feat: add startup janitor for stale bind-workdir task workspaces (#870)

- Add idle-time cleanup for stale bind-workdir task directories instead of cleaning them on the task execution path.
- Make cleanup behavior configurable with `runner.startup_cleanup_age` as the stale-age threshold (default: `24h`) and `runner.idle_cleanup_interval` as the idle cleanup cadence (default: `10m`).
- Restrict cleanup scope to numeric task directory names only, to avoid touching operator-managed folders.
- Document the cleanup settings in `config.example.yaml` and `README.md`.
- Add tests for stale-directory cleanup, idle cleanup throttling, and config default/override parsing.

## Why

When a runner or host crashes, normal per-task cleanup may not run, leaving stale task directories under the bind-workdir root. Running this cleanup only while the runner is idle recovers that disk space without adding overhead to active job execution.

If you want, I can also tighten the wording around `startup_cleanup_age`, since the key name now reads a bit misleadingly relative to the actual behavior.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/870
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/app/run/runner_idle_cleanup_test.go

@@ -0,0 +1,247 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package run
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+	"time"
+
+	"gitea.com/gitea/runner/internal/pkg/config"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRunnerCleanupStaleTaskDirs(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+	workdirRoot := filepath.Join(t.TempDir(), "workspace")
+	require.NoError(t, os.MkdirAll(workdirRoot, 0o700))
+
+	oldTask := filepath.Join(workdirRoot, "1001")
+	freshTask := filepath.Join(workdirRoot, "1002")
+	nonTask := filepath.Join(workdirRoot, "shared")
+	alphaNumericTask := filepath.Join(workdirRoot, "123abc")
+	for _, path := range []string{oldTask, freshTask, nonTask, alphaNumericTask} {
+		require.NoError(t, os.MkdirAll(path, 0o700))
+	}
+
+	require.NoError(t, os.Chtimes(oldTask, now.Add(-3*time.Hour), now.Add(-3*time.Hour)))
+	require.NoError(t, os.Chtimes(freshTask, now.Add(-30*time.Minute), now.Add(-30*time.Minute)))
+	require.NoError(t, os.Chtimes(nonTask, now.Add(-5*time.Hour), now.Add(-5*time.Hour)))
+	require.NoError(t, os.Chtimes(alphaNumericTask, now.Add(-5*time.Hour), now.Add(-5*time.Hour)))
+
+	r := &Runner{
+		cfg: &config.Config{
+			Runner: config.Runner{
+				WorkdirCleanupAge: 2 * time.Hour,
+			},
+		},
+		now: func() time.Time { return now },
+	}
+
+	r.cleanupStaleTaskDirs(context.Background(), workdirRoot)
+
+	assert.NoDirExists(t, oldTask)
+	assert.DirExists(t, freshTask)
+	assert.DirExists(t, nonTask)
+	assert.DirExists(t, alphaNumericTask)
+}
+
+func TestRunnerCleanupStaleTaskDirsMissingRoot(t *testing.T) {
+	r := &Runner{
+		cfg: &config.Config{
+			Runner: config.Runner{WorkdirCleanupAge: time.Hour},
+		},
+		now: time.Now,
+	}
+
+	// Must be a silent no-op rather than a warning or panic when the root
+	// has not yet been created (e.g. the runner has never executed a task).
+	r.cleanupStaleTaskDirs(context.Background(), filepath.Join(t.TempDir(), "missing"))
+}
+
+func TestRunnerCleanupStaleTaskDirsHonorsContext(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+	workdirRoot := filepath.Join(t.TempDir(), "workspace")
+	require.NoError(t, os.MkdirAll(workdirRoot, 0o700))
+
+	for i := 1001; i <= 1003; i++ {
+		dir := filepath.Join(workdirRoot, strconv.Itoa(i))
+		require.NoError(t, os.MkdirAll(dir, 0o700))
+		require.NoError(t, os.Chtimes(dir, now.Add(-3*time.Hour), now.Add(-3*time.Hour)))
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	r := &Runner{
+		cfg: &config.Config{
+			Runner: config.Runner{WorkdirCleanupAge: time.Hour},
+		},
+		now: func() time.Time { return now },
+	}
+
+	r.cleanupStaleTaskDirs(ctx, workdirRoot)
+
+	for i := 1001; i <= 1003; i++ {
+		assert.DirExists(t, filepath.Join(workdirRoot, strconv.Itoa(i)))
