Relay 21 drops @defer chunks whose `subPath` addresses a leaf inside the fragment (per-item non-Node lists, deduped fields, connection edges)

[jonreading81]

Summary

OperationExecutor._processIncrementalResponses looks up placeholders by exact (label, chunk.path.join('.')). Placeholders register at pendingPart.path — the fragment-spread path — with no subPath suffix. Any chunk arriving from a spec-compliant incremental-delivery server that carries a subPath (e.g. graphql-core 3.3+'s per-item non-Node list chunks, or field-dedup chunks addressed at a sub-record) therefore lands in a map bucket no placeholder ever registers at → buffered forever → silently dropped by the client store.

Repro shapes

All three are the same underlying lookup miss:

1) String-only subPath (deduplicated single sub-record)

Chunk carries data: {isBranded: false}, subPath: ['owner'] because the server dedupes fields the parent selection already covered.

Chunk arrives at path = ['cloudcast', 'owner']. Placeholder for CreatorActions_cloudcast @defer sits at pathKey = 'cloudcast'. Lookup misses. Field never lands in the owner record.

2) Numeric-in-subPath (non-Node list per-item chunk)

fragment Body on Cloudcast { tags { ..., position } ...ChartPosition_cloudcast @defer } where ChartPosition_cloudcast also selects tags { ..., bestPosition } and CloudcastTag has no id. graphql-core streams per item:

• {data: {bestPosition: 1}, subPath: ['tags', 0]}
• {data: {bestPosition: 1}, subPath: ['tags', 1]} …

Every chunk arrives at path = ['cloudcast', 'tags', N]. Placeholder sits at ['cloudcast']. Every chunk dropped. Card renders "Global chart history" header + 0 positions.

3) Numeric-in-subPath through a connection

fragment Sidebar on User { ...SideBarSpotlights_user @defer } where SideBarSpotlights_user selects spotlight.users(first: 4) @connection. Edges arrive at path = ['user', 'spotlight', 'users', 'edges', N, 'node', ...]. Placeholder sits at ['user']. Every edge dropped. Header reads "Spotlights (10)" (scalar sibling made it through the pre-existing paths) but the 4 rows are blank.

Case #3 is the @defer + @connection symptom (empty edges + count present); case #1 is the "deferred field never lands" symptom; case #2 is the non-Node array symptom.

The bug is one line

_processIncrementalResponses, packages/relay-runtime/store/OperationExecutor.js:

const pathKey = path.map(String).join('.');
let resultForPath = resultForLabel.get(pathKey);

Exact match only. There's no logic that says "if the chunk's path is longer than any placeholder's path, walk back."

Fix (PR)

Filed as #5370.

• Prefix-match placeholder lookup — walk shorter prefixes of the chunk's path until a placeholder is found; compute subPath = path.slice(prefix.length).
• Shape-specific recovery in _processDeferResponse:
  • String-only subPath → wrap data + normalise at placeholder root, seed the local source with a clone of the existing parent record so _normalizeLink's id-fallback re-uses the store's real linked-record IDs (avoids synthesising a client ID that would sever the parent's link on publish).
  • Numeric-in-subPath → walk the fragment's normalization AST alongside the store from parentID following each subPath key, resolve the child dataID + item-level selection + concrete type, and normalise chunk data directly INTO that child record (no plural-link clobber).
• normalizeResponse gains an optional existingRootRecord parameter; all callers unchanged unless they need it.

yarn typecheck clean; 78/78 existing defer + deferredStreamedConnection tests still pass; verified empirically against a graphql-core 3.3+ backend in a shipping app across all three shapes.

Context

Supersedes #5354 (closed) — same root cause, three distinct symptom shapes, one fix for all of them. The prior issue framed the two visible symptoms (fragment-nested @connection empty edges; non-Node array patches dropped) as separate bugs. They aren't. Opening this to link the PR to a clear description of the unified root cause.