feat: declared background workers + frankenphp_get_worker_handle()

Adds a minimal background-worker surface: long-lived non-HTTP PHP scripts
declared via WithWorkerBackground() / `background` in the Caddyfile worker
block. Each worker exposes a stop-pipe stream via
frankenphp_get_worker_handle(); on FrankenPHP shutdown / restart Go closes
the write end so the script's stream_select returns EOF and the loop can
exit cleanly. Crash-restart with quadratic backoff, halted by
max_consecutive_failures. Per-php_server scope isolation so two server
blocks can declare the same worker name without colliding.

Deferred to follow-ups (kept out for review surface):
- frankenphp_ensure_background_worker() / lazy-start machinery
- Catch-all (empty-name) workers
- Shared-state APIs (frankenphp_set_vars / frankenphp_get_vars)

What lands:
- frankenphp_get_worker_handle() zif + stop-fd plumbing in C
- WithWorkerBackground() / WithWorkerScope() Go options; num >= 1 required
- backgroundWorkerThread handler: boot, restart with backoff, cap-on-crash,
  graceful exit on stop-fd close
- Per-scope lookup map keyed by user-facing name
- Caddy `background` worker flag + scope-label cascade (host matcher ->
  first listener address) so the metric prefix m#<scope-label>:<name> is
  set before the very first metric emit.

Tests:
- TestBackgroundWorkerLifecycle: bg worker boots, touches sentinel, parks,
  exits within 10s of Shutdown.
- TestBackgroundWorkerCrashRestarts: exit(1) -> respawn -> restarted
  sentinel.
- TestBackgroundWorkerWithoutHTTP: a regular HTTP request alongside a bg
  worker still serves.
- TestBackgroundWorkerSameNameDifferentScope: two servers each declare
  "shared", both run, distinct *worker per scope.

