Bug 1866078 - Add InputToReadableStreamAlgorithms::MaybeGetInputStreamIfUnread r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D194186

dom/fetch/FetchStreamReader.cpp

@@ -169,6 +169,10 @@ void FetchStreamReader::StartConsuming(JSContext* aCx, ReadableStream* aStream,
                                        ErrorResult& aRv) {
   MOZ_DIAGNOSTIC_ASSERT(!mReader);
   MOZ_DIAGNOSTIC_ASSERT(aStream);
+  MOZ_ASSERT(!aStream->MaybeGetInputStreamIfUnread(),
+             "FetchStreamReader is for JS streams but we got a stream based on "
+             "nsIInputStream here. Extract nsIInputStream and read it instead "
+             "to reduce overhead.");
 
   aRv = MaybeGrabStrongWorkerRef(aCx);
   if (aRv.Failed()) {

dom/streams/UnderlyingSourceCallbackHelpers.cpp

@@ -511,6 +511,12 @@ void InputToReadableStreamAlgorithms::ReleaseObjects() {
   mPullPromise = nullptr;
 }
 
+nsIInputStream* InputToReadableStreamAlgorithms::MaybeGetInputStreamIfUnread() {
+  MOZ_ASSERT(!mStream->Disturbed(),
+             "Should be only called on non-disturbed streams");
+  return mInput->GetInputStream();
+}
+
 void InputToReadableStreamAlgorithms::ErrorPropagation(JSContext* aCx,
                                                        ReadableStream* aStream,
                                                        nsresult aError) {

dom/streams/UnderlyingSourceCallbackHelpers.h

@@ -59,7 +59,9 @@ class UnderlyingSourceAlgorithmsBase : public nsISupports {
   // from closed(canceled)/errored streams, without waiting for GC.
   virtual void ReleaseObjects() {}
 
-  // Fetch wants to special-case nsIInputStream-based streams
+  // Can be used to read chunks directly via nsIInputStream to skip JS-related
+  // overhead, if this readable stream is a wrapper of a native stream.
+  // Currently used by Fetch helper functions e.g. new Response(stream).text()
   virtual nsIInputStream* MaybeGetInputStreamIfUnread() { return nullptr; }
 
   // https://streams.spec.whatwg.org/#other-specs-rs-create
@@ -198,6 +200,8 @@ class InputStreamHolder final : public nsIInputStreamCallback,
     return mInput->CloseWithStatus(aStatus);
   }
 
+  nsIAsyncInputStream* GetInputStream() { return mInput; }
+
  private:
   ~InputStreamHolder();
 
@@ -241,6 +245,8 @@ class InputToReadableStreamAlgorithms final
 
   void ReleaseObjects() override;
 
+  nsIInputStream* MaybeGetInputStreamIfUnread() override;
+
  private:
   ~InputToReadableStreamAlgorithms() {
     if (mInput) {