refactor(poll): use per-worker backoff counters

- Introduce workerState holding consecutiveEmpty and consecutiveErrors
- Plumb workerState through pollOnce, fetchTask and calculateInterval
- Drop the shared atomic.Int64 counters from Poller

With Capacity > 1, the previous shared counters inflated whenever multiple
workers each saw a single empty response, triggering an unnecessarily long
backoff. Per-worker state keeps each goroutine's backoff independent.

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

internal/app/poll/poller_test.go

@@ -0,0 +1,108 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package poll
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	runnerv1 "code.gitea.io/actions-proto-go/runner/v1"
+	connect_go "connectrpc.com/connect"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+
+	"gitea.com/gitea/act_runner/internal/pkg/client/mocks"
+	"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) {
+	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) {
+			// Always return an empty response.
+			return connect_go.NewResponse(&runnerv1.FetchTaskResponse{}), nil
+		},
+	)
+
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	p := &Poller{client: client, cfg: cfg}
+
+	ctx := context.Background()
+	s1 := &workerState{}
+	s2 := &workerState{}
+
+	// Each worker independently observes one empty response.
+	_, ok := p.fetchTask(ctx, s1)
+	require.False(t, ok)
+	_, ok = p.fetchTask(ctx, s2)
+	require.False(t, ok)
+
+	assert.Equal(t, int64(1), s1.consecutiveEmpty, "worker 1 should only count its own empty response")
+	assert.Equal(t, int64(1), s2.consecutiveEmpty, "worker 2 should only count its own empty response")
+
+	// Worker 1 sees a second empty; worker 2 stays at 1.
+	_, ok = p.fetchTask(ctx, s1)
+	require.False(t, ok)
+	assert.Equal(t, int64(2), s1.consecutiveEmpty)
+	assert.Equal(t, int64(1), s2.consecutiveEmpty, "worker 2's counter must not be affected by worker 1's empty fetches")
+}
+
+// TestPoller_FetchErrorIncrementsErrorsOnly verifies that a fetch error
+// increments only the per-worker error counter, not the empty counter.
+func TestPoller_FetchErrorIncrementsErrorsOnly(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) {
+			return nil, errors.New("network unreachable")
+		},
+	)
+
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	p := &Poller{client: client, cfg: cfg}
+
+	s := &workerState{}
+	_, ok := p.fetchTask(context.Background(), s)
+	require.False(t, ok)
+	assert.Equal(t, int64(1), s.consecutiveErrors)
+	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.
+func TestPoller_CalculateInterval(t *testing.T) {
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	cfg.Runner.FetchInterval = 2 * time.Second
+	cfg.Runner.FetchIntervalMax = 60 * time.Second
+	p := &Poller{cfg: cfg}
+
+	cases := []struct {
+		name         string
+		empty, errs  int64
+		wantInterval time.Duration
+	}{
+		{"first poll, no backoff", 0, 0, 2 * time.Second},
+		{"single empty, still base", 1, 0, 2 * time.Second},
+		{"two empties, doubled", 2, 0, 4 * time.Second},
+		{"five empties, capped path", 5, 0, 32 * time.Second},
+		{"many empties, capped at max", 20, 0, 60 * time.Second},
+		{"errors drive backoff too", 0, 3, 8 * time.Second},
+		{"max(empty, errors) wins", 2, 4, 16 * time.Second},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			s := &workerState{consecutiveEmpty: tc.empty, consecutiveErrors: tc.errs}
+			assert.Equal(t, tc.wantInterval, p.calculateInterval(s))
+		})
+	}
+}