Merge pull request #3159 from cloudflare/jsnell/fixup-transformstream…

…-backpressure

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

@@ -7,6 +7,7 @@
 #include "readable.h"
 #include "writable.h"
 
+#include <workerd/io/features.h>
 #include <workerd/jsg/jsg.h>
 #include <workerd/util/weak-refs.h>
 
@@ -3634,6 +3635,13 @@ void TransformStreamDefaultController::enqueue(jsg::Lock& js, v8::Local<v8::Valu
   bool newBackpressure = readableController.hasBackpressure();
   if (newBackpressure != backpressure) {
     KJ_ASSERT(newBackpressure);
+    // Unfortunately the original implementation forgot to actually set the backpressure
+    // here so the backpressure signaling failed to work correctly. This is unfortunate
+    // because applying the backpressure here could break existing code, so we need to
+    // put the fix behind a compat flag. Doh!
+    if (FeatureFlags::get(js).getFixupTransformStreamBackpressure()) {
+      setBackpressure(js, true);
+    }
   }
 }
 

src/workerd/io/compatibility-date.capnp

@@ -660,4 +660,12 @@ struct CompatibilityFlags @0x8f8c1b68151b6cef {
   # When enabled, use of top-level await syntax in require() calls will be disallowed.
   # The ecosystem and runtimes are moving to a state where top level await in modules
   # is being strongly discouraged.
+
+  fixupTransformStreamBackpressure @68 :Bool
+      $compatEnableFlag("fixup-transform-stream-backpressure")
+      $compatDisableFlag("original-transform-stream-backpressure")
+      $compatEnableDate("2024-12-16");
+  # A bug in the original implementation of TransformStream failed to apply backpressure
+  # correctly. The fix, however, can break existing implementations that don't account
+  # for the bug so we need to put the fix behind a compat flag.
 }