+	}
+}
+
+func TestRunnerShouldRunIdleCleanupThrottles(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+	r := &Runner{
+		cfg: &config.Config{
+			Container: config.Container{
+				BindWorkdir: true,
+			},
+			Runner: config.Runner{
+				WorkdirCleanupAge:   24 * time.Hour,
+				IdleCleanupInterval: time.Hour,
+			},
+		},
+		now: func() time.Time { return now },
+	}
+
+	assert.True(t, r.shouldRunIdleCleanup())
+
+	now = now.Add(30 * time.Minute)
+	assert.False(t, r.shouldRunIdleCleanup())
+
+	now = now.Add(31 * time.Minute)
+	assert.True(t, r.shouldRunIdleCleanup())
+}
+
+func TestRunnerShouldRunIdleCleanupSkipsWhenJobRunning(t *testing.T) {
+	r := &Runner{
+		cfg: &config.Config{
+			Container: config.Container{
+				BindWorkdir: true,
+			},
+			Runner: config.Runner{
+				WorkdirCleanupAge:   24 * time.Hour,
+				IdleCleanupInterval: time.Minute,
+			},
+		},
+		now: time.Now,
+	}
+	r.runningCount.Store(1)
+
+	assert.False(t, r.shouldRunIdleCleanup())
+}
+
+func TestRunnerShouldRunIdleCleanupSkipsWhenBindWorkdirDisabled(t *testing.T) {
+	r := &Runner{
+		cfg: &config.Config{
+			Runner: config.Runner{
+				WorkdirCleanupAge:   24 * time.Hour,
+				IdleCleanupInterval: time.Minute,
+			},
+		},
+		now: time.Now,
+	}
+
+	assert.False(t, r.shouldRunIdleCleanup())
+}
+
+func TestRunnerShouldRunIdleCleanupSkipsWhenDisabled(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+
+	t.Run("cleanup age disabled", func(t *testing.T) {
+		r := &Runner{
+			cfg: &config.Config{
+				Container: config.Container{
+					BindWorkdir: true,
+				},
+				Runner: config.Runner{
+					WorkdirCleanupAge:   -1,
+					IdleCleanupInterval: time.Minute,
+				},
+			},
+			now: func() time.Time { return now },
+		}
+
+		assert.False(t, r.shouldRunIdleCleanup())
+	})
+
+	t.Run("idle interval disabled", func(t *testing.T) {
+		r := &Runner{
+			cfg: &config.Config{
+				Container: config.Container{
+					BindWorkdir: true,
+				},
+				Runner: config.Runner{
+					WorkdirCleanupAge:   24 * time.Hour,
+					IdleCleanupInterval: -1,
+				},
+			},
+			now: func() time.Time { return now },
+		}
+
+		assert.False(t, r.shouldRunIdleCleanup())
+	})
+}
+
+// TestRunnerOnIdleIntegratesCleanup wires the full OnIdle entry point and
+// confirms it walks workdir_parent (after the leading-slash trim that
+// matches the production path construction) and removes stale numeric dirs.
+func TestRunnerOnIdleIntegratesCleanup(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+	root := t.TempDir()
+	stale := filepath.Join(root, "1234")
+	require.NoError(t, os.MkdirAll(stale, 0o700))
+	require.NoError(t, os.Chtimes(stale, now.Add(-48*time.Hour), now.Add(-48*time.Hour)))
+
+	r := &Runner{
+		cfg: &config.Config{
+			Container: config.Container{
+				BindWorkdir:   true,
+				WorkdirParent: root, // leading slash absent, OnIdle reattaches it
+			},
+			Runner: config.Runner{
+				WorkdirCleanupAge:   24 * time.Hour,
+				IdleCleanupInterval: time.Minute,
+			},
+		},
+		now: func() time.Time { return now },
+	}
+
+	r.OnIdle(context.Background())
+
+	assert.NoDirExists(t, stale)
+}
+
+// TestRunnerOnIdleSkipsWhenAlreadyCancelled verifies a pre-cancelled ctx
+// short-circuits cleanup before any directory entry is touched.
+func TestRunnerOnIdleSkipsWhenAlreadyCancelled(t *testing.T) {
+	now := time.Date(2026, time.April, 29, 20, 0, 0, 0, time.UTC)
+	root := t.TempDir()
+	stale := filepath.Join(root, "1234")
+	require.NoError(t, os.MkdirAll(stale, 0o700))
+	require.NoError(t, os.Chtimes(stale, now.Add(-48*time.Hour), now.Add(-48*time.Hour)))
+
+	r := &Runner{
+		cfg: &config.Config{
+			Container: config.Container{
+				BindWorkdir:   true,
+				WorkdirParent: root,
+			},
+			Runner: config.Runner{
+				WorkdirCleanupAge:   24 * time.Hour,
+				IdleCleanupInterval: time.Minute,
+			},
+		},
+		now: func() time.Time { return now },
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	r.OnIdle(ctx)
+
+	assert.DirExists(t, stale)
+}