Bug fix for GRXConcurrentWriter

src/objective-c/RxLibrary/GRXConcurrentWriteable.m

@@ -27,8 +27,15 @@
 
 @implementation GRXConcurrentWriteable {
   dispatch_queue_t _writeableQueue;
-  // This ensures that writesFinishedWithError: is only sent once to the writeable.
+
+  // This ivar ensures that writesFinishedWithError: is only sent once to the writeable. Protected
+  // by _writeableQueue.
   BOOL _alreadyFinished;
+
+  // This ivar ensures that a cancelWithError: call prevents further values to be sent to
+  // self.writeable. It must support manipulation outside of _writeableQueue and thus needs to be
+  // protected by self lock.
+  BOOL _cancelled;
 }
 
 - (instancetype)init {
@@ -42,6 +49,7 @@
     _writeableQueue = queue;
     _writeable = writeable;
     _alreadyFinished = NO;
+    _cancelled = NO;
   }
   return self;
 }
@@ -56,6 +64,12 @@
       return;
     }
 
+    @synchronized (self) {
+       if (self->_cancelled) {
+         return;
+       }
+    }
+
     [self.writeable writeValue:value];
     handler();
   });
@@ -63,13 +77,18 @@
 
 - (void)enqueueSuccessfulCompletion {
   dispatch_async(_writeableQueue, ^{
-    @synchronized(self) {
-      if (self->_alreadyFinished) {
+    if (self->_alreadyFinished) {
+      return;
+    }
+    @synchronized (self) {
+      if (self->_cancelled) {
         return;
       }
     }
     [self.writeable writesFinishedWithError:nil];
+
     // Skip any possible message to the wrapped writeable enqueued after this one.
+    self->_alreadyFinished = YES;
     self.writeable = nil;
   });
 }
@@ -77,14 +96,17 @@
 - (void)cancelWithError:(NSError *)error {
   NSAssert(error != nil, @"For a successful completion, use enqueueSuccessfulCompletion.");
   @synchronized(self) {
+    self->_cancelled = YES;
+  }
+  dispatch_async(_writeableQueue, ^{
     if (self->_alreadyFinished) {
+      // a cancel or a successful completion is already issued
       return;
     }
-  }
-  dispatch_async(_writeableQueue, ^{
-    // If enqueueSuccessfulCompletion is already issued, self.writeable is nil and the following
-    // line is no-op.
     [self.writeable writesFinishedWithError:error];
+
+    // Skip any possible message to the wrapped writeable enqueued after this one.
+    self->_alreadyFinished = YES;
     self.writeable = nil;
   });
 }