@Override
 public void closed(Status status, Metadata trailers) {
   Deadline deadline = context.getDeadline();
   if (status.getCode() == Status.Code.CANCELLED && deadline != null) {
     // When the server's deadline expires, it can only reset the stream with CANCEL and no
     // description. Since our timer may be delayed in firing, we double-check the deadline and
     // turn the failure into the likely more helpful DEADLINE_EXCEEDED status.
     if (deadline.isExpired()) {
       status = DEADLINE_EXCEEDED;
       // Replace trailers to prevent mixing sources of status and trailers.
       trailers = new Metadata();
     }
   }
   final Status savedStatus = status;
   final Metadata savedTrailers = trailers;
   callExecutor.execute(
       new ContextRunnable(context) {
         @Override
         public final void runInContext() {
           try {
             closed = true;
             contextListenerShouldBeRemoved = true;
             observer.onClose(savedStatus, savedTrailers);
           } finally {
             context.removeListener(ClientCallImpl.this);
           }
         }
       });
 }
  @Override
  public void start(final Listener<RespT> observer, Metadata headers) {
    checkState(stream == null, "Already started");
    checkNotNull(observer, "observer");
    checkNotNull(headers, "headers");

    // Create the context
    final Deadline effectiveDeadline = min(callOptions.getDeadline(), parentContext.getDeadline());
    if (effectiveDeadline != parentContext.getDeadline()) {
      context = parentContext.withDeadline(effectiveDeadline, deadlineCancellationExecutor);
    } else {
      context = parentContext.withCancellation();
    }

    if (context.isCancelled()) {
      // Context is already cancelled so no need to create a real stream, just notify the observer
      // of cancellation via callback on the executor
      stream = NoopClientStream.INSTANCE;
      callExecutor.execute(
          new ContextRunnable(context) {
            @Override
            public void runInContext() {
              observer.onClose(statusFromCancelled(context), new Metadata());
            }
          });
      return;
    }
    final String compressorName = callOptions.getCompressor();
    Compressor compressor = null;
    if (compressorName != null) {
      compressor = compressorRegistry.lookupCompressor(compressorName);
      if (compressor == null) {
        stream = NoopClientStream.INSTANCE;
        callExecutor.execute(
            new ContextRunnable(context) {
              @Override
              public void runInContext() {
                observer.onClose(
                    Status.INTERNAL.withDescription(
                        String.format("Unable to find compressor by name %s", compressorName)),
                    new Metadata());
              }
            });
        return;
      }
    } else {
      compressor = Codec.Identity.NONE;
    }

    prepareHeaders(headers, callOptions, userAgent, decompressorRegistry, compressor);

    final boolean deadlineExceeded = effectiveDeadline != null && effectiveDeadline.isExpired();
    if (!deadlineExceeded) {
      updateTimeoutHeaders(
          effectiveDeadline, callOptions.getDeadline(), parentContext.getDeadline(), headers);
      ClientTransport transport = clientTransportProvider.get(callOptions);
      stream = transport.newStream(method, headers);
    } else {
      stream = new FailingClientStream(DEADLINE_EXCEEDED);
    }

    if (callOptions.getAuthority() != null) {
      stream.setAuthority(callOptions.getAuthority());
    }
    stream.setCompressor(compressor);

    stream.start(new ClientStreamListenerImpl(observer));
    if (compressor != Codec.Identity.NONE) {
      stream.setMessageCompression(true);
    }

    // Delay any sources of cancellation after start(), because most of the transports are broken if
    // they receive cancel before start. Issue #1343 has more details

    // Propagate later Context cancellation to the remote side.
    context.addListener(this, directExecutor());
    if (contextListenerShouldBeRemoved) {
      // Race detected! ClientStreamListener.closed may have been called before
      // deadlineCancellationFuture was set, thereby preventing the future from being cancelled.
      // Go ahead and cancel again, just to be sure it was cancelled.
      context.removeListener(this);
    }
  }