Synchronize the in-memory checkpoint manager

[PratikDhanave]

Summary

inMemoryManager (workflow/checkpoint/manager.go) has an unsynchronized Store map[string]*SessionCache and per-session caches. The manager is keyed by session ID, and NewInMemoryManager documents no single-use restriction — so sharing one manager across concurrent workflow runs (distinct sessions committing at the same time) is a natural pattern. Under that use, concurrent Commit calls race on the map and can crash the process with fatal: concurrent map writes.

Under -race, a handful of concurrent commits reliably report data races on the Store access in sessionStore.

Fix

Guard Store with a sync.Mutex in Commit, Lookup, and RetrieveIndex. The critical sections are pure in-memory map/cache operations (no I/O), so holding the lock across each method is cheap.

Design note: I fixed this by making the in-memory manager safe for concurrent use, since its session-keyed design invites sharing. The alternative is to document it as not concurrency-safe, mirroring FileSystemJSONStore's existing caveat, and leave callers to serialize. Happy to switch to the doc-only approach if you'd prefer the stores be consistent in that direction instead.

Public API

No exported symbols change.

Tests

Adds TestInMemoryManager_ConcurrentSessions_NoRace (internal test): 64 goroutines Commit and RetrieveIndex distinct sessions concurrently. It reports data races under -race before this change and passes after.

go test -race ./workflow/checkpoint passes; go build ./... and go vet clean; gofumpt clean.

[Copilot]

Pull request overview

Synchronizes the in-memory checkpoint Manager implementation to prevent concurrent access races when a single inMemoryManager is shared across multiple concurrent workflow sessions.

Changes:

• Added a mutex to inMemoryManager and guarded Commit, Lookup, and RetrieveIndex to protect the session Store map and per-session caches.
• Added an internal concurrency test intended to reproduce the prior race under -race and validate the fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
workflow/checkpoint/manager.go	Adds a sync.Mutex to inMemoryManager and locks around Store/cache access in key methods.
workflow/checkpoint/manager_race_internal_test.go	Adds a concurrent multi-session test to validate race-free behavior under -race.

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