[Bug] eager backend: guard groups/tasks maps with a single mutex

[madirey]

Summary

• The v1 eager backend's groups and tasks maps were read without holding any lock — updateState was the only place that acquired stateMutex, and only while writing tasks. Concurrent callers can trip Go's "concurrent map read and map write" fatal.
• Replaces stateMutex with a single sync.Mutex (mu) that guards both maps across every public method, and introduces getStateLocked so GroupCompleted/GroupTaskStates can re-read tasks without re-acquiring the lock.
• A coarse single mutex is intentional: the eager backend is for tests and local development, contention is low, and one lock keeps the "single caller at a time touches internal state" invariant easy to reason about.

This is the same root cause reported upstream as RichardKnop/machinery#737 (PR #740). Upstream's fix uses two sync.RWMutex (one per map). Ours is simpler and also catches the v1 PurgeGroupMeta case that upstream PR #740 missed.

v2 status

The same bug exists in v2/backends/eager/eager.go but is not addressed here. The v2 module has a pre-existing build break (v2/config/config.go:65 references MongoDBConfig, which was deleted in commit 8617130 "Drop mongo support" #23 and never replaced). CI's go list ./... runs from the root module and doesn't descend into v2, so v2 is effectively unmaintained dead code in this fork. The v2 race-fix would need to come with a v2 build fix; leaving that as a separate decision.

How did you validate this change

Added TestEagerBackend_ConcurrentAccess (v1/backends/eager/eager_test.go) — spawns 50 goroutines x 200 iterations and exercises every public surface: SetState{Pending,Received,Started,Success}, GetState, InitGroup, GroupCompleted, GroupTaskStates, PurgeState, PurgeGroupMeta. Without the mutex changes this test deterministically trips go test -race; with them it passes cleanly.

• Assisted by Claude 🤖

[Copilot]

Pull request overview

This PR makes the v1 eager in-memory backend safe for concurrent use by serializing access to its task and group metadata maps.

Changes:

• Replaces the previous task-only mutex with a single mutex guarding both groups and tasks.
• Avoids recursive locking by adding getStateLocked for callers that already hold the mutex.
• Adds a concurrent access regression test covering the eager backend’s map read/write paths.

Reviewed changes

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

File	Description
v1/backends/eager/eager.go	Adds mutex protection around eager backend group/task map access.
v1/backends/eager/eager_test.go	Adds a concurrency regression test for eager backend public operations.

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