Bug 1282577 - Guard against multiple intr messages causes us to multi-delete GMP actors. r=jesup.

This is a regression from bug 1162358.

We must be hitting the #ifndef SHMEM_ALLOC_IN_CHILD block in
GMPVideoDecoderChild::Alloc() with multiple allocs doing intr calls at once.
If this happens when a DecodingComplete() comes in, we'll end up sending one
task to re-call RecvDecodingComplete for every Alloc() blocked on an intr
response.  This would result in us ending up trying to Send__delete__() in
RecvDecodingComplete() twice.  Which causes the runtime abort we're seeing
here.

I think that could happen in the WidevineVideoDecoder if a Decode message comes
in, and goes into a ReturnOutput(), tries to alloc a frame and has to spin on
an intr message response, and another Decode message comes in and does the
same, so GMPVideoDecoderChild::mNeedShmemIntrCount will be 2, and then a
DecodingComplete comes in, and when two tasks on the stack in
GMPVideoDecoderChild::Alloc() finish they both end up dispatching a task to
re-call GMPVideoDecoderChild::RecvDecodingComplete(). So we end up trying to
Send__delete__() in RecvDecodingComplete() twice.

I expect the same problem exists in the GMPVideoEncoder too.

intr, or spinning event loops in general for that matter, is evil.


MozReview-Commit-ID: AKsvP62G3Cx

--HG--
extra : rebase_source : 53ca12dbbbf3ab071788e2322b7c926ec7c0325f

dom/media/gmp/GMPVideoDecoderChild.cpp

@@ -225,7 +225,8 @@ GMPVideoDecoderChild::Alloc(size_t aSize,
   ++mNeedShmemIntrCount;
   rv = CallNeedShmem(aSize, aMem);
   --mNeedShmemIntrCount;
-  if (mPendingDecodeComplete) {
+  if (mPendingDecodeComplete && mNeedShmemIntrCount == 0) {
+    mPendingDecodeComplete = false;
     mPlugin->GMPMessageLoop()->PostTask(
       NewRunnableMethod(this, &GMPVideoDecoderChild::RecvDecodingComplete));
   }

dom/media/gmp/GMPVideoEncoderChild.cpp

@@ -206,7 +206,8 @@ GMPVideoEncoderChild::Alloc(size_t aSize,
   ++mNeedShmemIntrCount;
   rv = CallNeedShmem(aSize, aMem);
   --mNeedShmemIntrCount;
-  if (mPendingEncodeComplete) {
+  if (mPendingEncodeComplete && mNeedShmemIntrCount == 0) {
+    mPendingEncodeComplete = false;
     mPlugin->GMPMessageLoop()->PostTask(
       NewRunnableMethod(this, &GMPVideoEncoderChild::RecvEncodingComplete));
   }