fix(mergeAllOf): prevent infinite loop on mutually recursive allOf schemas

[vwong]

Problem

When a schema contains mutually recursive allOf references (e.g. A = { allOf: [{ $ref: '#/B' }] }, B = { allOf: [{ $ref: '#/A' }] }), mergeAllOf enters an infinite loop and hangs the process.

The root cause is in the do-while loop in mergeAllOf:

do {
  merged = _mergeAllOf(merged, path, resolveRef, seen);
} while ('allOf' in merged);

resolveAllOf (from @stoplight/json-schema-merge-allof, called with deep: false) returns a new object on every invocation. Because seen is a WeakMap keyed by object identity, the cache never hits for these new intermediate objects. The loop runs forever, consuming 100% CPU without ever overflowing the call stack.

This surfaces as an InternalError: too much recursion (Firefox) or RangeError: Maximum call stack size exceeded (Chrome) in consumers like @stoplight/elements when rendering OpenAPI specs that contain circular schema references.

Fix

Track which $refs have been resolved across do-while iterations using a Set<string> created once per mergeAllOf call. Each iteration records the $refs it resolves; subsequent iterations check against that accumulated set. If a $ref from a prior iteration appears again, the chain is circular and we throw ResolvingError — which the walker already catches and handles gracefully by falling back to the raw unmerged fragment.

Key properties of this approach:

• No false positives for siblings: the same $ref appearing multiple times within a single allOf (resolved in the same iteration) is allowed — only cross-iteration repeats are blocked.
• Long legitimate chains work: A → allOf → B → allOf → C → (no allOf) resolves #/B then #/C in successive iterations with no repeated refs.
• Simplifies the code: removes the store WeakMap (which tracked schemaRefs per (resolveRef, fragment) pair) in favour of the per-call set.

Test

src/__tests__/mergeAllOf.spec.ts calls mergeAllOf directly with a mutually recursive A ↔ B schema and asserts toThrow(ResolvingError). Without the fix the test hangs; with it the test completes in ~3 ms.