[Flight Reply] Decode FormData part ids as hex - #37879

Summary

A FormData passed to a Server Action is deserialized as an empty FormData whenever its part id is 10 or higher, silently dropping every field.

The wire format is $K<id> with id in hex, like every other reference in the protocol. But the backing entries are written by the client under <formFieldPrefix>_<refId>_<key>, where refId is interpolated as a decimal number:

// react-client/src/ReactFlightReplyClient.js
const refId = nextPartId++;
const prefix = formFieldPrefix + '_' + refId + '_';
value.forEach((originalValue, originalKey) => {
  data.append(prefix + originalKey, originalValue);
});
return serializeFormDataReference(refId); // '$K' + refId.toString(16)

parseModelString then used the raw hex substring as the key prefix instead of converting it back:

const stringId = value.slice(2);
const formPrefix = response._prefix + '_' + stringId + '_';

The two representations only agree below 10, so for any id >= 10 the lookup matches nothing. consumeBackingEntries returns an empty FormData and, because it doesn't advance the cursor, the entries are left behind for the rest of the reply.

Every other id-carrying branch in the same function already decodes hex before building a backing key — parseTypedArray (parseInt(reference.slice(2), 16)), 'B' (parseInt(value.slice(2), 16)), getOutlinedModel (parseInt(path[0], 16)) and getChunk (takes a number). 'K' was the only exception.

This is reachable from encodeReply whenever ten or more parts are outlined before the FormData (promises, Blobs, typed arrays, iterators, other FormDatas...), and from bound Server Actions whose bound arguments include one.

Test

Added a case to ReactFlightDOMReply-test.js that outlines nine promises first, so the FormData becomes part 10 — the first id whose hex form differs from its decimal form. Failing output before the fix:

● ReactFlightDOMReply › can pass FormData after at least ten outlined parts

  expect(received).toEqual(expected) // deep equality

  - Expected  - 6
  + Received  + 1

  - Array [
  -   Array [
  -     "hello",
  -     "world",
  -   ],
  - ]
  + Array []

Verification

• ReactFlightDOMReply (webpack / Node / Edge), ReactFlightDOMForm, ReactFlightTurbopackDOMReply, ReactFlightDOMNode — all pass on experimental, plus -r=stable and --prod for the reply suites.
• yarn linc and prettier check-changed clean.
• No overlap with [Flight Reply] Check that a typed array's backing entry is a Blob #37822, which touches parseTypedArray further down the same file and adds its tests earlier in the same describe block.