Make the call retain cycle explicit and encapsulated within GRPCCall.m

3 files changed, 50 insertions, 56 deletions

diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index 0bfc864de3..9435bf2b35 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -81,6 +81,10 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
   GRXConcurrentWriteable *_responseWriteable;
   GRXWriter *_requestWriter;
 
+  // To create a retain cycle when a call is started, up until it finishes. See
+  // |startWithWriteable:| and |finishWithError:|.
+  GRPCCall *_self;
+
   NSMutableDictionary *_requestMetadata;
   NSMutableDictionary *_responseMetadata;
 }
@@ -143,8 +147,13 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 #pragma mark Finish
 
 - (void)finishWithError:(NSError *)errorOrNil {
+  // If the call isn't retained anywhere else, it can be deallocated now.
+  _self = nil;
+
+  // If there were still request messages coming, stop them.
   _requestWriter.state = GRXWriterStateFinished;
   _requestWriter = nil;
+
   if (errorOrNil) {
     [_responseWriteable cancelWithError:errorOrNil];
   } else {
@@ -276,6 +285,7 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 }
 
 - (void)writesFinishedWithError:(NSError *)errorOrNil {
+  _requestWriter = nil;
   if (errorOrNil) {
     [self cancel];
   } else {
@@ -335,12 +345,14 @@ NSString * const kGRPCStatusMetadataKey = @"io.grpc.StatusMetadataKey";
 #pragma mark GRXWriter implementation
 
 - (void)startWithWriteable:(id<GRXWriteable>)writeable {
-  // The following produces a retain cycle self:_responseWriteable:self, which is only
-  // broken when writesFinishedWithError: is sent to the wrapped writeable.
-  // Care is taken not to retain self strongly in any of the blocks used in
-  // the implementation of GRPCCall, so that the life of the instance is
-  // determined by this retain cycle.
-  _responseWriteable = [[GRXConcurrentWriteable alloc] initWithWriteable:writeable writer:self];
+  // Create a retain cycle so that this instance lives until the RPC finishes (or is cancelled).
+  // This makes RPCs in which the call isn't externally retained possible (as long as it is started
+  // before being autoreleased).
+  // Care is taken not to retain self strongly in any of the blocks used in this implementation, so
+  // that the life of the instance is determined by this retain cycle.
+  _self = self;
+
+  _responseWriteable = [[GRXConcurrentWriteable alloc] initWithWriteable:writeable];
   [self sendHeaders:_requestMetadata];
   [self invokeCall];
 }
diff --git a/src/objective-c/RxLibrary/GRXConcurrentWriteable.h b/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
index 018af46150..1080001905 100644
--- a/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
+++ b/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
@@ -36,44 +36,36 @@
 #import "GRXWriter.h"
 #import "GRXWriteable.h"
 
-// This is a thread-safe wrapper over a GRXWriteable instance. It lets one
-// enqueue calls to a GRXWriteable instance for the main thread, guaranteeing
-// that writesFinishedWithError: is the last message sent to it (no matter what
-// messages are sent to the wrapper, in what order, nor from which thread). It
-// also guarantees that, if cancelWithError: is called from the main thread
-// (e.g. by the app cancelling the writes), no further messages are sent to the
-// writeable except writesFinishedWithError:.
+// This is a thread-safe wrapper over a GRXWriteable instance. It lets one enqueue calls to a
+// GRXWriteable instance for the main thread, guaranteeing that writesFinishedWithError: is the last
+// message sent to it (no matter what messages are sent to the wrapper, in what order, nor from
+// which thread). It also guarantees that, if cancelWithError: is called from the main thread (e.g.
+// by the app cancelling the writes), no further messages are sent to the writeable except
+// writesFinishedWithError:.
 //
-// TODO(jcanizales): Let the user specify another queue for the writeable
-// callbacks.
+// TODO(jcanizales): Let the user specify another queue for the writeable callbacks.
 @interface GRXConcurrentWriteable : NSObject
 
 // The GRXWriteable passed is the wrapped writeable.
-// Both the GRXWriter instance and the GRXWriteable instance are retained until
-// writesFinishedWithError: is sent to the writeable, and released after that.
-// This is used to create a retain cycle that keeps both objects alive until the
-// writing is explicitly finished.
-- (instancetype)initWithWriteable:(id<GRXWriteable>)writeable writer:(GRXWriter *)writer
-    NS_DESIGNATED_INITIALIZER;
+// The GRXWriteable instance is retained until writesFinishedWithError: is sent to it, and released
+// after that.
+- (instancetype)initWithWriteable:(id<GRXWriteable>)writeable NS_DESIGNATED_INITIALIZER;
 
 // Enqueues writeValue: to be sent to the writeable in the main thread.
 // The passed handler is invoked from the main thread after writeValue: returns.
 - (void)enqueueValue:(id)value completionHandler:(void (^)())handler;
 
