Address review from server interceptor executor design comments.

Author: kannanjgithub

api/src/main/java/io/grpc/ServerCall.java

@@ -279,7 +279,8 @@ public String getAuthority() {
    * Triggers a custom event to be processed by the listener.
    * The event will be delivered to {@link Listener#onEvent(Object)} on the call's executor.
    *
-   * <p>This method is thread-safe and can be called from any thread.
+   * <p>This method is thread-safe and can be called from any thread. No events will be delivered
+   * after the RPC is cancelled or completed.
    *
    * @param event the event to trigger.
    */

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

@@ -178,9 +178,7 @@ public final void triggerEvent(final Object event) {
     transportState().runOnTransportThread(new Runnable() {
       @Override
       public void run() {
-        if (transportState().listener != null) {
-          transportState().listener.triggerEvent(event);
-        }
+        transportState().triggerEvent(event);
       }
     });
   }
@@ -271,6 +269,13 @@ public void deframerClosed(boolean hasPartialMessage) {
 
 
 
+    public final void triggerEvent(Object event) {
+      if (listenerClosed) {
+        return;
+      }
+      listener().triggerEvent(event);
+    }
+
     @Override
     protected ServerStreamListener listener() {
       return listener;