perf(actions): debounce runner heartbeat writes and throttle task picks (#38281)

3 reductions in the DB load generated by many runners polling
`FetchTask`:

**1. Debounce runner heartbeat writes**
Every poll wrote `last_online`, and every `UpdateTask`/`UpdateLog` wrote
`last_active` — while a runner streams logs that is many writes per
second per runner. These are now persisted only when stale enough to
actually affect the active/offline status (`ShouldPersistLastOnline` /
`ShouldPersistLastActive`), using the existing columns.

**2. Throttle concurrent task picks**
A new in-process semaphore (`MAX_CONCURRENT_TASK_PICKS`) bounds how many
runners run the task-assignment transaction at once, so a fleet polling
together cannot stampede the query. Throttled polls retry on their next
poll without advancing the runner's tasks version.

**3. Paginate the task-pick query**
`CreateTaskForRunner` previously loaded every waiting job in the
runner's scope into memory on each poll (no `LIMIT`). Now it pages
through the waiting backlog oldest-first with `LIMIT`, claiming the
first label-matching job.

---------

Co-authored-by: Zettat123 <zettat123@gmail.com>

services/actions/task.go

@@ -7,16 +7,47 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"sync"
 
 	runnerv1 "gitea.dev/actions-proto-go/runner/v1"
 	actions_model "gitea.dev/models/actions"
 	"gitea.dev/models/db"
 	secret_model "gitea.dev/models/secret"
 	"gitea.dev/modules/log"
+	"gitea.dev/modules/setting"
 
 	"google.golang.org/protobuf/types/known/structpb"
 )
 
+var (
+	taskPickSem     chan struct{}
+	taskPickSemOnce sync.Once
+)
+
+func taskPickLimiter() chan struct{} {
+	taskPickSemOnce.Do(func() {
+		taskPickSem = make(chan struct{}, setting.Actions.MaxConcurrentTaskPicks)
+	})
+	return taskPickSem
+}
+
+// TryPickTask attempts to assign a task to the runner, bounding the number of
+// concurrent assignment transactions to avoid a thundering herd when many
+// runners poll at once. When the concurrency limit is reached it returns
+// throttled=true without touching the DB, so the caller can let the runner
+// retry on its next poll instead of advancing its tasks version.
+func TryPickTask(ctx context.Context, runner *actions_model.ActionRunner) (task *runnerv1.Task, ok, throttled bool, err error) {
+	sem := taskPickLimiter()
+	select {
+	case sem <- struct{}{}:
+		defer func() { <-sem }()
+	default:
+		return nil, false, true, nil
+	}
+	task, ok, err = PickTask(ctx, runner)
+	return task, ok, false, err
+}
+
 func PickTask(ctx context.Context, runner *actions_model.ActionRunner) (*runnerv1.Task, bool, error) {
 	var (
 		task       *runnerv1.Task

services/actions/task_test.go

@@ -0,0 +1,34 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"testing"
+
+	actions_model "gitea.dev/models/actions"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTryPickTaskThrottled(t *testing.T) {
+	sem := taskPickLimiter()
+
+	// Saturate every assignment slot so the next attempt must be throttled.
+	for range cap(sem) {
+		sem <- struct{}{}
+	}
+	defer func() {
+		for range cap(sem) {
+			<-sem
+		}
+	}()
+
+	// No DB access happens on the throttled path, so this is safe without fixtures.
+	task, ok, throttled, err := TryPickTask(t.Context(), &actions_model.ActionRunner{})
+	require.NoError(t, err)
+	assert.Nil(t, task)
+	assert.False(t, ok)
+	assert.True(t, throttled)
+}