feat(scheduler): add admission cost, dedup, and RunSync cancellation

Add per-job admission cost floor to prevent volume-based DoS by charging
a minimum fairness cost regardless of actual job duration.

Deduplicate jobs by (type, id): Submit silently drops duplicates,
RunSync coalesces callers onto the existing job. Sync and async jobs
can be mixed on the same key. When all RunSync waiters cancel and no
Submit owns the job, the job's context is cancelled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alecthomas

internal/scheduler/README.md

@@ -20,7 +20,9 @@ All work — foreground and background — goes through a single scheduler. The
 
 **Cost**: A `time.Duration` representing the relative system impact of a job. The scheduler automatically learns the cost of each `(job_type, job_id)` pair using an exponential moving average of observed execution time. On first encounter, a small default (1 second) is used. Callers never specify cost explicitly.
 
-**Accumulated cost**: A running total of cost consumed per fairness key. Every time a job is admitted, its estimated cost is added to its fairness key's accumulated cost. The scheduler always picks the job whose fairness key has the lowest accumulated cost — whoever has consumed the least goes next.
+**Admission cost**: A configurable minimum fairness cost charged per job admission (`Config.AdmissionCost`, default 5 seconds). At admission time, the fairness charge is `max(estimated_cost, admission_cost)`. This prevents a client from monopolising the scheduler by submitting many cheap jobs — even a job that completes in 100ms still costs 5 seconds of fairness budget, so mass-submission is expensive in fairness terms without affecting actual execution time.
+
+**Accumulated cost**: A running total of cost consumed per fairness key. Every time a job is admitted, the admission cost (the max of estimated and configured minimum) is added to its fairness key's accumulated cost. The scheduler always picks the job whose fairness key has the lowest accumulated cost — whoever has consumed the least goes next.
 
 **Fairness key**: An opaque string on the job, populated by the caller. For foreground jobs, this is typically the client IP or identity. For background jobs, this is empty. The scheduler doesn't know what it represents — it just uses it for ordering.
 
@@ -159,6 +161,17 @@ scheduler.Submit(
 
 Both enter the same pending queue and the same admission logic. `RunSync` blocks on a completion signal before returning to the caller.
 
+### Deduplication
+
+Jobs are deduplicated by `(job_type, job_id)`. If a job with the same key is already pending or running:
+
+- **`Submit`**: the call is silently dropped. The existing job's `fn` will be used.
+- **`RunSync`**: the caller coalesces onto the existing job and receives its result when it completes. Multiple callers can wait on the same job simultaneously.
+
+Sync and async jobs can be mixed for the same `(job_type, job_id)`. A `RunSync` onto an active `Submit` job attaches a waiter. A `Submit` onto an active `RunSync` job marks it as submitted so it survives waiter cancellation.
+
+This means if ten clients request the same clone concurrently, only one clone runs — all ten callers receive the result. If a coalesced `RunSync` caller's context is cancelled, that caller is detached but the job continues for the remaining waiters. When **all** waiters cancel and no `Submit` created the job, the job itself is cancelled via its context — both pending and running jobs are stopped.
+
 ## Dispatch Algorithm
 
