ClientStream>() {
+
+          @Override
+          public ClientStream answer(InvocationOnMock invocation) throws Throwable {
+            transport1Lock.lock();
+            return mockStream4;
+          }
+        });
+
+    hedgingStream = newThrottledHedgingStream(null, fakeClock.getScheduledExecutorService());
+    hedgingStream.start(masterListener);
+    assertEquals(1, fakeClock.numPendingTasks());
+    ArgumentCaptor<ClientStreamListener> sublistenerCaptor1 =
+        ArgumentCaptor.forClass(ClientStreamListener.class);
+    inOrder.verify(mockStream1).start(sublistenerCaptor1.capture());
+    inOrder.verify(mockStream1).isReady();
+    inOrder.verifyNoMoreInteractions();
+
+    fakeClock.forwardTime(HEDGING_DELAY_IN_SECONDS, TimeUnit.SECONDS);
+    assertEquals(1, fakeClock.numPendingTasks());
+    ArgumentCaptor<ClientStreamListener> sublistenerCaptor2 =
+        ArgumentCaptor.forClass(ClientStreamListener.class);
+    inOrder.verify(mockStream2).start(sublistenerCaptor2.capture());
+    inOrder.verify(mockStream2).isReady();
+    inOrder.verifyNoMoreInteractions();
+
+    fakeClock.forwardTime(HEDGING_DELAY_IN_SECONDS, TimeUnit.SECONDS);
+    assertEquals(1, fakeClock.numPendingTasks());
+    ArgumentCaptor<ClientStreamListener> sublistenerCaptor3 =
+        ArgumentCaptor.forClass(ClientStreamListener.class);
+    inOrder.verify(mockStream3).start(sublistenerCaptor3.capture());
+    inOrder.verify(mockStream3).isReady();
+    inOrder.verifyNoMoreInteractions();
+
+    doAnswer(new Answer<Void>() {
+      @Override
+      @SuppressWarnings("LockNotBeforeTry")
+      public Void answer(InvocationOnMock invocation) throws Throwable {
+        transport2Lock.lock();
+        transport2Lock.unlock();
+        return null;
+      }
+    }).when(mockStream3).cancel(any(Status.class));
+
+    doAnswer(new Answer<Void>() {
+      @Override
+      @SuppressWarnings("LockNotBeforeTry")
+      public Void answer(InvocationOnMock invocation) throws Throwable {
+        transport2Lock.lock();
+        transport2Lock.unlock();
+        return null;
+      }
+    }).when(mockStream2).cancel(any(Status.class));
+
+    CountDownLatch latch = new CountDownLatch(1);
+    Thread transport1Activity = new Thread(new Runnable() {
+      @Override
+      public void run() {
+        transport1Lock.lock();
+        try {
+          sublistenerCaptor1.getValue().headersRead(new Metadata());
+          latch.countDown();
+        } finally {
+          transport1Lock.unlock();
+        }
+      }
+    }, "Thread-transport1");
+    transport1Activity.start();
+    Thread transport2Activity = new Thread(new Runnable() {
+      @Override
+      public void run() {
+        transport2Lock.lock();
+        try {
+          sublistenerCaptor2.getValue()
+              .closed(Status.fromCode(NON_FATAL_STATUS_CODE_1), REFUSED, new Metadata());
+        } finally {
+          transport2Lock.unlock();
+          if (transport1Lock.tryLock()) {
+            transport1Lock.unlock();
+          }
+        }
+      }
+    }, "Thread-transport2");
+    transport2Activity.start();
+    latch.await();
+    fakeClock.runDueTasks();
+    transport2Activity.join();
+    transport1Activity.join();
+  }
+
   @Test
   public void hedging_pushback_positive() {
     ClientStream mockStream1 = mock(ClientStream.class);