refactor: use single poller with semaphore-based capacity control

Previously, capacity=N spawned N independent polling goroutines, each
making FetchTask RPCs to the Gitea server concurrently. This caused
unnecessary connection load on the server proportional to the runner's
capacity setting.

Replace the N-goroutine model with a single polling loop that uses a
buffered channel as a semaphore to control concurrent task execution.
The poller acquires a capacity slot before fetching; when at capacity,
it blocks without issuing RPCs. Fetched tasks are dispatched to
independent goroutines that release their slot on completion.

Also fix a pre-existing bug in Shutdown() where the timeout branch
used a blocking receive on p.done instead of a non-blocking select,
which prevented shutdownJobs() from ever being called on timeout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/app/poll/poller.go

@@ -37,16 +37,15 @@ type Poller struct {
 	done chan struct{}
 }
 
-// workerState holds per-goroutine polling state. Backoff counters are
+// workerState holds the single poller's backoff state. Consecutive empty or
-// per-worker so that with Capacity > 1, N workers each seeing one empty
+// error responses drive exponential backoff; a successful task fetch resets
-// response don't combine into a "consecutive N empty" reading on a shared
+// both counters so the next poll fires immediately.
-// counter and trigger an unnecessarily long backoff.
 type workerState struct {
 	consecutiveEmpty  int64
 	consecutiveErrors int64
-	// lastBackoff is the last interval reported to the PollBackoffSeconds gauge
+	// lastBackoff is the last interval reported to the PollBackoffSeconds gauge;
-	// from this worker; used to suppress redundant no-op Set calls when the
+	// used to suppress redundant no-op Set calls when the backoff plateaus
-	// backoff plateaus (e.g. at FetchIntervalMax).
+	// (e.g. at FetchIntervalMax).
 	lastBackoff time.Duration
 }
 
@@ -73,22 +72,57 @@ func New(cfg *config.Config, client client.Client, runner *run.Runner) *Poller {
 }
 
 func (p *Poller) Poll() {
+	sem := make(chan struct{}, p.cfg.Runner.Capacity)
 	wg := &sync.WaitGroup{}
-	for i := 0; i < p.cfg.Runner.Capacity; i++ {
+	s := &workerState{}
-		wg.Add(1)
-		go p.poll(wg)
-	}
-	wg.Wait()
 
-	// signal that we shutdown
+	defer func() {
-	close(p.done)
+		wg.Wait()
+		close(p.done)
+	}()
+
+	for {
+		select {
+		case sem <- struct{}{}:
+		case <-p.pollingCtx.Done():
+			return
+		}
+
+		task, ok := p.fetchTask(p.pollingCtx, s)
+		if !ok {
+			<-sem
+			if !p.waitBackoff(s) {
+				return
+			}
+			continue
+		}
+
+		s.resetBackoff()
+
+		wg.Add(1)
+		go func(t *runnerv1.Task) {
+			defer wg.Done()
+			defer func() { <-sem }()
+			p.runTaskWithRecover(p.jobsCtx, t)
+		}(task)
+	}
 }
 
 func (p *Poller) PollOnce() {
-	p.pollOnce(&workerState{})
+	defer close(p.done)
-
+	s := &workerState{}
-	// signal that we're done
+	for {
-	close(p.done)
+		task, ok := p.fetchTask(p.pollingCtx, s)
+		if !ok {
+			if !p.waitBackoff(s) {
+				return
+			}
+			continue
+		}
+		s.resetBackoff()
+		p.runTaskWithRecover(p.jobsCtx, task)
+		return
+	}
 }
 
 func (p *Poller) Shutdown(ctx context.Context) error {
@@ -101,13 +135,13 @@ func (p *Poller) Shutdown(ctx context.Context) error {
 
 	// our timeout for shutting down ran out
 	case <-ctx.Done():
-		// when both the timeout fires and the graceful shutdown
+		// Both the timeout and the graceful shutdown may fire
-		// completed succsfully, this branch of the select may
+		// simultaneously. Do a non-blocking check to avoid forcing
-		// fire. Do a non-blocking check here against the graceful
+		// a shutdown when graceful already completed.
-		// shutdown status to avoid sending an error if we don't need to.
+		select {
-		_, ok := <-p.done
+		case <-p.done:
-		if !ok {
 			return nil
+		default:
 		}
 