 The entire scheduling algorithm:
@@ -174,7 +187,7 @@ for each job in sorted order:
     if type_running_count >= type_slots → skip
     if any running job has same job_id AND same non-empty conflict_group → skip
     admit job
-    estimated_cost = cost_estimates[(job_type, job_id)] or 1s
+    estimated_cost = max(cost_estimates[(job_type, job_id)] or 1s, config.admission_cost)
     accumulated_cost[fairness_key] += estimated_cost
 ```
 
@@ -194,6 +207,7 @@ Key properties of this algorithm:
 - **Per-type limits**: within a tier, individual job types can be capped to a fraction of the tier's allocation, preventing expensive operations from monopolising the tier.
 - **Fairness**: within a priority level, jobs from the fairness key with the lowest accumulated cost go first. A client that has consumed a lot of capacity yields to one that has consumed little.
 - **Cost-awareness**: expensive jobs advance accumulated cost faster, so they naturally yield to cheaper work from other clients. A `linux.git` clone that takes 60 seconds advances the client's accumulated cost by ~60s, while a `git.git` clone that takes 5 seconds advances it by ~5s.
+- **Volume-awareness**: the admission cost floor ensures that submitting many cheap jobs is still expensive in fairness terms. A client submitting 1000 trivial jobs accumulates at least `1000 * admission_cost`, causing it to yield to clients that have submitted fewer jobs.
 - **Adaptive**: the scheduler automatically learns the cost of each `(job_type, job_id)` pair. No manual cost tuning required. After one execution, estimates are already meaningful.
 - **Conflict safety**: conflicting jobs on the same resource stay in the pending queue, not consuming concurrency slots while they wait.
 - **No head-of-line blocking**: if the next job by ordering is blocked (conflict or concurrency limit), the scheduler skips it and admits the next admissible job.

internal/scheduler/scheduler.go

@@ -75,6 +75,15 @@ type Config struct {
 	CostTTL          time.Duration `hcl:"cost-ttl,optional" help:"TTL for cost estimate entries." default:"1h"`
 	FairnessTTL      time.Duration `hcl:"fairness-ttl,optional" help:"TTL for accumulated cost entries." default:"10m"`
 	CleanupInterval  time.Duration `hcl:"cleanup-interval,optional" help:"How often to run TTL cleanup." default:"1m"`
+	// AdmissionCost is the minimum fairness cost charged per job admission, regardless of the
+	// job's estimated duration. This prevents a client from monopolising the scheduler by
+	// submitting many cheap jobs.
+	AdmissionCost time.Duration `hcl:"admission-cost,optional" help:"Minimum fairness cost per job admission." default:"5s"`
+}
+
+type dedupKey struct {
+	jobType JobType
+	jobID   string
 }
 
 type job struct {
@@ -83,7 +92,10 @@ type job struct {
 	fairnessKey string
 	fn          func(ctx context.Context) error
 	arrivalTime time.Time
-	done        chan error // non-nil for RunSync
+	submitted   bool               // true if created or referenced by Submit
+	waiters     []chan error       // RunSync callers waiting for the result
+	ctx         context.Context    // context passed to fn; cancellable for RunSync jobs
+	cancel      context.CancelFunc // non-nil for RunSync jobs
 }
 
 func (j *job) String() string { return j.jobType.String() + ":" + j.jobID }
@@ -115,6 +127,7 @@ type Scheduler struct {
 	types      map[JobType]JobTypeConfig
 	pending    []*job
 	running    []*runningJob
+	active     map[dedupKey]*job // all pending + running jobs for dedup
 	fairness   map[string]*fairnessEntry
 	costs      map[costKey]*costEntry
 	config     Config
@@ -142,6 +155,7 @@ func New(ctx context.Context, config Config, ns *metadatadb.Namespace) (*Schedul
 	s := &Scheduler{
 		priorities:    make(map[Priority]bool),
 		types:         make(map[JobType]JobTypeConfig),
+		active:        make(map[dedupKey]*job),
 		fairness:      make(map[string]*fairnessEntry),
 		costs:         make(map[costKey]*costEntry),
 		lastRunsLocal: make(map[string]time.Time),
@@ -199,55 +213,73 @@ func (s *Scheduler) validateType(jt JobType) {
 }
 
 // Submit queues a background job for async execution. Returns immediately.
+// If a job with the same (type, id) is already pending or running, the
+// submission is silently deduplicated.
 func (s *Scheduler) Submit(jobType JobType, jobID string, fn func(ctx context.Context) error) {
 	s.mu.Lock()
 	s.validateType(jobType)
-	s.pending = append(s.pending, &job{
+	key := dedupKey{jobType, jobID}
