dagger/dagger · error

missing engine locker

Error message

missing engine locker

What it means

After obtaining the query, lockMountedCaches requires the engine's locker (query.Locker()) to serialize cache volume access. If the locker is nil the engine cannot grant cache locks and this error is returned, preventing unsafe concurrent cache writes.

Source

Thrown at core/container_exec.go:571

	ActiveRef       bkcache.MutableRef
	OutputMutable   bkcache.MutableRef
	OutputImmutable bkcache.ImmutableRef
}

type materializedExecPlan struct {
	Root   executor.Mount
	Mounts []executor.Mount
	States []*execMountState
}

func lockMountedCaches(ctx context.Context, mounts []ContainerMount) (func(), error) {
	query, err := CurrentQuery(ctx)
	if err != nil {
		return nil, fmt.Errorf("get current query for cache locks: %w", err)
	}
	locker := query.Locker()
	if locker == nil {
		return nil, fmt.Errorf("missing engine locker")
	}

	lockSet := make(map[string]struct{})
	for i, ctrMount := range mounts {
		if ctrMount.Readonly || ctrMount.CacheSource == nil || ctrMount.CacheSource.Volume.Self() == nil {
			continue
		}
		cacheSelf := ctrMount.CacheSource.Volume.Self()
		if !cacheSharingModeLocksWrites(cacheSelf.Sharing) {
			continue
		}
		lockKey, err := cacheSelf.lockKey()
		if err != nil {
			return nil, fmt.Errorf("cache lock key for mount %d: %w", i, err)
		}
		lockSet[lockKey] = struct{}{}
	}
	if len(lockSet) == 0 {

View on GitHub (pinned to 82ba2681db)

Solutions

1. Run through a fully initialized dagger engine session where the locker is configured
2. Check engine initialization code so Locker() is set on the query
3. If the mounts don't need write-locking semantics, mark them readonly or remove the cache mount

Example fix

// before
query without locker -> startContainer with cache share SHARED
// after
initialize engine locker before exec, or set cache sharing to PRIVATE/LOCKED-free readonly mount

if q.Locker() == nil { return errors.New("engine locker unavailable") }

func lockerReady(q *Query) bool { return q.Locker() != nil }

When it happens

Trigger: Executing a container with writable cache mounts while the engine session has no locker configured (locker-less engine or misinitialized query).

Common situations: Running core exec code outside a fully initialized engine, or engine configurations that skip locker setup (e.g. certain embedded/test setups).

Related errors

• get current query for cache locks: %w
• cache lock key for mount %d: %w
• cache pragma %q, must be a valid string
• get parent name: %w
• get fn name: %w

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/1b7d8ecf175b0ab6. Report an issue: GitHub.