fix request#clone not working with js-backed streams

[anonrig]

Fixes #5175

[jasnell]

This is looking at the issue from the wrong side.. the problem is really in AllReader::loop in standard.c++... it's detaching the underlying ArrayBuffer which breaks the other branch since they're sharing a reference to the same object.

The correct fix is:

diff --git a/src/workerd/api/streams/standard.c++ b/src/workerd/api/streams/standard.c++
index df5adf3f7..1b476b077 100644
--- a/src/workerd/api/streams/standard.c++
+++ b/src/workerd/api/streams/standard.c++
@@ -2735,14 +2735,13 @@ class AllReader {
           }
 
           jsg::BufferSource bufferSource(js, handle);
-          jsg::BackingStore backing = bufferSource.detach(js);
 
-          if (backing.size() == 0) {
+          if (bufferSource.size() == 0) {
             // Weird but allowed, we'll skip it.
             return loop(js);
           }
 
-          if ((runningTotal + backing.size()) > limit) {
+          if ((runningTotal + bufferSource.size()) > limit) {
             auto error = js.v8TypeError("Memory limit exceeded before EOF.");
             auto rs = kj::mv(readable);
             state.template init<StreamStates::Errored>(js.v8Ref(error));
@@ -2750,8 +2749,8 @@ class AllReader {
                 js, [&](jsg::Lock& js) { return loop(js); });
           }
 
-          runningTotal += backing.size();
-          parts.add(jsg::BufferSource(js, kj::mv(backing)));
+          runningTotal += bufferSource.size();
+          parts.add(bufferSource.copy(js));
           return loop(js);
         };

[anonrig]

thanks @jasnell - duh! you're right.