Author: nicolas-grekas

bgworker.go

@@ -0,0 +1,116 @@
+package frankenphp
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+	"sync/atomic"
+)
+
+// Scope isolates background workers between php_server blocks; the
+// zero value is the global/embed scope. Obtain values via NextScope.
+type Scope uint64
+
+var scopeCounter atomic.Uint64
+
+// NextScope returns a fresh scope value. Each php_server block should
+// call this once during provisioning.
+func NextScope() Scope {
+	return Scope(scopeCounter.Add(1))
+}
+
+// scopeLabels maps Scope -> human-readable label registered by the
+// embedder (e.g. the Caddy module).
+var scopeLabels sync.Map
+
+// SetScopeLabel attaches a human-readable label to a scope; the bg-worker
+// metric emitter renders it as e.g. server="api.example.com" instead of
+// an opaque numeric id. Empty labels are ignored.
+func SetScopeLabel(s Scope, label string) {
+	if label == "" {
+		return
+	}
+	scopeLabels.Store(s, label)
+}
+
+// lookupScopeLabel reports whether a label has been registered for s,
+// returning ("", false) when none has. Distinguishes "unset" from
+// "explicitly empty" without the numeric fallback.
+func lookupScopeLabel(s Scope) (string, bool) {
+	v, ok := scopeLabels.Load(s)
+	if !ok {
+		return "", false
+	}
+	return v.(string), true
+}
+
+// bgWorkerMetricName formats the metric label for a background worker:
+// "m#<scopeLabel>:<runtimeName>". scopeLabel is empty when the scope
+// has no registered label (embed/global, or before the embedder calls
+// SetScopeLabel). The "m#" prefix mirrors the m# convention used for
+// module workers; the colon keeps the format uniform so a single regex
+// (m#([^:]*):(.+)) parses both labelled and unlabelled forms.
+func bgWorkerMetricName(scope Scope, runtimeName string) string {
+	label, _ := lookupScopeLabel(scope)
+	return "m#" + label + ":" + runtimeName
+}
+
+// backgroundLookups maps scope -> name -> *worker. Scope 0 is the
+// global/embed scope. nil when no background worker is declared.
+var backgroundLookups map[Scope]map[string]*worker
+
+// buildBackgroundWorkerLookups maps each declared bg worker into its scope's
+// lookup. Per-scope name collisions are caught here because bg workers
+// intentionally skip the global workersByName map (so two scopes can share
+// a user-facing name). Names are not allowed to be empty in this minimal
+// build; catch-all bg workers are deferred to a follow-up PR.
+func buildBackgroundWorkerLookups(workers []*worker, opts []workerOpt) (map[Scope]map[string]*worker, error) {
+	lookups := make(map[Scope]map[string]*worker)
+
+	for i, o := range opts {
+		if !o.isBackgroundWorker {
+			continue
+		}
+		w := workers[i]
+		w.scope = o.scope
+
+		phpName := strings.TrimPrefix(w.name, "m#")
+		if phpName == "" || phpName == w.fileName {
+			return nil, fmt.Errorf("background worker must have an explicit name (got %q)", w.name)
+		}
+
+		byName := lookups[o.scope]
+		if byName == nil {
+			byName = make(map[string]*worker)
+			lookups[o.scope] = byName
+		}
+		if _, exists := byName[phpName]; exists {
+			return nil, fmt.Errorf("duplicate background worker name %q in the same scope", phpName)
+		}
+		byName[phpName] = w
+	}
+
+	if len(lookups) == 0 {
+		return nil, nil
+	}
+	return lookups, nil
+}
+
+// reserveBackgroundWorkerThreads returns the thread budget to add to the
+// pool for declared bg workers, and pre-registers totalWorkers so a bg-only
+// deployment has the metric initialised. num must be >= 1 for bg workers
+// in this build.
+func reserveBackgroundWorkerThreads(opt *opt) (int, error) {
+	reserved := 0
+	for _, w := range opt.workers {
+		if !w.isBackgroundWorker {
+			continue
+		}
+		if w.num < 1 {
+			return 0, fmt.Errorf("background worker %q must declare num >= 1 (lazy/ensure() machinery is not in this build)", w.name)
+		}
+		reserved += w.num
+		metrics.TotalWorkers(bgWorkerMetricName(w.scope, w.name), w.num)
+	}
+	return reserved, nil
+}

bgworker_test.go

@@ -0,0 +1,86 @@
+package frankenphp_test
+
+import (
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/dunglas/frankenphp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestBackgroundWorkerLifecycle boots a background worker that touches a
+// sentinel file then parks on the stop pipe. It proves the bg worker runs
+// (sentinel appears) and that Shutdown returns within a reasonable time.
+func TestBackgroundWorkerLifecycle(t *testing.T) {
+	tmp := t.TempDir()
+	sentinel := filepath.Join(tmp, "bg-lifecycle.sentinel")
+
+	require.NoError(t, frankenphp.Init(
+		frankenphp.WithWorkers("bg-lifecycle", "testdata/bgworker/basic.php", 1,
+			frankenphp.WithWorkerBackground(),
+			frankenphp.WithWorkerEnv(map[string]string{"BG_SENTINEL": sentinel}),
+		),
+		frankenphp.WithNumThreads(2),
+	))
+	// Note: this test asserts on Shutdown timing, so it manages Shutdown
+	// itself instead of using setupFrankenPHP's t.Cleanup hook.
+
+	requireFileEventually(t, sentinel, "background worker did not touch sentinel")
+
+	done := make(chan struct{})
+	go func() {
+		frankenphp.Shutdown()
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatalf("Shutdown did not return within 10s")
+	}
+}
+
+// TestBackgroundWorkerCrashRestarts boots a worker that exit(1)s on its
+// first run and touches a "restarted" sentinel on its second run. The
+// sentinel proves the crash-restart loop fired.
+func TestBackgroundWorkerCrashRestarts(t *testing.T) {
+	tmp := t.TempDir()
+	crashMarker := filepath.Join(tmp, "bg-crash.marker")
+	restarted := filepath.Join(tmp, "bg-crash.restarted")
+
+	setupFrankenPHP(t,
+		frankenphp.WithWorkers("bg-crash", "testdata/bgworker/crash.php", 1,
+			frankenphp.WithWorkerBackground(),
+			frankenphp.WithWorkerEnv(map[string]string{
+				"BG_CRASH_MARKER":       crashMarker,
+				"BG_RESTARTED_SENTINEL": restarted,
+			}),
+		),
+		frankenphp.WithNumThreads(2),
+	)
+
+	requireFileEventually(t, restarted, "background worker did not restart after crash")
+}
+
+// TestBackgroundWorkerWithoutHTTP confirms that a request to a script
+// unrelated to the bg worker still works: the bg worker doesn't intercept
+// HTTP traffic.
+func TestBackgroundWorkerWithoutHTTP(t *testing.T) {
+	tmp := t.TempDir()
+	sentinel := filepath.Join(tmp, "bg-nohttp.sentinel")
+
+	testDataDir := setupFrankenPHP(t,
+		frankenphp.WithWorkers("bg-nohttp", "testdata/bgworker/basic.php", 1,
+			frankenphp.WithWorkerBackground(),
+			frankenphp.WithWorkerEnv(map[string]string{"BG_SENTINEL": sentinel}),
+		),
+		frankenphp.WithNumThreads(2),
+	)
+
+	requireFileEventually(t, sentinel, "background worker did not touch sentinel")
+
+	body := serveBody(t, testDataDir, "index.php")
+	assert.NotEmpty(t, body, "expected non-empty body from index.php")
+}

