Future now clears cycle nodes more aggressively, and has a comment that it's okay if they aren't

Author: Strilanc

Diff for: src/TOCFutureAndSource.m

@@ -84,7 +84,15 @@ -(enum StartUnwrapResult) _ForSource_tryStartUnwrapping:(TOCFuture*)targetFuture
     // look for flattening cycles
     @synchronized(getSharedCycleDetectionLock()) {
         bool didMerge = [self._getInitCycleNode unionWith:targetFuture._getInitCycleNode];
-        if (!didMerge) return StartUnwrapResult_CycleDetected;
+        if (!didMerge) {
+            // it's not necessary to clear the cycle nodes, but it frees up some memory
+            // other futures in the cycle won't get their cycle nodes cleared
+            // but node chains should be in EXTREMELY shallow trees
+            // and the nodes can't keep futures alive, so there's no reference cycle
+            _cycleNode = nil;
+            targetFuture->_cycleNode = nil;
+            return StartUnwrapResult_CycleDetected;
+        }
     }
 
     return StartUnwrapResult_Started;