Have internal streams return empty Uint8Array on end of byob stream

src/workerd/api/streams/internal.c++

@@ -9,6 +9,7 @@
 #include <kj/vector.h>
 #include <workerd/api/util.h>
 #include <workerd/util/string-buffer.h>
+#include <workerd/io/features.h>
 
 namespace workerd::api {
 
@@ -543,6 +544,15 @@ kj::Maybe<jsg::Promise<ReadResult>> ReadableStreamInternalController::read(
 
   KJ_SWITCH_ONEOF(state) {
     KJ_CASE_ONEOF(closed, StreamStates::Closed) {
+      if (maybeByobOptions != kj::none && FeatureFlags::get(js).getInternalStreamByobReturn()) {
+        // When using the BYOB reader, we must return a sized-0 Uint8Array that is backed
+        // by the ArrayBuffer passed in the options.
+        auto u8 = v8::Uint8Array::New(v8::ArrayBuffer::New(js.v8Isolate, store), 0, 0);
+        return js.resolvedPromise(ReadResult {
+          .value = js.v8Ref(u8.As<v8::Value>()),
+          .done = true,
+        });
+      }
       return js.resolvedPromise(ReadResult { .done = true });
     }
     KJ_CASE_ONEOF(errored, StreamStates::Errored) {
@@ -582,8 +592,10 @@ kj::Maybe<jsg::Promise<ReadResult>> ReadableStreamInternalController::read(
       // That's a larger refactor, though.
       auto& ioContext = IoContext::current();
       return ioContext.awaitIoLegacy(js, kj::mv(promise)).then(js,
-          ioContext.addFunctor([this,store = kj::mv(store), byteOffset, byteLength]
-          (jsg::Lock& js, size_t amount) mutable -> jsg::Promise<ReadResult> {
+          ioContext.addFunctor(
+              [this,store = kj::mv(store), byteOffset, byteLength,
+               isByob = maybeByobOptions != kj::none]
+              (jsg::Lock& js, size_t amount) mutable -> jsg::Promise<ReadResult> {
         readPending = false;
         KJ_ASSERT(amount <= byteLength);
         if (amount == 0) {
@@ -593,6 +605,15 @@ kj::Maybe<jsg::Promise<ReadResult>> ReadableStreamInternalController::read(
           KJ_IF_SOME(o, owner) {
             o.signalEof(js);
           }
+          if (isByob && FeatureFlags::get(js).getInternalStreamByobReturn()) {
+            // When using the BYOB reader, we must return a sized-0 Uint8Array that is backed
+            // by the ArrayBuffer passed in the options.
+            auto u8 = v8::Uint8Array::New(v8::ArrayBuffer::New(js.v8Isolate, store), 0, 0);
+            return js.resolvedPromise(ReadResult {
+              .value = js.v8Ref(u8.As<v8::Value>()),
+              .done = true,
+            });
+          }
           return js.resolvedPromise(ReadResult { .done = true });
         }
         // Return a slice so the script can see how many bytes were read.

src/workerd/api/tests/streams-test.js

@@ -427,6 +427,27 @@ export const abortWriterAfterGc = {
   }
 };
 
+export const finalReadOnInternalStreamReturnsBuffer = {
+  async test() {
+    const { readable, writable } = new IdentityTransformStream();
+    const writer = writable.getWriter();
+    await writer.close();
+
+    const reader = readable.getReader({ mode: 'byob' });
+    let result = await reader.read(new Uint8Array(10));
+    strictEqual(result.done, true);
+    ok(result.value instanceof Uint8Array);
+    strictEqual(result.value.byteLength, 0);
+    strictEqual(result.value.buffer.byteLength, 10);
+
+    result = await reader.read(new Uint8Array(10));
+    strictEqual(result.done, true);
+    ok(result.value instanceof Uint8Array);
+    strictEqual(result.value.byteLength, 0);
+    strictEqual(result.value.buffer.byteLength, 10);
+  }
+};
+
 export default {
   async fetch(request, env) {
     strictEqual(request.headers.get('content-length'), '10');

src/workerd/api/tests/streams-test.wd-test

@@ -9,7 +9,7 @@ const unitTests :Workerd.Config = (
           (name = "worker", esModule = embed "streams-test.js")
         ],
         compatibilityDate = "2023-01-15",
-        compatibilityFlags = ["nodejs_compat"],
+        compatibilityFlags = ["nodejs_compat", "internal_stream_byob_return_view"],
         bindings = [
           (name = "subrequest", service = "streams-test")
         ]

src/workerd/io/compatibility-date.capnp

@@ -415,4 +415,15 @@ struct CompatibilityFlags @0x8f8c1b68151b6cef {
   # type of Durable Object stubs -- support RPC. If so, this type will have a wildcard method, so
   # it will appear that all possible property names are present on any fetcher instance. This could
   # break code that tries to infer types based on the presence or absence of methods.
+
+  internalStreamByobReturn @47 :Bool
+      $compatEnableFlag("internal_stream_byob_return_view")
+      $compatDisableFlag("internal_stream_byob_return_undefined")
+      $compatEnableDate("2024-05-13");
+  # Sadly, the original implementation of ReadableStream (now called "internal" streams), did not
+  # properly implement the result of ReadableStreamBYOBReader's read method. When done = true,
+  # per the spec, the result `value` must be an empty ArrayBufferView whose underlying ArrayBuffer
+  # is the same as the one passed to the read method. Our original implementation returned
+  # undefined instead. This flag changes the behavior to match the spec and to match the behavior
+  # implemented by the JS-backed ReadableStream implementation.
 }