core: Delay client listener exception notification until normal close

This should avoid messages being leaked when a Listener throws an exception and
the executor is shut down immediately after the call completes. This is related
to #7105 but a different scenario and we aren't aware of any user having
observed the previous behavior.

Note also this does _not_ fix the similar case of reordering caused by
delayedCancelOnDeadlineExceeded().

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

@@ -584,12 +584,24 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
 
   private class ClientStreamListenerImpl implements ClientStreamListener {
     private final Listener<RespT> observer;
-    private boolean closed;
+    private Status exceptionStatus;
 
     public ClientStreamListenerImpl(Listener<RespT> observer) {
       this.observer = checkNotNull(observer, "observer");
     }
 
+    /**
+     * Cancels call and schedules onClose() notification. May only be called from the application
+     * thread.
+     */
+    private void exceptionThrown(Status status) {
+      // Since each RPC can have its own executor, we can only call onClose() when we are sure there
+      // will be no further callbacks. We set the status here and overwrite the onClose() details
+      // when it arrives.
+      exceptionStatus = status;
+      stream.cancel(status);
+    }
+
     @Override
     public void headersRead(final Metadata headers) {
       PerfMark.startTask("ClientStreamListener.headersRead", tag);
@@ -612,16 +624,14 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
         }
 
         private void runInternal() {
-          if (closed) {
+          if (exceptionStatus != null) {
             return;
           }
           try {
             observer.onHeaders(headers);
           } catch (Throwable t) {
-            Status status =
+            exceptionThrown(
-                Status.CANCELLED.withCause(t).withDescription("Failed to read headers");
+                Status.CANCELLED.withCause(t).withDescription("Failed to read headers"));
-            stream.cancel(status);
-            close(status, new Metadata());
           }
         }
       }
@@ -655,7 +665,7 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
         }
 
         private void runInternal() {
-          if (closed) {
+          if (exceptionStatus != null) {
             GrpcUtil.closeQuietly(producer);
             return;
           }
@@ -672,10 +682,8 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
             }
           } catch (Throwable t) {
             GrpcUtil.closeQuietly(producer);
-            Status status =
+            exceptionThrown(
-                Status.CANCELLED.withCause(t).withDescription("Failed to read message.");
+                Status.CANCELLED.withCause(t).withDescription("Failed to read message."));
-            stream.cancel(status);
-            close(status, new Metadata());
           }
         }
       }
@@ -687,20 +695,6 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
       }
     }
 
-    /**
-     * Must be called from application thread.
-     */
-    private void close(Status status, Metadata trailers) {
-      closed = true;
-      cancelListenersShouldBeRemoved = true;
-      try {
-        closeObserver(observer, status, trailers);
-      } finally {
-        removeContextListenerAndCancelDeadlineFuture();
-        channelCallsTracer.reportCallEnded(status.isOk());
-      }
-    }
-
     @Override
     public void closed(Status status, Metadata trailers) {
       closed(status, RpcProgress.PROCESSED, trailers);
@@ -752,11 +746,25 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
         }
 
         private void runInternal() {
-          if (closed) {
+          Status status = savedStatus;
-            // We intentionally don't keep the status or metadata from the server.
+          Metadata trailers = savedTrailers;
-            return;
+          if (exceptionStatus != null) {
+            // Ideally exceptionStatus == savedStatus, as exceptionStatus was passed to cancel().
+            // However the cancel is racy and this closed() may have already been queued when the
+            // cancellation occurred. Since other calls like onMessage() will throw away data if
+            // exceptionStatus != null, it is semantically essential that we _not_ use a status
+            // provided by the server.
+            status = exceptionStatus;
+            // Replace trailers to prevent mixing sources of status and trailers.
+            trailers = new Metadata();
+          }
+          cancelListenersShouldBeRemoved = true;
+          try {
+            closeObserver(observer, status, trailers);
+          } finally {
+            removeContextListenerAndCancelDeadlineFuture();
+            channelCallsTracer.reportCallEnded(status.isOk());
           }
-          close(savedStatus, savedTrailers);
         }
       }
 
@@ -789,13 +797,14 @@ final class ClientCallImpl<ReqT, RespT> extends ClientCall<ReqT, RespT> {
         }
 
         private void runInternal() {
+          if (exceptionStatus != null) {
+            return;
+          }
           try {
             observer.onReady();
           } catch (Throwable t) {
-            Status status =
+            exceptionThrown(
-                Status.CANCELLED.withCause(t).withDescription("Failed to call onReady.");
+                Status.CANCELLED.withCause(t).withDescription("Failed to call onReady."));
-            stream.cancel(status);
-            close(status, new Metadata());
           }
         }
       }

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

@@ -256,6 +256,51 @@ public class ClientCallImplTest {
     verify(stream).cancel(same(callListenerStatus));
   }
 
+  @Test
+  public void exceptionInOnHeadersHasOnCloseQueuedLast() {
+    class PointOfNoReturnExecutor implements Executor {
+      boolean rejectNewRunnables;
+
+      @Override public void execute(Runnable command) {
+        assertThat(rejectNewRunnables).isFalse();
+        command.run();
+      }
+    }
+
+    final PointOfNoReturnExecutor executor = new PointOfNoReturnExecutor();
+    ClientCallImpl<Void, Void> call = new ClientCallImpl<>(
+        method,
+        executor,
+        baseCallOptions,
+        provider,
+        deadlineCancellationExecutor,
+        channelCallTracer,
+        /* retryEnabled= */ false);
+    callListener = new NoopClientCall.NoopClientCallListener<Void>() {
+      private final RuntimeException failure = new RuntimeException("bad");
+
+      @Override public void onHeaders(Metadata metadata) {
+        throw failure;
+      }
+
+      @Override public void onClose(Status status, Metadata metadata) {
+        verify(stream).cancel(same(status));
+        assertThat(status.getCode()).isEqualTo(Status.Code.CANCELLED);
+        assertThat(status.getCause()).isSameInstanceAs(failure);
+        // At the point onClose() is called the user may shut down the executor, so no further
+        // Runnables may be scheduled. The only thread-safe way of guaranteeing that is for
+        // onClose() to be queued last.
+        executor.rejectNewRunnables = true;
+      }
+    };
+    call.start(callListener, new Metadata());
+    verify(stream).start(listenerArgumentCaptor.capture());
+    final ClientStreamListener streamListener = listenerArgumentCaptor.getValue();
+
+    streamListener.headersRead(new Metadata());
+    streamListener.closed(Status.OK, new Metadata());
+  }
+
   @Test
   public void exceptionInOnReadyTakesPrecedenceOverServer() {
     DelayedExecutor executor = new DelayedExecutor();