Sync writes with queue

src/objective-c/RxLibrary/GRXBufferedPipe.m

@@ -38,6 +38,7 @@
   NSMutableArray *_queue;
   BOOL _inputIsFinished;
   NSError *_errorOrNil;
+  dispatch_queue_t _writeQueue;
 }
 
 @synthesize state = _state;
@@ -50,6 +51,7 @@
   if (self = [super init]) {
     _queue = [NSMutableArray array];
     _state = GRXWriterStateNotStarted;
+    _writeQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
   }
   return self;
 }
@@ -61,35 +63,51 @@
 }
 
 - (void)writeBufferUntilPausedOrStopped {
-  while (_state == GRXWriterStateStarted && _queue.count > 0) {
-    [_writeable writeValue:[self popValue]];
-  }
-  if (_inputIsFinished && _queue.count == 0) {
-    // Our writer finished normally while we were paused or not-started-yet.
-    [self finishWithError:_errorOrNil];
-  }
+  dispatch_async(_writeQueue, ^(void) {
+    while (_queue.count > 0) {
+      BOOL started;
+      @synchronized (self) {
+        started = (_state == GRXWriterStateStarted);
+      }
+      if (started) {
+        [_writeable writeValue:[self popValue]];
+      } else {
+        break;
+      }
+    }
+    if (_inputIsFinished && _queue.count == 0) {
+      // Our writer finished normally while we were paused or not-started-yet.
+      [self finishWithError:_errorOrNil];
+    }
+  });
 }
 
 #pragma mark GRXWriteable implementation
 
 // Returns whether events can be simply propagated to the other end of the pipe.
 - (BOOL)shouldFastForward {
-  return _state == GRXWriterStateStarted && _queue.count == 0;
+  BOOL started;
+  @synchronized (self) {
+    started = (_state == GRXWriterStateStarted);
+  }
+  return _state == started && _queue.count == 0;
 }
 
 - (void)writeValue:(id)value {
-  if (self.shouldFastForward) {
-    // Skip the queue.
-    [_writeable writeValue:value];
-  } else {
-    // Even if we're paused and with enqueued values, we can't excert back-pressure to our writer.
-    // So just buffer the new value.
-    // We need a copy, so that it doesn't mutate before it's written at the other end of the pipe.
-    if ([value respondsToSelector:@selector(copy)]) {
-      value = [value copy];
-    }
-    [_queue addObject:value];
+  if ([value respondsToSelector:@selector(copy)]) {
+    value = [value copy];
   }
+  dispatch_async(_writeQueue, ^(void) {
+    if (self.shouldFastForward) {
+      // Skip the queue.
+      [_writeable writeValue:value];
+    } else {
+      // Even if we're paused and with enqueued values, we can't excert back-pressure to our writer.
+      // So just buffer the new value.
+      // We need a copy, so that it doesn't mutate before it's written at the other end of the pipe.
+      [_queue addObject:value];
+    }
+  });
 }
 
 - (void)writesFinishedWithError:(NSError *)errorOrNil {