test(poll): add concurrency test for single-poller capacity control

- Introduce TaskRunner interface to decouple Poller from concrete run.Runner
- Add TestPoller_ConcurrencyLimitedByCapacity verifying max concurrent
  tasks respects capacity and FetchTask is never called concurrently
- Mock runner respects context cancellation for proper shutdown testing

internal/app/poll/poller.go

@@ -16,15 +16,20 @@ import (
 	"connectrpc.com/connect"
 	log "github.com/sirupsen/logrus"
 
-	"gitea.com/gitea/act_runner/internal/app/run"
 	"gitea.com/gitea/act_runner/internal/pkg/client"
 	"gitea.com/gitea/act_runner/internal/pkg/config"
 	"gitea.com/gitea/act_runner/internal/pkg/metrics"
 )
 
+// TaskRunner abstracts task execution so the poller can be tested
+// without a real runner.
+type TaskRunner interface {
+	Run(ctx context.Context, task *runnerv1.Task) error
+}
+
 type Poller struct {
 	client       client.Client
-	runner       *run.Runner
+	runner       TaskRunner
 	cfg          *config.Config
 	tasksVersion atomic.Int64 // tasksVersion used to store the version of the last task fetched from the Gitea.
 
@@ -37,20 +42,19 @@ type Poller struct {
 	done chan struct{}
 }
 
-// workerState holds per-goroutine polling state. Backoff counters are
-// per-worker so that with Capacity > 1, N workers each seeing one empty
-// response don't combine into a "consecutive N empty" reading on a shared
-// counter and trigger an unnecessarily long backoff.
+// workerState holds the single poller's backoff state. Consecutive empty or
+// error responses drive exponential backoff; a successful task fetch resets
+// both counters so the next poll fires immediately.
 type workerState struct {
 	consecutiveEmpty  int64
 	consecutiveErrors int64
-	// lastBackoff is the last interval reported to the PollBackoffSeconds gauge
-	// from this worker; used to suppress redundant no-op Set calls when the
-	// backoff plateaus (e.g. at FetchIntervalMax).
+	// lastBackoff is the last interval reported to the PollBackoffSeconds gauge;
+	// used to suppress redundant no-op Set calls when the backoff plateaus
+	// (e.g. at FetchIntervalMax).
 	lastBackoff time.Duration
 }
 
-func New(cfg *config.Config, client client.Client, runner *run.Runner) *Poller {
+func New(cfg *config.Config, client client.Client, runner TaskRunner) *Poller {
 	pollingCtx, shutdownPolling := context.WithCancel(context.Background())
 
 	jobsCtx, shutdownJobs := context.WithCancel(context.Background())
@@ -73,22 +77,57 @@ 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++ {
-		wg.Add(1)
-		go p.poll(wg)
-	}
-	wg.Wait()
+	s := &workerState{}
 
-	// signal that we shutdown
-	close(p.done)
+	defer func() {
+		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{})
-
-	// signal that we're done
-	close(p.done)
+	defer close(p.done)
+	s := &workerState{}
+	for {
+		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 +140,13 @@ 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
-		// completed succsfully, this branch of the select may
-		// fire. Do a non-blocking check here against the graceful
-		// shutdown status to avoid sending an error if we don't need to.
-		_, ok := <-p.done
-		if !ok {
+		// Both the timeout and the graceful shutdown may fire
+		// simultaneously. Do a non-blocking check to avoid forcing
+		// a shutdown when graceful already completed.
+		select {
+		case <-p.done:
 			return nil
+		default:
 		}
 
 		// force a shutdown of all running jobs
@@ -120,18 +159,27 @@ func (p *Poller) Shutdown(ctx context.Context) error {
 	}
 }
 
-func (p *Poller) poll(wg *sync.WaitGroup) {
-	defer wg.Done()
-	s := &workerState{}
-	for {
-		p.pollOnce(s)
+func (s *workerState) resetBackoff() {
+	s.consecutiveEmpty = 0
+	s.consecutiveErrors = 0
+	s.lastBackoff = 0
+}
 
-		select {
-		case <-p.pollingCtx.Done():
-			return
-		default:
-			continue
-		}
+// waitBackoff sleeps for the current backoff interval (with jitter).
+// Returns false if the polling context was cancelled during the wait.
+func (p *Poller) waitBackoff(s *workerState) bool {
+	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:
+		return true
+	case <-p.pollingCtx.Done():
+		timer.Stop()
+		return false
 	}
 }
 
@@ -167,34 +215,6 @@ 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

@@ -6,6 +6,8 @@ package poll
 import (
 	"context"
 	"errors"
+	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -19,11 +21,10 @@ import (
 	"gitea.com/gitea/act_runner/internal/pkg/config"
 )
 
