Polish interceptor/transport manager logic

src/objective-c/GRPCClient/GRPCInterceptor.h

@@ -182,6 +182,8 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Notify the manager that the interceptor has shut down and the manager should release references
  * to other interceptors and stop forwarding requests/responses.
+ *
+ * The method can only be called by the manager's associated interceptor.
  */
 - (void)shutDown;
 

src/objective-c/GRPCClient/GRPCInterceptor.m

@@ -68,12 +68,11 @@
 }
 
 - (void)shutDown {
-  dispatch_async(_dispatchQueue, ^{
-    self->_nextInterceptor = nil;
-    self->_previousInterceptor = nil;
-    self->_thisInterceptor = nil;
-    self->_shutDown = YES;
-  });
+  // immediately releases reference; should not queue to dispatch queue.
+  _nextInterceptor = nil;
+  _previousInterceptor = nil;
+  _thisInterceptor = nil;
+  _shutDown = YES;
 }
 
 - (void)createNextInterceptor {
@@ -203,6 +202,8 @@
     return;
   }
   id<GRPCResponseHandler> copiedPreviousInterceptor = _previousInterceptor;
+  // no more callbacks should be issued to the previous interceptor
+  _previousInterceptor = nil;
   dispatch_async(copiedPreviousInterceptor.dispatchQueue, ^{
     [copiedPreviousInterceptor didCloseWithTrailingMetadata:trailingMetadata error:error];
   });

src/objective-c/GRPCClient/private/GRPCTransport+Private.h

@@ -43,6 +43,8 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Notify the manager that the transport has shut down and the manager should release references to
  * its response handler and stop forwarding requests/responses.
+ *
+ * The method can only be called by the manager's associated transport instance.
  */
 - (void)shutDown;
 

src/objective-c/GRPCClient/private/GRPCTransport+Private.m

@@ -46,8 +46,8 @@
   return self;
 }
 
-// Must be called on _dispatchQueue or queues targeted by _dispatchQueue
 - (void)shutDown {
+  // immediately releases reference; should not queue to dispatch queue.
   _transport = nil;
   _previousInterceptor = nil;
 }
@@ -111,6 +111,8 @@
     return;
   }
   id<GRPCResponseHandler> copiedPreviousInterceptor = _previousInterceptor;
+  // no more callbacks should be issued to the previous interceptor
+  _previousInterceptor = nil;
   dispatch_async(copiedPreviousInterceptor.dispatchQueue, ^{
     [copiedPreviousInterceptor didCloseWithTrailingMetadata:trailingMetadata error:error];
   });