Bug 1359172 - RemoteInputStream must create a correct inputStream when used and sliced in a separate process, r=smaug

dom/file/ipc/Blob.cpp

@@ -50,6 +50,7 @@
 #include "nsStringStream.h"
 #include "nsThreadUtils.h"
 #include "nsXULAppAPI.h"
+#include "SlicedInputStream.h"
 #include "StreamBlobImpl.h"
 #include "WorkerPrivate.h"
 #include "WorkerRunnable.h"
@@ -2765,19 +2766,44 @@ CreateStreamHelper::GetStream(nsIInputStream** aInputStream)
   MOZ_ASSERT(baseRemoteBlobImpl);
 
   if (EventTargetIsOnCurrentThread(baseRemoteBlobImpl->GetActorEventTarget())) {
+    // RunInternal will populate mInputStream using the correct mStart/mLength
+    // value.
     RunInternal(baseRemoteBlobImpl, false);
   } else if (PBackgroundChild* manager = mozilla::ipc::BackgroundChild::GetForCurrentThread()) {
+    // In case we are on a PBackground thread and this is not the owning thread,
+    // we need to create a new actor here. This actor must be created for the
+    // baseRemoteBlobImpl, which can be the mRemoteBlobImpl or the parent one,
+    // in case we are dealing with a sliced blob.
     BlobChild* blobChild = BlobChild::GetOrCreate(manager, baseRemoteBlobImpl);
     MOZ_ASSERT(blobChild);
 
-    RefPtr<BlobImpl> blobImpl = blobChild->GetBlobImpl();
-    MOZ_ASSERT(blobImpl);
+    // Note that baseBlobImpl is generated by the actor, and the actor is
+    // created from the baseRemoteBlobImpl. This means that baseBlobImpl is the
+    // remote blobImpl on PBackground of baseRemoteBlobImpl.
+    RefPtr<BlobImpl> baseBlobImpl = blobChild->GetBlobImpl();
+    MOZ_ASSERT(baseBlobImpl);
 
     ErrorResult rv;
-    blobImpl->GetInternalStream(aInputStream, rv);
+    nsCOMPtr<nsIInputStream> baseInputStream;
+    baseBlobImpl->GetInternalStream(getter_AddRefs(baseInputStream), rv);
+    if (NS_WARN_IF(rv.Failed())) {
+      return rv.StealNSResult();
+    }
+
+    // baseInputStream is the stream of the baseRemoteBlobImpl. If
+    // mRemoteBlobImpl is a slice of baseRemoteBlobImpl, here we need to slice
+    // baseInputStream.
+    if (mRemoteBlobImpl->IsSlice()) {
+      RefPtr<SlicedInputStream> slicedInputStream =
+        new SlicedInputStream(baseInputStream, mStart, mLength);
+      slicedInputStream.forget(aInputStream);
+    } else {
+      baseInputStream.forget(aInputStream);
+    }
+
     mRemoteBlobImpl = nullptr;
     mDone = true;
-    return rv.StealNSResult();
+    return NS_OK;
   } else {
     nsresult rv = baseRemoteBlobImpl->DispatchToTarget(this);
     if (NS_WARN_IF(NS_FAILED(rv))) {