bgworkerhelpers_test.go

@@ -0,0 +1,57 @@
+package frankenphp_test
+
+import (
+	"io"
+	"net/http/httptest"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/dunglas/frankenphp"
+	"github.com/stretchr/testify/require"
+)
+
+// requireFileEventually asserts that `path` appears on disk before the
+// deadline. Wraps require.Eventually so call sites stay short.
+func requireFileEventually(t testing.TB, path string, msgAndArgs ...any) {
+	t.Helper()
+	require.Eventually(t, func() bool {
+		_, err := os.Stat(path)
+		return err == nil
+	}, 5*time.Second, 25*time.Millisecond, msgAndArgs...)
+}
+
+// setupFrankenPHP boots FrankenPHP with the given options, registers
+// Shutdown as a t.Cleanup, and returns the absolute path to the testdata
+// directory. Saves the boilerplate every bg-worker test repeats.
+func setupFrankenPHP(t *testing.T, opts ...frankenphp.Option) (testDataDir string) {
+	t.Helper()
+	cwd, err := os.Getwd()
+	require.NoError(t, err)
+	testDataDir = cwd + "/testdata/"
+	require.NoError(t, frankenphp.Init(opts...))
+	t.Cleanup(frankenphp.Shutdown)
+	return
+}
+
+// serveBody runs `script` (relative to testDataDir, may include a query
+// string) through FrankenPHP and returns the response body. ErrRejected is
+// treated as a non-fatal outcome so worker-mode quirks don't fail tests
+// that only care about the script's stdout.
+func serveBody(t *testing.T, testDataDir, scriptAndQuery string, opts ...frankenphp.RequestOption) string {
+	t.Helper()
+	req := httptest.NewRequest("GET", "http://example.com/"+scriptAndQuery, nil)
+	reqOpts := append([]frankenphp.RequestOption{
+		frankenphp.WithRequestDocumentRoot(testDataDir, false),
+	}, opts...)
+	fr, err := frankenphp.NewRequestWithContext(req, reqOpts...)
+	require.NoError(t, err)
+
+	w := httptest.NewRecorder()
+	if err := frankenphp.ServeHTTP(w, fr); err != nil {
+		require.ErrorAs(t, err, &frankenphp.ErrRejected{})
+	}
+	body, err := io.ReadAll(w.Result().Body)
+	require.NoError(t, err)
+	return string(body)
+}

