Miniflare supports more TransformStream functionality than Workers

[leviwolfe]

Miniflare supports passing the transformer argument to the TransformStream constructor. This worked for me and I could see chunks being passed to my transformer's transform method. Once I deployed this code to Cloudflare, the transform method is no longer called.

Cloudflare's Worker documentation for the TransformStream constructor makes no mention of the transformer argument. It only mentions:

The Workers platform currently only implements an identity transform stream, a type of transform stream which forwards all chunks written to its writable side to its readable side, without any changes.

It seems like Cloudflare is ignoring the transformer argument, so perhaps Miniflare should as well? It is a platform difference that cost quite a bit of time to debug.

[leviwolfe]

I created a small repo at https://github.com/leviwolfe/miniflare-168 which I think reproduces this issue.

[mrbbot]

Hey! 👋 This is an issue with all the stream constructors (ReadableStream, WritableStream, TransformStream). Implementations of the ReadableStream and WritableStream constructors were added recently, but are currently behind a compatibility flag: https://community.cloudflare.com/t/2021-12-10-workers-runtime-release-notes/334982. We should probably change this to throw when the flag isn't set. 👍

[leviwolfe]

Thanks, I'll have to look into that flag! Funny enough the Cloudflare team said almost the exact same thing a few years back: https://community.cloudflare.com/t/running-into-unimplemented-functionality/77343/6

[mrbbot]

Hey again! 👋 The latest Workers Runtime adds a transformstream_enable_standard_constructor compatibility flag to enable the standard (Miniflare) behaviour. Will try implement the non-standard behaviour (also available as IdentityTransformStream) and that flag.