inprocess: Fix listener race if transport is shutdown while starting

Returning the runnable did nothing, as both the start method and the
runnable are run within the synchronization context. I believe the
Runnable used to be required in the previous implementation of
ManagedChannelImpl (the lock-based implementation before we created
SynchronizationContext).

This fixes a NPE seen in ServerImpl because the server expects proper
ordering of transport lifecycle events.
```
Uncaught exception in the SynchronizationContext. Panic!
java.lang.NullPointerException: Cannot invoke "java.util.concurrent.Future.cancel(boolean)" because "this.handshakeTimeoutFuture" is null
	at io.grpc.internal.ServerImpl$ServerTransportListenerImpl.transportReady(ServerImpl.java:440)
	at io.grpc.inprocess.InProcessTransport$4.run(InProcessTransport.java:215)
	at io.grpc.SynchronizationContext.drain(SynchronizationContext.java:94)
```

b/338445186

core/src/testFixtures/java/io/grpc/internal/AbstractTransportTest.java

@@ -350,6 +350,26 @@ public void clientStartStop() throws Exception {
     verify(mockClientTransportListener, never()).transportInUse(anyBoolean());
   }
 
+  @Test
+  public void clientShutdownBeforeStartRunnable() throws Exception {
+    server.start(serverListener);
+    client = newClientTransport(server);
+    Runnable runnable = client.start(mockClientTransportListener);
+    // Shutdown before calling 'runnable'
+    client.shutdown(Status.UNAVAILABLE.withDescription("shutdown called"));
+    runIfNotNull(runnable);
+    verify(mockClientTransportListener, timeout(TIMEOUT_MS)).transportTerminated();
+    // We should verify that clients don't call transportReady() after transportTerminated(), but
+    // transports do this today and nothing cares. ServerImpl, on the other hand, doesn't appreciate
+    // the out-of-order calls.
+    MockServerTransportListener serverTransportListener
+        = serverListener.takeListenerOrFail(TIMEOUT_MS, TimeUnit.MILLISECONDS);
+    assertTrue(serverTransportListener.waitForTermination(TIMEOUT_MS, TimeUnit.MILLISECONDS));
+    // Allow any status as some transports (e.g., Netty) don't communicate the original status when
+    // shutdown while handshaking. It won't be used anyway, so no big deal.
+    verify(mockClientTransportListener).transportShutdown(any(Status.class));
+  }
+
   @Test
   public void clientStartAndStopOnceConnected() throws Exception {
     server.start(serverListener);
@@ -2251,6 +2271,7 @@ public void streamCreated(ServerStream stream, String method, Metadata headers)
 
     @Override
     public Attributes transportReady(Attributes attributes) {
+      assertFalse(terminated.isDone());
       return Attributes.newBuilder()
           .setAll(attributes)
           .set(ADDITIONAL_TRANSPORT_ATTR_KEY, "additional attribute value")

inprocess/src/main/java/io/grpc/inprocess/InProcessTransport.java

@@ -203,21 +203,14 @@ public void run() {
         }
       };
     }
-    return new Runnable() {
-      @Override
-      @SuppressWarnings("deprecation")
-      public void run() {
-        synchronized (InProcessTransport.this) {
-          Attributes serverTransportAttrs = Attributes.newBuilder()
-              .set(Grpc.TRANSPORT_ATTR_REMOTE_ADDR, address)
-              .set(Grpc.TRANSPORT_ATTR_LOCAL_ADDR, address)
-              .build();
-          serverStreamAttributes = serverTransportListener.transportReady(serverTransportAttrs);
-          attributes = clientTransportListener.filterTransport(attributes);
-          clientTransportListener.transportReady();
-        }
-      }
-    };
+    Attributes serverTransportAttrs = Attributes.newBuilder()
+        .set(Grpc.TRANSPORT_ATTR_REMOTE_ADDR, address)
+        .set(Grpc.TRANSPORT_ATTR_LOCAL_ADDR, address)
+        .build();
+    serverStreamAttributes = serverTransportListener.transportReady(serverTransportAttrs);
+    attributes = clientTransportListener.filterTransport(attributes);
+    clientTransportListener.transportReady();
+    return null;
   }
 
   @Override

servlet/src/jettyTest/java/io/grpc/servlet/JettyTransportTest.java

@@ -262,4 +262,9 @@ public void clientChecksInboundMetadataSize_header() {}
   @Ignore("https://github.com/jetty/jetty.project/issues/11822")
   @Test
   public void clientChecksInboundMetadataSize_trailer() {}
+
+  @Override
+  @Ignore("Not yet investigated, but has been seen for multiple servlet containers")
+  @Test
+  public void clientShutdownBeforeStartRunnable() {}
 }

servlet/src/tomcatTest/java/io/grpc/servlet/TomcatTransportTest.java

@@ -274,4 +274,9 @@ public void earlyServerClose_serverFailure_withClientCancelOnListenerClosed() {}
   @Ignore("regression since bumping grpc v1.46 to v1.53")
   @Test
   public void messageProducerOnlyProducesRequestedMessages() {}
+
+  @Override
+  @Ignore("Not yet investigated, but has been seen for multiple servlet containers")
+  @Test
+  public void clientShutdownBeforeStartRunnable() {}
 }

servlet/src/undertowTest/java/io/grpc/servlet/UndertowTransportTest.java

@@ -308,4 +308,9 @@ public void clientCancel() {}
   @Ignore("regression since bumping grpc v1.46 to v1.53")
   @Test
   public void messageProducerOnlyProducesRequestedMessages() {}
+
+  @Override
+  @Ignore("Not yet investigated, but has been seen for multiple servlet containers")
+  @Test
+  public void clientShutdownBeforeStartRunnable() {}
 }