From a1983d6c8177ad01665aadec03b9fe56dba65edf Mon Sep 17 00:00:00 2001
From: Jun Futagawa <jfut@integ.jp>
Date: Tue, 23 Jun 2026 22:24:20 +0900
Subject: [PATCH] feat: prevent duplicate precd instances

Co-authored-by: Codex <noreply@openai.com>
---
 README.md                        |  5 +++
 cmd/precd/main.go                | 10 +++++
 cmd/precd/singleton_lock.go      | 68 ++++++++++++++++++++++++++++++++
 cmd/precd/singleton_lock_test.go | 49 +++++++++++++++++++++++
 4 files changed, 132 insertions(+)
 create mode 100644 cmd/precd/singleton_lock.go
 create mode 100644 cmd/precd/singleton_lock_test.go

diff --git a/README.md b/README.md
index 3c25311..6e6fa27 100644
--- a/README.md
+++ b/README.md
@@ -162,6 +162,11 @@ Default config path: `/etc/prec/precd.conf`
 
 See: [packaging/precd.conf.example](packaging/precd.conf.example)
 
+Single-instance behavior:
+
+- `precd` acquires an exclusive `flock` on `<log_path>.lock` at startup
+- if the lock is already held, startup fails immediately to prevent duplicate collection
+
 Compression modes:
 
 - `compress = "no"` plain JSONL
diff --git a/cmd/precd/main.go b/cmd/precd/main.go
index 7b711bc..a2d6289 100644
--- a/cmd/precd/main.go
+++ b/cmd/precd/main.go
@@ -4,6 +4,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -62,6 +63,15 @@ func main() {
 		log.Fatalf("load config: %v", err)
 	}
 
+	lock, err := acquireSingletonLock(cfg.LogPath)
+	if err != nil {
+		if errors.Is(err, errPrecdAlreadyRunning) {
+			log.Fatalf("another precd instance is already running: %v", err)
+		}
+		log.Fatalf("init singleton lock: %v", err)
+	}
+	defer lock.Close()
+
 	w, err := logger.NewJSONLWriter(cfg.LogPath, cfg.Compress, cfg.CompressLevel)
 	if err != nil {
 		log.Fatalf("init logger: %v", err)
diff --git a/cmd/precd/singleton_lock.go b/cmd/precd/singleton_lock.go
new file mode 100644
index 0000000..0cf51ce
--- /dev/null
+++ b/cmd/precd/singleton_lock.go
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright contributors to the prec project.
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+var errPrecdAlreadyRunning = errors.New("precd already running")
+
+// singletonLock holds a process-wide lock file descriptor to prevent duplicate daemons.
+type singletonLock struct {
+	f *os.File
+}
+
+func lockPathFromLogPath(logPath string) string {
+	return logPath + ".lock"
+}
+
+func acquireSingletonLock(logPath string) (*singletonLock, error) {
+	lockPath := lockPathFromLogPath(logPath)
+	lockDir := filepath.Dir(lockPath)
+	if err := os.MkdirAll(lockDir, 0o750); err != nil {
+		return nil, fmt.Errorf("create lock dir: %w", err)
+	}
+
+	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0o640)
+	if err != nil {
+		return nil, fmt.Errorf("open lock file: %w", err)
+	}
+	if err := f.Chmod(0o640); err != nil {
+		f.Close()
+		return nil, fmt.Errorf("chmod lock file: %w", err)
+	}
+
+	if err := unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB); err != nil {
+		f.Close()
+		if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+			return nil, fmt.Errorf("%w: %s", errPrecdAlreadyRunning, lockPath)
+		}
+		return nil, fmt.Errorf("acquire lock: %w", err)
+	}
+
+	return &singletonLock{f: f}, nil
+}
+
+func (l *singletonLock) Close() error {
+	if l == nil || l.f == nil {
+		return nil
+	}
+
+	var firstErr error
+	if err := unix.Flock(int(l.f.Fd()), unix.LOCK_UN); err != nil {
+		firstErr = err
+	}
+	if err := l.f.Close(); err != nil && firstErr == nil {
+		firstErr = err
+	}
+
+	l.f = nil
+	return firstErr
+}
diff --git a/cmd/precd/singleton_lock_test.go b/cmd/precd/singleton_lock_test.go
new file mode 100644
index 0000000..3e52c87
--- /dev/null
+++ b/cmd/precd/singleton_lock_test.go
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright contributors to the prec project.
+
+package main
+
+import (
+	"errors"
+	"path/filepath"
+	"testing"
+)
+
+func TestAcquireSingletonLockRejectsSecondInstance(t *testing.T) {
+	logPath := filepath.Join(t.TempDir(), "prec.log")
+
+	first, err := acquireSingletonLock(logPath)
+	if err != nil {
+		t.Fatalf("acquire first lock: %v", err)
+	}
+	defer first.Close()
+
+	second, err := acquireSingletonLock(logPath)
+	if second != nil {
+		second.Close()
+		t.Fatalf("second lock must be nil when lock acquisition fails")
+	}
+	if !errors.Is(err, errPrecdAlreadyRunning) {
+		t.Fatalf("second lock error=%v, want errPrecdAlreadyRunning", err)
+	}
+}
+
+func TestAcquireSingletonLockAllowsNextAfterClose(t *testing.T) {
+	logPath := filepath.Join(t.TempDir(), "prec.log")
+
+	first, err := acquireSingletonLock(logPath)
+	if err != nil {
+		t.Fatalf("acquire first lock: %v", err)
+	}
+	if err := first.Close(); err != nil {
+		t.Fatalf("close first lock: %v", err)
+	}
+
+	second, err := acquireSingletonLock(logPath)
+	if err != nil {
+		t.Fatalf("acquire second lock after close: %v", err)
+	}
+	if err := second.Close(); err != nil {
+		t.Fatalf("close second lock: %v", err)
+	}
+}