core: fast-path return in ServerCallImpl.triggerEvent when closeCalled is true

Check closeCalled before dispatching triggerEvent to the transport
stream, avoiding unnecessary task allocations and transport hops if
the call has already been closed.

TAG=agy
CONV=e1bfa5a2-e855-4f79-abdd-ef2b264977be

Author: kannanjgithub

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

@@ -256,6 +256,9 @@ public MethodDescriptor<ReqT, RespT> getMethodDescriptor() {
 
   @Override
   public void triggerEvent(Object event) {
+    if (closeCalled) {
+      return;
+    }
     try (TaskCloseable ignore = PerfMark.traceTask("ServerCall.triggerEvent")) {
       PerfMark.attachTag(tag);
       stream.triggerEvent(event);

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

@@ -500,6 +500,14 @@ public void triggerEvent_propagatesToStream() {
     verify(stream).triggerEvent(event);
   }
 
+  @Test
+  public void triggerEvent_afterClose_noop() {
+    call.close(Status.OK, new Metadata());
+    Object event = new Object();
+    call.triggerEvent(event);
+    verify(stream, never()).triggerEvent(event);
+  }
+
   @Test
   public void streamListener_triggerEvent() {
     ServerStreamListenerImpl<Long> streamListener =