Fix crash in MergeChannel when removing subscription to closed receiver

Summary: We currently call closeReceiver() on a receiver in receiversBySubscriptionId_ that has already been removed from receivers_ (and freed). The fix is to check to see whether the receiver in receiversBySubscriptionId_ is still present in receivers_ before invoking any functions on it. If the receiver is not present in receivers_, that means it has already been closed, so we do not need to close it.

Reviewed By: aary

Differential Revision: D32233062

fbshipit-source-id: 0aa763b509897db7de970e12e2f01cffc1b49fc3

folly/experimental/channels/MergeChannel-inl.h

@@ -158,11 +158,11 @@ class MergeChannelProcessor
       auto [unbufferedReceiver, buffer] =
           detail::receiverUnbuffer(std::move(receiver));
       auto existingReceiverIt = receiversBySubscriptionId_.find(subscriptionId);
-      if (existingReceiverIt != receiversBySubscriptionId_.end() &&
-          receivers_.contains(existingReceiverIt->second)) {
-        // We already have a receiver with the given subscription ID. Trigger
-        // cancellation on that previous receiver.
-        if (!existingReceiverIt->second->isReceiverCancelled()) {
+      if (existingReceiverIt != receiversBySubscriptionId_.end()) {
+        if (receivers_.contains(existingReceiverIt->second) &&
+            !existingReceiverIt->second->isReceiverCancelled()) {
+          // We already have a receiver with the given subscription ID. Trigger
+          // cancellation on that previous receiver.
           existingReceiverIt->second->receiverCancel();
         }
         receiversBySubscriptionId_.erase(existingReceiverIt);
@@ -187,7 +187,8 @@ class MergeChannelProcessor
       if (receiverIt == receiversBySubscriptionId_.end()) {
         return;
       }
-      if (!receiverIt->second->isReceiverCancelled()) {
+      if (receivers_.contains(receiverIt->second) &&
+          !receiverIt->second->isReceiverCancelled()) {
         receiverIt->second->receiverCancel();
       }
       receiversBySubscriptionId_.erase(receiverIt);
@@ -390,10 +391,18 @@ class MergeChannelProcessor
 
   ChannelBridgePtr<TValue> sender_;
   folly::Executor::KeepAlive<folly::SequencedExecutor> executor_;
+  bool handleDestroyed_{false};
+
+  // The set of receivers that feed into this MergeChannel. This set "owns" its
+  // receivers. MergeChannelProcessor must free any receiver removed from this
+  // set.
   folly::F14FastSet<ChannelBridge<TValue>*> receivers_;
-  folly::F14FastMap<TSubscriptionId, ChannelBridge<TValue>*>
+
+  // A non-owning map from subscription ID to receiver. If the receiver for a
+  // given subscription ID is not present in receivers_, it has been freed and
+  // must not be used.
+  folly::F14FastMap<TSubscriptionId, ChannelBridge<TValue>*> //
       receiversBySubscriptionId_;
-  bool handleDestroyed_{false};
 };
 } // namespace detail
 

folly/experimental/channels/test/MergeChannelTest.cpp

@@ -159,6 +159,69 @@ TEST_F(
   executor_.drain();
 }
 
+TEST_F(MergeChannelFixture, ReceiveValues_RemoveReceiver) {
+  auto [receiver1, sender1] = Channel<int>::create();
+  auto [receiver2, sender2] = Channel<int>::create();
+  auto [mergedReceiver, mergeChannel] =
+      createMergeChannel<int, std::string>(&executor_);
+
+  mergeChannel.addNewReceiver("sub1", std::move(receiver1));
+  mergeChannel.addNewReceiver("sub2", std::move(receiver2));
+  executor_.drain();
+
+  EXPECT_CALL(onNext_, onValue(1));
+  EXPECT_CALL(onNext_, onValue(2));
+  EXPECT_CALL(onNext_, onValue(3));
+  EXPECT_CALL(onNext_, onClosed());
+
+  auto callbackHandle = processValues(std::move(mergedReceiver));
+
+  sender1.write(1);
+  sender2.write(2);
+  executor_.drain();
+
+  mergeChannel.removeReceiver("sub2");
+
+  sender1.write(3);
+  sender2.write(4);
+  executor_.drain();
+
+  std::move(mergeChannel).close();
+  executor_.drain();
+}
+
+TEST_F(MergeChannelFixture, ReceiveValues_RemoveReceiver_AfterClose) {
+  auto [receiver1, sender1] = Channel<int>::create();
+  auto [receiver2, sender2] = Channel<int>::create();
+  auto [mergedReceiver, mergeChannel] =
+      createMergeChannel<int, std::string>(&executor_);
+
+  mergeChannel.addNewReceiver("sub1", std::move(receiver1));
+  mergeChannel.addNewReceiver("sub2", std::move(receiver2));
+  executor_.drain();
+
+  EXPECT_CALL(onNext_, onValue(1));
+  EXPECT_CALL(onNext_, onValue(2));
+  EXPECT_CALL(onNext_, onValue(3));
+  EXPECT_CALL(onNext_, onClosed());
+
+  auto callbackHandle = processValues(std::move(mergedReceiver));
+
+  sender1.write(1);
+  sender2.write(2);
+  executor_.drain();
+
+  std::move(sender2).close();
+  executor_.drain();
+
+  mergeChannel.removeReceiver("sub2");
+  sender1.write(3);
+  executor_.drain();
+
+  std::move(mergeChannel).close();
+  executor_.drain();
+}
+
 TEST_F(MergeChannelFixture, OneInputClosed_ContinuesMerging) {
   auto [receiver1, sender1] = Channel<int>::create();
   auto [receiver2, sender2] = Channel<int>::create();