 		// force a shutdown of all running jobs
@@ -120,18 +154,27 @@ func (p *Poller) Shutdown(ctx context.Context) error {
 	}
 }
 
-func (p *Poller) poll(wg *sync.WaitGroup) {
+func (s *workerState) resetBackoff() {
-	defer wg.Done()
+	s.consecutiveEmpty = 0
-	s := &workerState{}
+	s.consecutiveErrors = 0
-	for {
+	s.lastBackoff = 0
-		p.pollOnce(s)
+}
 
-		select {
+// waitBackoff sleeps for the current backoff interval (with jitter).
-		case <-p.pollingCtx.Done():
+// Returns false if the polling context was cancelled during the wait.
-			return
+func (p *Poller) waitBackoff(s *workerState) bool {
-		default:
+	base := p.calculateInterval(s)
-			continue
+	if base != s.lastBackoff {
-		}
+		metrics.PollBackoffSeconds.Set(base.Seconds())
+		s.lastBackoff = base
+	}
+	timer := time.NewTimer(addJitter(base))
+	select {
+	case <-timer.C:
+		return true
+	case <-p.pollingCtx.Done():
+		timer.Stop()
+		return false
 	}
 }
 
@@ -167,34 +210,6 @@ func addJitter(d time.Duration) time.Duration {
 	return d + time.Duration(jitter)
 }
 
-func (p *Poller) pollOnce(s *workerState) {
-	for {
-		task, ok := p.fetchTask(p.pollingCtx, s)
-		if !ok {
-			base := p.calculateInterval(s)
-			if base != s.lastBackoff {
-				metrics.PollBackoffSeconds.Set(base.Seconds())
-				s.lastBackoff = base
-			}
-			timer := time.NewTimer(addJitter(base))
-			select {
-			case <-timer.C:
-			case <-p.pollingCtx.Done():
-				timer.Stop()
-				return
-			}
-			continue
-		}
-
-		// Got a task — reset backoff counters for fast subsequent polling.
-		s.consecutiveEmpty = 0
-		s.consecutiveErrors = 0
-
-		p.runTaskWithRecover(p.jobsCtx, task)
-		return
-	}
-}
-
 func (p *Poller) runTaskWithRecover(ctx context.Context, task *runnerv1.Task) {
 	defer func() {
 		if r := recover(); r != nil {

internal/app/poll/poller_test.go

@@ -19,11 +19,10 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// TestPoller_PerWorkerCounters verifies that each worker maintains its own
+// TestPoller_WorkerStateCounters verifies that workerState correctly tracks
-// backoff counters. With a shared counter, N workers each seeing one empty
+// consecutive empty responses independently per state instance, and that
-// response would inflate the counter to N and trigger an unnecessarily long
+// fetchTask increments only the relevant counter.
-// backoff. With per-worker state, each worker only sees its own count.
+func TestPoller_WorkerStateCounters(t *testing.T) {
-func TestPoller_PerWorkerCounters(t *testing.T) {
 	client := mocks.NewClient(t)
 	client.On("FetchTask", mock.Anything, mock.Anything).Return(
 		func(_ context.Context, _ *connect_go.Request[runnerv1.FetchTaskRequest]) (*connect_go.Response[runnerv1.FetchTaskResponse], error) {
@@ -77,8 +76,8 @@ func TestPoller_FetchErrorIncrementsErrorsOnly(t *testing.T) {
 	assert.Equal(t, int64(0), s.consecutiveEmpty)
 }
 
-// TestPoller_CalculateInterval verifies the per-worker exponential backoff
+// TestPoller_CalculateInterval verifies the exponential backoff math is
-// math is correctly driven by the worker's own counters.
+// correctly driven by the workerState counters.
 func TestPoller_CalculateInterval(t *testing.T) {
 	cfg, err := config.LoadDefault("")
 	require.NoError(t, err)

internal/pkg/metrics/metrics.go

@@ -89,7 +89,7 @@ var (
 		Namespace: Namespace,
 		Subsystem: "poll",
 		Name:      "backoff_seconds",
-		Help:      "Last observed polling backoff interval. With Capacity > 1, reflects whichever worker wrote last.",
+		Help:      "Last observed polling backoff interval in seconds.",
 	})
 
 	JobsTotal = prometheus.NewCounterVec(prometheus.CounterOpts{