+	if existing, ok := s.active[key]; ok {
+		existing.submitted = true
+		s.mu.Unlock()
+		return
+	}
+	j := &job{
 		jobType:     jobType,
 		jobID:       jobID,
 		fn:          fn,
 		arrivalTime: s.now(),
-	})
+		submitted:   true,
+	}
+	s.active[key] = j
+	s.pending = append(s.pending, j)
 	s.mu.Unlock()
 	s.signal()
 }
 
 // RunSync submits a foreground job and blocks until it completes or ctx is
-// cancelled. The fn receives a context that is cancelled when either the
-// caller's ctx or the scheduler's context is done.
+// cancelled. If a job with the same (type, id) is already pending or running,
+// the caller coalesces onto the existing job and receives its result. When all
+// coalesced callers cancel, the job itself is cancelled.
 func (s *Scheduler) RunSync(ctx context.Context, jobType JobType, jobID, fairnessKey string, fn func(ctx context.Context) error) error {
-	jobCtx, jobCancel := context.WithCancel(ctx)
-	stop := context.AfterFunc(s.ctx, jobCancel)
+	done := make(chan error, 1)
 
 	s.mu.Lock()
 	s.validateType(jobType)
-	s.mu.Unlock()
-
-	done := make(chan error, 1)
+	key := dedupKey{jobType, jobID}
+	if existing, ok := s.active[key]; ok {
+		existing.waiters = append(existing.waiters, done)
+		s.mu.Unlock()
+		return s.awaitDone(ctx, existing, done)
+	}
+	jobCtx, jobCancel := context.WithCancel(s.ctx)
 	j := &job{
 		jobType:     jobType,
 		jobID:       jobID,
 		fairnessKey: fairnessKey,
-		fn:          func(_ context.Context) error { return fn(jobCtx) },
+		fn:          fn,
 		arrivalTime: s.now(),
-		done:        done,
+		waiters:     []chan error{done},
+		ctx:         jobCtx,
+		cancel:      jobCancel,
 	}
-	s.mu.Lock()
+	s.active[key] = j
 	s.pending = append(s.pending, j)
 	s.mu.Unlock()
 	s.signal()
 
+	return s.awaitDone(ctx, j, done)
+}
+
+func (s *Scheduler) awaitDone(ctx context.Context, j *job, done chan error) error {
 	select {
 	case err := <-done:
-		stop()
-		jobCancel()
 		return err
 	case <-ctx.Done():
 		s.mu.Lock()
-		s.removePendingLocked(j)
+		s.removeWaiterLocked(j, done)
+		s.maybeRemoveJobLocked(j)
 		s.mu.Unlock()
-		stop()
-		jobCancel()
 		return errors.WithStack(ctx.Err())
 	}
 }
@@ -383,7 +415,11 @@ func (s *Scheduler) tierSlotsLocked(weight float64) int {
 func (s *Scheduler) executeJob(j *job) {
 	start := s.now()
 	s.logger.InfoContext(s.ctx, "Starting job", "job", j)
-	err := j.fn(s.ctx)
+	fnCtx := s.ctx
+	if j.ctx != nil {
+		fnCtx = j.ctx
+	}
+	err := j.fn(fnCtx)
 	elapsed := s.now().Sub(start)
 
 	if err != nil {
@@ -395,11 +431,14 @@ func (s *Scheduler) executeJob(j *job) {
 	s.mu.Lock()
 	s.updateCostEstimateLocked(j.jobType, j.jobID, elapsed)
 	s.removeFromRunningLocked(j)
+	delete(s.active, dedupKey{j.jobType, j.jobID})
+	waiters := j.waiters
+	j.waiters = nil
 	s.recordMetricsLocked()
 	s.mu.Unlock()
 
-	if j.done != nil {
-		j.done <- err
+	for _, w := range waiters {
+		w <- err
 	}
 	s.signal()
 }
@@ -450,10 +489,11 @@ func (s *Scheduler) hasConflictLocked(j *job) bool {
 const defaultCost = time.Second
 
 func (s *Scheduler) estimatedCostLocked(j *job) time.Duration {
+	est := defaultCost
 	if entry, ok := s.costs[costKey{j.jobType, j.jobID}]; ok {
-		return entry.estimate
+		est = entry.estimate
 	}
-	return defaultCost
+	return max(est, s.config.AdmissionCost)
 }
 
 func (s *Scheduler) updateCostEstimateLocked(jt JobType, jobID string, elapsed time.Duration) {
@@ -530,6 +570,37 @@ func (s *Scheduler) removePendingLocked(j *job) {
 	s.pending = slices.DeleteFunc(s.pending, func(pj *job) bool { return pj == j })
 }
 
+func (s *Scheduler) removeWaiterLocked(j *job, done chan error) {
+	j.waiters = slices.DeleteFunc(j.waiters, func(w chan error) bool { return w == done })
+}
+
+func (s *Scheduler) isRunningLocked(j *job) bool {
+	for _, rj := range s.running {
+		if rj.job == j {
+			return true
+		}
+	}
+	return false
+}
+
+// maybeRemoveJobLocked is called when a RunSync waiter is removed. If no
+// waiters remain it cancels the job's context. Pending jobs are also removed
+// from the queue and active map; running jobs are left for executeJob to
+// clean up after the fn returns.
+func (s *Scheduler) maybeRemoveJobLocked(j *job) {
+	if len(j.waiters) > 0 || j.submitted {
+		return
+	}
+	if j.cancel != nil {
+		j.cancel()
+	}
+	if s.isRunningLocked(j) {
+		return
+	}
+	s.removePendingLocked(j)
+	delete(s.active, dedupKey{j.jobType, j.jobID})
+}
+
 // --- TTL cleanup ---
 
 func (s *Scheduler) cleanupLoop() {