Bug 1681529 - Part 3: Apply InputStreamLengthWrapper after DelayedStartInputStream, r=baku

Previously, we would apply the InputStreamLengthWrapper before
DelayedStartInputStream, which meant that a stream serialized with aDelayedStart
would not correctly implement nsIInputStreamLength. By inverting the order of
these wrappers, the nsIInputStreamLength implementation is visible, without
impacting the functionality of the DelayedStartInputStream wrapper.

Differential Revision: https://phabricator.services.mozilla.com/D101802

ipc/glue/IPCStreamDestination.cpp

@@ -330,26 +330,30 @@ void IPCStreamDestination::SetLength(int64_t aLength) {
   mLengthSet = true;
 #endif
 
-  if (aLength != -1) {
-    nsCOMPtr<nsIInputStream> finalStream;
-    finalStream = new InputStreamLengthWrapper(mReader.forget(), aLength);
-    mReader = do_QueryInterface(finalStream);
-    MOZ_ASSERT(mReader);
-  }
+  mLength = aLength;
 }
 
 already_AddRefed<nsIInputStream> IPCStreamDestination::TakeReader() {
   MOZ_ASSERT(mReader);
   MOZ_ASSERT(!mDelayedStartInputStream);
 
+  nsCOMPtr<nsIAsyncInputStream> reader{mReader.forget()};
   if (mDelayedStart) {
     mDelayedStartInputStream =
-        new DelayedStartInputStream(this, std::move(mReader));
-    RefPtr<nsIAsyncInputStream> inputStream = mDelayedStartInputStream;
-    return inputStream.forget();
+        new DelayedStartInputStream(this, std::move(reader));
+    reader = mDelayedStartInputStream;
+    MOZ_ASSERT(reader);
   }
 
-  return mReader.forget();
+  if (mLength != -1) {
+    MOZ_ASSERT(mLengthSet);
+    nsCOMPtr<nsIInputStream> finalStream =
+        new InputStreamLengthWrapper(reader.forget(), mLength);
+    reader = do_QueryInterface(finalStream);
+    MOZ_ASSERT(reader);
+  }
+
+  return reader.forget();
 }
 
 bool IPCStreamDestination::IsOnOwningThread() const {

ipc/glue/IPCStreamDestination.h

@@ -80,6 +80,7 @@ class IPCStreamDestination {
   // away. If that happens, the reading of data will not be possible anymore.
   class DelayedStartInputStream;
   RefPtr<DelayedStartInputStream> mDelayedStartInputStream;
+  int64_t mLength = -1;
 
   nsCOMPtr<nsIThread> mOwningThread;
   bool mDelayedStart;