Initial commit.

flock.go

@@ -0,0 +1,77 @@
+//go:build linux
+
+// The flock package provides a file-system mediated locking mechanism on linux
+// using the `flock` system call.
+
+package flock
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+var ErrLocked = errors.New("locked")
+
+// Lock gets an exclusive lock on the file at the given path. If the file
+// doesn't exist, it's created.
+func Lock(path string) (*os.File, error) {
+	return lock(path, syscall.LOCK_EX)
+}
+
+// TryLock attempts to lock the file at path. It will return ErrLocked if the
+// file is already locked.
+func TryLock(path string) (*os.File, error) {
+	return lock(path, syscall.LOCK_EX|syscall.LOCK_NB)
+}
+
+// LockFile is like Lock, but takes an *os.File.
+func LockFile(f *os.File) error {
+	return lockFile(f, syscall.LOCK_EX)
+}
+
+// TryLockFile is like TryLock, but takes an *os.File.
+func TryLockFile(f *os.File) error {
+	return lockFile(f, syscall.LOCK_EX|syscall.LOCK_NB)
+}
+
+func lockFile(f *os.File, flags int) error {
+	if err := flock(int(f.Fd()), flags); err != nil {
+		if flags&syscall.LOCK_NB != 0 && errors.Is(err, syscall.EAGAIN) {
+			return ErrLocked
+		}
+		return err
+	}
+	return nil
+}
+
+func flock(fd int, how int) error {
+	_, _, e1 := syscall.Syscall(syscall.SYS_FLOCK, uintptr(fd), uintptr(how), 0)
+	if e1 != 0 {
+		return syscall.Errno(e1)
+	}
+	return nil
+}
+
+func lock(path string, flags int) (*os.File, error) {
+	perm := os.O_CREATE | os.O_RDWR
+	f, err := os.OpenFile(path, perm, 0600)
+	if err != nil {
+		return nil, err
+	}
+
+	err = lockFile(f, flags)
+	if err != nil {
+		f.Close()
+		f = nil
+	}
+	return f, err
+}
+
+// Unlock releases the lock acquired via the Lock function.
+func Unlock(f *os.File) error {
+	if err := flock(int(f.Fd()), syscall.LOCK_UN); err != nil {
+		return err
+	}
+	return f.Close()
+}

flock_test.go

@@ -0,0 +1,111 @@
+package flock
+
+import (
+	"errors"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestLock_basic(t *testing.T) {
+	const path = "/tmp/fsutil-test-lock"
+
+	f, err := Lock(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// locked is closed once the goroutine acquires the lock.
+	locked := make(chan struct{})
+	go func() {
+		f2, err := Lock(path) // blocks until f is unlocked
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		close(locked)
+		Unlock(f2)
+	}()
+
+	// Goroutine should be blocked while we hold the lock.
+	select {
+	case <-locked:
+		t.Fatal("goroutine acquired lock while we hold it")
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	Unlock(f)
+
+	// Now the goroutine should unblock and acquire the lock.
+	select {
+	case <-locked:
+	case <-time.After(time.Second):
+		t.Fatal("goroutine never acquired lock after release")
+	}
+}
+
+func TestLock_badPath(t *testing.T) {
+	_, err := Lock("./dne/file.lock")
+	if err == nil {
+		t.Fatal("expected error for bad path")
+	}
+}
+
+func TestTryLock(t *testing.T) {
+	const path = "/tmp/fsutil-test-lock"
+
+	f, err := TryLock(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Unlock(f)
+
+	// While f holds the lock, TryLock should return ErrLocked.
+	_, err = TryLock(path)
+	if !errors.Is(err, ErrLocked) {
+		t.Fatalf("expected ErrLocked, got %v", err)
+	}
+
+	// After unlocking, TryLock should succeed.
+	if err := Unlock(f); err != nil {
+		t.Fatal(err)
+	}
+
+	f2, err := TryLock(path)
+	if err != nil {
+		t.Fatalf("expected lock after unlock, got %v", err)
+	}
+	Unlock(f2)
+}
+
+func TestTryLockFile(t *testing.T) {
+	const path = "/tmp/fsutil-test-lockfile"
+
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	if err := TryLockFile(f); err != nil {
+		t.Fatal(err)
+	}
+
+	f2, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f2.Close()
+
+	if err := TryLockFile(f2); !errors.Is(err, ErrLocked) {
+		t.Fatalf("expected ErrLocked, got %v", err)
+	}
+
+	if err := Unlock(f); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := TryLockFile(f2); err != nil {
+		t.Fatalf("expected lock after unlock, got %v", err)
+	}
+}

go.mod

@@ -0,0 +1,3 @@
+module git.crumpington.com/lib/flock
+
+go 1.25.1