-// Enqueues writesFinishedWithError:nil to be sent to the writeable in the main
-// thread. After that message is sent to the writeable, all other methods of
-// this object are effectively noops.
+// Enqueues writesFinishedWithError:nil to be sent to the writeable in the main thread. After that
+// message is sent to the writeable, all other methods of this object are effectively noops.
 - (void)enqueueSuccessfulCompletion;
 
-// If the writeable has not yet received a writesFinishedWithError: message, this
-// will enqueue one to be sent to it in the main thread, and cancel all other
-// pending messages to the writeable enqueued by this object (both past and
-// future).
+// If the writeable has not yet received a writesFinishedWithError: message, this will enqueue one
+// to be sent to it in the main thread, and cancel all other pending messages to the writeable
+// enqueued by this object (both past and future).
 // The error argument cannot be nil.
 - (void)cancelWithError:(NSError *)error;
 
-// Cancels all pending messages to the writeable enqueued by this object (both
-// past and future). Because the writeable won't receive writesFinishedWithError:,
-// this also releases the writeable and the writer.
+// Cancels all pending messages to the writeable enqueued by this object (both past and future).
+// Because the writeable won't receive writesFinishedWithError:, this also releases the writeable.
 - (void)cancelSilently;
 @end
diff --git a/src/objective-c/RxLibrary/GRXConcurrentWriteable.m b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
index 735df3137b..08bd079aea 100644
--- a/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
+++ b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
@@ -36,9 +36,8 @@
 #import <RxLibrary/GRXWriteable.h>
 
 @interface GRXConcurrentWriteable ()
-// These are atomic so that cancellation can nillify them from any thread.
+// This is atomic so that cancellation can nillify it from any thread.
 @property(atomic, strong) id<GRXWriteable> writeable;
-@property(atomic, strong) GRXWriter *writer;
 @end
 
 @implementation GRXConcurrentWriteable {
@@ -48,25 +47,23 @@
 }
 
 - (instancetype)init {
-  return [self initWithWriteable:nil writer:nil];
+  return [self initWithWriteable:nil];
 }
 
 // Designated initializer
-- (instancetype)initWithWriteable:(id<GRXWriteable>)writeable writer:(GRXWriter *)writer {
+- (instancetype)initWithWriteable:(id<GRXWriteable>)writeable {
   if (self = [super init]) {
     _writeableQueue = dispatch_get_main_queue();
     _writeable = writeable;
-    _writer = writer;
   }
   return self;
 }
 
 - (void)enqueueValue:(id)value completionHandler:(void (^)())handler {
   dispatch_async(_writeableQueue, ^{
-    // We're racing a possible cancellation performed by another thread. To turn
-    // all already-enqueued messages into noops, cancellation nillifies the
-    // writeable property. If we get it before it's nil, we won
-    // the race.
+    // We're racing a possible cancellation performed by another thread. To turn all already-
+    // enqueued messages into noops, cancellation nillifies the writeable property. If we get it
+    // before it's nil, we won the race.
     id<GRXWriteable> writeable = self.writeable;
     if (writeable) {
       [writeable writeValue:value];
@@ -78,13 +75,11 @@
 - (void)enqueueSuccessfulCompletion {
   dispatch_async(_writeableQueue, ^{
     dispatch_once(&_alreadyFinished, ^{
-      // Cancellation is now impossible. None of the other three blocks can run
-      // concurrently with this one.
+      // Cancellation is now impossible. None of the other three blocks can run concurrently with
+      // this one.
       [self.writeable writesFinishedWithError:nil];
-      // Break the retain cycle with writer, and skip any possible message to the
-      // wrapped writeable enqueued after this one.
+      // Skip any possible message to the wrapped writeable enqueued after this one.
       self.writeable = nil;
-      self.writer = nil;
     });
   });
 }
@@ -92,29 +87,24 @@
 - (void)cancelWithError:(NSError *)error {
   NSAssert(error, @"For a successful completion, use enqueueSuccessfulCompletion.");
   dispatch_once(&_alreadyFinished, ^{
-    // Skip any of the still-enqueued messages to the wrapped writeable. We use
-    // the atomic setter to nillify writer and writeable because we might be
-    // running concurrently with the blocks in _writeableQueue, and assignment
-    // with ARC isn't atomic.
+    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
+    // nillify writeable because we might be running concurrently with the blocks in
+    // _writeableQueue, and assignment with ARC isn't atomic.
     id<GRXWriteable> writeable = self.writeable;
     self.writeable = nil;
 
     dispatch_async(_writeableQueue, ^{
       [writeable writesFinishedWithError:error];
-      // Break the retain cycle with writer.
-      self.writer = nil;
     });
   });
 }
 
 - (void)cancelSilently {
   dispatch_once(&_alreadyFinished, ^{
-    // Skip any of the still-enqueued messages to the wrapped writeable. We use
-    // the atomic setter to nillify writer and writeable because we might be
-    // running concurrently with the blocks in _writeableQueue, and assignment
-    // with ARC isn't atomic.
+    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
+    // nillify writeable because we might be running concurrently with the blocks in
+    // _writeableQueue, and assignment with ARC isn't atomic.
     self.writeable = nil;
-    self.writer = nil;
   });
 }
 @end