Greptile Summary

This PR introduces promise-level deduplication for the getSharedHighlighter call inside prepareHighlighter. Previously, concurrent renders for the same appearance:language key could each kick off their own getSharedHighlighter() call; now they all share a single in-flight Promise<Highlighter> stored in the new highlighterPromiseCache. Once the promise resolves, every subsequent caller receives the already-settled promise immediately, and on rejection the entry is evicted so the next caller gets a clean retry. The existing highlighterOptionsByKey option cache is preserved as-is.

Key changes:

• Added type Highlighter = Awaited<ReturnType<typeof getSharedHighlighter>> for type-safe Map annotation.
• Added highlighterPromiseCache: Map<string, Promise<Highlighter>> at module scope.
• prepareHighlighter now returns the cached promise for in-flight and already-resolved entries, skipping option computation and a second getSharedHighlighter call.
• A .catch() chained to getSharedHighlighter deletes the cache key before re-throwing, ensuring transient errors don't permanently poison the cache.

Confidence Score: 5/5

Safe to merge — the change is a clean promise-deduplication optimization with correct error-eviction semantics and no behavioral regressions.

No P0 or P1 issues found. The new cache correctly shares in-flight and resolved promises, the .catch() eviction prevents stale rejected entries, the highlighterOptionsByKey invariant is preserved, and the bounded key space (appearance × language) prevents unbounded cache growth. All edge cases (concurrent miss, concurrent hit before rejection, post-rejection retry) behave as intended.

No files require special attention.

Important Files Changed

Sequence Diagram

sequenceDiagram
    participant C1 as Caller A
    participant C2 as Caller B (concurrent)
    participant P as prepareHighlighter
    participant Cache as highlighterPromiseCache
    participant SH as getSharedHighlighter

    C1->>P: prepareHighlighter(lang, appearance)
    P->>Cache: get(cacheKey) → miss
    P->>SH: getSharedHighlighter(options)
    SH-->>P: Promise<Highlighter> (pending)
    P->>Cache: set(cacheKey, promise)
    P-->>C1: promise (pending)

    C2->>P: prepareHighlighter(lang, appearance)
    P->>Cache: get(cacheKey) → hit
    P-->>C2: same promise (pending)

    SH-->>C1: resolves → Highlighter
    SH-->>C2: resolves → Highlighter (shared)

    Note over Cache: Entry stays for future callers

    rect rgb(255, 220, 220)
        Note over SH,Cache: On failure path
        SH-->>P: rejects
        P->>Cache: delete(cacheKey)
        P-->>C1: rethrows error
        P-->>C2: rethrows error (same promise)
        Note over Cache: Entry cleared — next caller retries cleanly
    end

_{Reviews (1): Last reviewed commit: "perf: cache highlighter promises by key" | Re-trigger Greptile}