Bug 1709138 - Fix a crash in MutableBlobStorage related to a wrong error propagation logic, r=asuth

Differential Revision: https://phabricator.services.mozilla.com/D236168

dom/file/MutableBlobStorage.cpp

@@ -339,8 +339,6 @@ void MutableBlobStorage::GetBlobImplWhenReady(
       return;
     }
 
-    MOZ_ASSERT(mActor);
-
     // We want to wait until all the WriteRunnable are completed. The way we do
     // this is to go to the I/O thread and then we come back: the runnables are
     // executed in order and this LastRunnable will be... the last one.
@@ -594,9 +592,19 @@ void MutableBlobStorage::AskForBlob(TemporaryIPCBlobChildCallback* aCallback,
 
   MutexAutoLock lock(mMutex);
   MOZ_ASSERT(mStorageState == eClosed);
+  MOZ_ASSERT(aCallback);
+
+  // If something went wrong in one of the previous WriteRunnable runnables,
+  // it's time to report the error.
+  if (NS_FAILED(mErrorResult)) {
+    MOZ_ASSERT(!mFD);
+    MOZ_ASSERT(!mActor);
+    aCallback->OperationFailed(mErrorResult);
+    return;
+  }
+
   MOZ_ASSERT(mFD);
   MOZ_ASSERT(mActor);
-  MOZ_ASSERT(aCallback);
 
   // Let's pass the FileDescriptor to the parent actor in order to keep the file
   // locked on windows.