RACSubject: avoid retaining the subscribers strongly in the disposable.

The `-subscribe:` method of `RACSubject` is returning a disposable that
removes the new subscriber from the collection of active subscribers to
that subject. The disposable is disposed when the new subscription is
disposed, either explicitly or by having the subject send a completion
or error.

However, for infinite subjects that are not manually disposed, the
subscriber will be kept in memory forever. This happens because the
subscriber holds the disposable, which in turns holds the subscriber.

To break the cycle, the subscriber and the collection of subscribers
needs to be held weakly.

Based on joomcode@9b27795.

Author: StatusReport

ReactiveObjC/RACSubject.m

@@ -61,8 +61,14 @@ - (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
   @synchronized (subscribers) {
     [subscribers addObject:subscriber];
   }
-  
+
+  @rac_weakify(subscribers, subscriber);
   [disposable addDisposable:[RACDisposable disposableWithBlock:^{
+    @rac_strongify(subscribers, subscriber);
+    if (!subscribers || !subscriber) {
+      return;
+    }
+
     @synchronized (subscribers) {
       // Since newer subscribers are generally shorter-lived, search
       // starting from the end of the list.

ReactiveObjCTests/RACSubjectSpec.m

@@ -72,14 +72,28 @@ - (void)didSubscribeWithDisposable:(RACCompoundDisposable *)disposable {
   });
 
   qck_it(@"should dispose the paired disposable when a subscription terminates", ^{
-    RACSubject* subject = [RACSubject new];
-    RACTestSubscriber* subscriber = [RACTestSubscriber new];
+    RACSubject *subject = [RACSubject new];
+    RACTestSubscriber *subscriber = [RACTestSubscriber new];
 
     [[subject subscribe:subscriber] dispose];
 
     expect(@(subscriber.disposable.disposed)).to(beTruthy());
   });
 
+  qck_it(@"should release the subscriber after subject deallocation", ^{
+    __weak RACTestSubscriber *weakSubscriber;
+
+    @autoreleasepool {
+      RACTestSubscriber *subscriber = [RACTestSubscriber new];
+      weakSubscriber = subscriber;
+      RACSubject *subject = [RACSubject subject];
+
+      [subject subscribe:subscriber];
+    }
+
+    expect(weakSubscriber).to(beNil());
+  });
+
   qck_itBehavesLike(RACSubscriberExamples, ^{
     return @{
       RACSubscriberExampleSubscriber: subject,