-// TestPoller_PerWorkerCounters verifies that each worker maintains its own
-// backoff counters. With a shared counter, N workers each seeing one empty
-// response would inflate the counter to N and trigger an unnecessarily long
-// backoff. With per-worker state, each worker only sees its own count.
-func TestPoller_PerWorkerCounters(t *testing.T) {
+// TestPoller_WorkerStateCounters verifies that workerState correctly tracks
+// consecutive empty responses independently per state instance, and that
+// fetchTask increments only the relevant counter.
+func TestPoller_WorkerStateCounters(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 +78,8 @@ func TestPoller_FetchErrorIncrementsErrorsOnly(t *testing.T) {
 	assert.Equal(t, int64(0), s.consecutiveEmpty)
 }
 
-// TestPoller_CalculateInterval verifies the per-worker exponential backoff
-// math is correctly driven by the worker's own counters.
+// TestPoller_CalculateInterval verifies the exponential backoff math is
+// correctly driven by the workerState counters.
 func TestPoller_CalculateInterval(t *testing.T) {
 	cfg, err := config.LoadDefault("")
 	require.NoError(t, err)
@@ -106,3 +107,96 @@ func TestPoller_CalculateInterval(t *testing.T) {
 		})
 	}
 }
+
+// atomicMax atomically updates target to max(target, val).
+func atomicMax(target *atomic.Int64, val int64) {
+	for {
+		old := target.Load()
+		if val <= old || target.CompareAndSwap(old, val) {
+			break
+		}
+	}
+}
+
+type mockRunner struct {
+	delay          time.Duration
+	running        atomic.Int64
+	maxConcurrent  atomic.Int64
+	totalCompleted atomic.Int64
+}
+
+func (m *mockRunner) Run(ctx context.Context, _ *runnerv1.Task) error {
+	atomicMax(&m.maxConcurrent, m.running.Add(1))
+	select {
+	case <-time.After(m.delay):
+	case <-ctx.Done():
+	}
+	m.running.Add(-1)
+	m.totalCompleted.Add(1)
+	return nil
+}
+
+// TestPoller_ConcurrencyLimitedByCapacity verifies that with capacity=3 and
+// 6 available tasks, at most 3 tasks run concurrently, and FetchTask is
+// never called concurrently (single poller).
+func TestPoller_ConcurrencyLimitedByCapacity(t *testing.T) {
+	const (
+		capacity   = 3
+		totalTasks = 6
+		taskDelay  = 50 * time.Millisecond
+	)
+
+	var (
+		tasksReturned  atomic.Int64
+		fetchConcur    atomic.Int64
+		maxFetchConcur atomic.Int64
+	)
+
+	cli := mocks.NewClient(t)
+	cli.On("FetchTask", mock.Anything, mock.Anything).Return(
+		func(_ context.Context, _ *connect_go.Request[runnerv1.FetchTaskRequest]) (*connect_go.Response[runnerv1.FetchTaskResponse], error) {
+			atomicMax(&maxFetchConcur, fetchConcur.Add(1))
+			defer fetchConcur.Add(-1)
+
+			n := tasksReturned.Add(1)
+			if n <= totalTasks {
+				return connect_go.NewResponse(&runnerv1.FetchTaskResponse{
+					Task: &runnerv1.Task{Id: n},
+				}), nil
+			}
+			return connect_go.NewResponse(&runnerv1.FetchTaskResponse{}), nil
+		},
+	)
+
+	runner := &mockRunner{delay: taskDelay}
+
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	cfg.Runner.Capacity = capacity
+	cfg.Runner.FetchInterval = 10 * time.Millisecond
+	cfg.Runner.FetchIntervalMax = 10 * time.Millisecond
+
+	poller := New(cfg, cli, runner)
+
+	var wg sync.WaitGroup
+	wg.Go(poller.Poll)
+
+	require.Eventually(t, func() bool {
+		return runner.totalCompleted.Load() >= totalTasks
+	}, 2*time.Second, 10*time.Millisecond, "all tasks should complete")
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	err = poller.Shutdown(ctx)
+	require.NoError(t, err)
+	wg.Wait()
+
+	assert.LessOrEqual(t, runner.maxConcurrent.Load(), int64(capacity),
+		"concurrent running tasks must not exceed capacity")
+	assert.GreaterOrEqual(t, runner.maxConcurrent.Load(), int64(2),
+		"with 6 tasks and capacity 3, at least 2 should overlap")
+	assert.Equal(t, int64(1), maxFetchConcur.Load(),
+		"FetchTask must never be called concurrently (single poller)")
+	assert.Equal(t, int64(totalTasks), runner.totalCompleted.Load(),
+		"all tasks should have been executed")
+}

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{