bgworkerscope_test.go

@@ -0,0 +1,89 @@
+package frankenphp
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// setupBgWorker boots FrankenPHP with the given options (internal-package
+// variant), registers Shutdown as a t.Cleanup, and returns the absolute
+// path to the testdata directory.
+func setupBgWorker(t *testing.T, opts ...Option) (testDataDir string) {
+	t.Helper()
+	cwd, err := os.Getwd()
+	require.NoError(t, err)
+	testDataDir = cwd + "/testdata/"
+	require.NoError(t, Init(opts...))
+	t.Cleanup(Shutdown)
+	return
+}
+
+// requireSentinelEventually asserts that `path` appears on disk before the
+// deadline. Wraps require.Eventually so call sites stay short.
+func requireSentinelEventually(t testing.TB, path string, msgAndArgs ...any) {
+	t.Helper()
+	require.Eventually(t, func() bool {
+		_, err := os.Stat(path)
+		return err == nil
+	}, 5*time.Second, 10*time.Millisecond, msgAndArgs...)
+}
+
+// TestNextScopeIsDistinct verifies the scope counter
+// hands out unique values on consecutive calls.
+func TestNextScopeIsDistinct(t *testing.T) {
+	a := NextScope()
+	b := NextScope()
+	assert.NotEqual(t, a, b, "consecutive scopes must differ")
+	assert.NotZero(t, a, "scopes must be non-zero (zero is the global scope)")
+	assert.NotZero(t, b, "scopes must be non-zero (zero is the global scope)")
+}
+
+// TestBackgroundWorkerSameNameDifferentScope declares two named bg
+// workers with the same user-facing name in distinct scopes. Both must
+// Init successfully (the global workersByName collision check must
+// recognize bg workers as scope-isolated), each must produce its own
+// sentinel under its scope-specific directory.
+func TestBackgroundWorkerSameNameDifferentScope(t *testing.T) {
+	scopeA := NextScope()
+	scopeB := NextScope()
+
+	tmp := t.TempDir()
+	dirA := filepath.Join(tmp, "a")
+	dirB := filepath.Join(tmp, "b")
+	require.NoError(t, os.MkdirAll(dirA, 0o755))
+	require.NoError(t, os.MkdirAll(dirB, 0o755))
+
+	setupBgWorker(t,
+		WithWorkers("shared", "testdata/bgworker/named.php", 1,
+			WithWorkerBackground(),
+			WithWorkerScope(scopeA),
+			WithWorkerEnv(map[string]string{"BG_SENTINEL_DIR": dirA}),
+		),
+		WithWorkers("shared", "testdata/bgworker/named.php", 1,
+			WithWorkerBackground(),
+			WithWorkerScope(scopeB),
+			WithWorkerEnv(map[string]string{"BG_SENTINEL_DIR": dirB}),
+		),
+		WithNumThreads(4),
+	)
+
+	// Both lookups must exist and resolve "shared" to a *worker.
+	require.NotNil(t, backgroundLookups[scopeA], "scope A lookup missing")
+	require.NotNil(t, backgroundLookups[scopeB], "scope B lookup missing")
+	assert.NotNil(t, backgroundLookups[scopeA]["shared"], "scope A must resolve 'shared'")
+	assert.NotNil(t, backgroundLookups[scopeB]["shared"], "scope B must resolve 'shared'")
+	// And they must be distinct *worker instances (not the same pointer).
+	assert.NotSame(t,
+		backgroundLookups[scopeA]["shared"],
+		backgroundLookups[scopeB]["shared"],
+		"each scope must own a distinct *worker for the same name")
+
+	// Each scope's worker writes its sentinel under its own dir.
+	requireSentinelEventually(t, filepath.Join(dirA, "shared"), "scope A worker did not touch sentinel")
+	requireSentinelEventually(t, filepath.Join(dirB, "shared"), "scope B worker did not touch sentinel")
+}