api: fix scheduler deadlocks

api/api.go

@@ -66,7 +66,7 @@ func New(deps Dependencies) *API {
 	if err := api.scheduler.Sync(context.Background(), api.db); err != nil {
 		panic(err)
 	}
-	api.scheduler.Start()
+	go api.scheduler.Start()
 
 	go api.StartSubredditDownloadPubsub(ch)
 	return api

api/scheduler/delete.go

@@ -4,10 +4,16 @@ package scheduler
 //
 // If job does not exist, it will be a no-op.
 func (scheduler *Scheduler) Delete(subreddit string) {
-	scheduler.mu.Lock()
-	defer scheduler.mu.Unlock()
+	scheduler.delete(subreddit, true)
+}
 
-	job := scheduler.Get(subreddit)
+func (scheduler *Scheduler) delete(subreddit string, lock bool) {
+	if lock {
+		scheduler.mu.Lock()
+		defer scheduler.mu.Unlock()
+	}
+
+	job := scheduler.get(subreddit, false)
 	if job != nil {
 		scheduler.scheduler.Remove(job.ID)
 	}

api/scheduler/get.go

@@ -2,11 +2,17 @@ package scheduler
 
 // List returns a copy of the list of jobs.
 func (scheduler *Scheduler) List() map[string]*Job {
-	scheduler.mu.RLock()
-	defer scheduler.mu.RUnlock()
+	return scheduler.list(true)
+}
 
-	m := make(map[string]*Job, len(scheduler.list))
-	for k, v := range scheduler.list {
+func (scheduler *Scheduler) list(lock bool) map[string]*Job {
+	if lock {
+		scheduler.mu.RLock()
+		defer scheduler.mu.RUnlock()
+	}
+
+	m := make(map[string]*Job, len(scheduler.entries))
+	for k, v := range scheduler.entries {
 		m[k] = v.clone()
 	}
 
@@ -17,10 +23,16 @@ func (scheduler *Scheduler) List() map[string]*Job {
 //
 // Returns nil if the subreddit is not found or active.
 func (scheduler *Scheduler) Get(subreddit string) *Job {
-	scheduler.mu.RLock()
-	defer scheduler.mu.RUnlock()
+	return scheduler.get(subreddit, true)
+}
 
-	schedule := scheduler.list[subreddit]
+func (scheduler *Scheduler) get(subreddit string, lock bool) *Job {
+	if lock {
+		scheduler.mu.RLock()
+		defer scheduler.mu.RUnlock()
+	}
+
+	schedule := scheduler.entries[subreddit]
 	if schedule != nil {
 		return schedule.clone()
 	}
@@ -28,10 +40,16 @@ func (scheduler *Scheduler) Get(subreddit string) *Job {
 }
 
 func (scheduler *Scheduler) Iter(f func(string, *Job) bool) {
-	scheduler.mu.RLock()
-	defer scheduler.mu.RUnlock()
+	scheduler.iter(f, true)
+}
 
-	for k, v := range scheduler.list {
+func (scheduler *Scheduler) iter(f func(string, *Job) bool, lock bool) {
+	if lock {
+		scheduler.mu.RLock()
+		defer scheduler.mu.RUnlock()
+	}
+
+	for k, v := range scheduler.entries {
 		if !f(k, v.clone()) {
 			break
 		}

api/scheduler/put.go

@@ -11,6 +11,10 @@ import (
 //
 // If job already exists, it will be replaced.
 func (scheduler *Scheduler) Put(subreddit string, schedule string) (job *Job, err error) {
+	return scheduler.put(subreddit, schedule, true)
+}
+
+func (scheduler *Scheduler) put(subreddit string, schedule string, lock bool) (job *Job, err error) {
 	sched, err := cron.ParseStandard(schedule)
 	if err != nil {
 		return nil, errs.
@@ -18,17 +22,19 @@ func (scheduler *Scheduler) Put(subreddit string, schedule string) (job *Job, er
 			Code(http.StatusBadRequest)
 	}
 
-	scheduler.Delete(subreddit)
+	scheduler.delete(subreddit, false)
 
 	id := scheduler.scheduler.Schedule(sched, cron.FuncJob(func() { scheduler.run(subreddit) }))
 
 	e := scheduler.scheduler.Entry(id)
 	job = &Job{ID: id, Entry: e}
 
-	scheduler.mu.Lock()
-	defer scheduler.mu.Unlock()
+	if lock {
+		scheduler.mu.Lock()
+		defer scheduler.mu.Unlock()
+	}
 
-	scheduler.list[subreddit] = job
+	scheduler.entries[subreddit] = job
 
 	return job, nil
 }

api/scheduler/rebalance.go

@@ -33,7 +33,7 @@ func (scheduler *Scheduler) Sync(ctx context.Context, db bob.Executor) (err erro
 	var errcoll error
 
 	for _, sub := range subs {
-		_, err := scheduler.Put(sub.Name, sub.Schedule)
+		_, err := scheduler.put(sub.Name, sub.Schedule, false)
 		if err != nil {
 			errcoll = errors.Join(errcoll, errs.Wrapw(err, "scheduler: rebalance: failed to add job", "subreddit", sub.Name, "schedule", sub.Schedule))
 		}

api/scheduler/scheduler.go

@@ -11,7 +11,7 @@ type Runner = func(subreddit string)
 type Scheduler struct {
 	scheduler *cron.Cron
 	mu        sync.RWMutex
-	list      map[string]*Job
+	entries   map[string]*Job
 	run       Runner
 }
 
@@ -30,7 +30,7 @@ func (job *Job) clone() *Job {
 func New(runner Runner) *Scheduler {
 	return &Scheduler{
 		scheduler: cron.New(),
-		list:      make(map[string]*Job),
+		entries:   make(map[string]*Job),
 		run:       runner,
 	}
 }