[pull] main from facebook:main#381
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…35127) This PR fixes a critical bug where `ReadableStream({type: 'bytes'})` instances passed through React Server Components (RSC) would stall after reading only the first chunk or the first few chunks in the client. This issue was masked by using `web-streams-polyfill` in tests, but manifests with native Web Streams implementations. The root cause is that when a chunk is enqueued to a `ReadableByteStreamController`, the spec requires the underlying ArrayBuffer to be synchronously transferred/detached. In the React Flight Client's chunk parsing, embedded byte stream chunks are created as views into the incoming RSC stream chunk buffer using `new Uint8Array(chunk.buffer, offset, length)`. When embedded byte stream chunks are enqueued, they can detach the shared buffer, leaving the RSC stream parsing in a broken state. The fix is to copy embedded byte stream chunks before enqueueing them, preventing buffer detachment from affecting subsequent parsing. To not affect performance too much, we use a zero-copy optimization: when a chunk ends exactly at the end of the RSC stream chunk, or when the row spans into the next RSC chunk, no further parsing will access that buffer, so we can safely enqueue the view directly without copying. We now also enqueue embedded byte stream chunks immediately as they are parsed, without waiting for the full row to complete. To simplify the logic in the client, we introduce a new `'b'` protocol tag specifically for byte stream chunks. The server now emits `'b'` instead of `'o'` for `Uint8Array` chunks from byte streams (detected via `supportsBYOB`). This allows the client to recognize byte stream chunks without needing to track stream IDs. Tests now use the proper Jest environment with native Web Streams instead of polyfills, exposing and validating the fix for this issue.
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