create filelock utility

Signed-off-by: Avi Deitcher <avi@deitcher.net>

src/cmd/linuxkit/util/filelock.go

@@ -0,0 +1,9 @@
+package util
+
+import (
+	"os"
+)
+
+type FileLock struct {
+	file *os.File
+}

src/cmd/linuxkit/util/filelock_other.go

@@ -0,0 +1,19 @@
+//go:build !unix
+
+package util
+
+// Lock opens the file (creating it if needed) and sets an exclusive lock.
+// Returns a FileLock that can later be unlocked.
+func Lock(path string) (*FileLock, error) {
+	return &FileLock{}, nil
+}
+
+// Unlock releases the lock and closes the file.
+func (l *FileLock) Unlock() error {
+	return nil
+}
+
+// CheckLock attempts to detect if the file is locked by another process.
+func CheckLock(path string) (locked bool, holderPID int, err error) {
+	return false, 0, nil
+}

src/cmd/linuxkit/util/filelock_unix.go

@@ -0,0 +1,101 @@
+//go:build unix
+
+package util
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// Lock opens the file (creating it if needed) and sets an exclusive lock.
+// Returns a FileLock that can later be unlocked.
+func Lock(path string) (*FileLock, error) {
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		return nil, fmt.Errorf("open file: %w", err)
+	}
+
+	flock := unix.Flock_t{
+		Type:   unix.F_WRLCK,
+		Whence: int16(io.SeekStart),
+		Start:  0,
+		Len:    0,
+	}
+
+	if err := unix.FcntlFlock(f.Fd(), unix.F_SETLKW, &flock); err != nil {
+		_ = f.Close()
+		return nil, fmt.Errorf("set lock: %w", err)
+	}
+
+	return &FileLock{file: f}, nil
+}
+
+// Unlock releases the lock and closes the file.
+func (l *FileLock) Unlock() error {
+	flock := unix.Flock_t{
+		Type:   unix.F_UNLCK,
+		Whence: int16(io.SeekStart),
+		Start:  0,
+		Len:    0,
+	}
+	if err := unix.FcntlFlock(l.file.Fd(), unix.F_SETLKW, &flock); err != nil {
+		return fmt.Errorf("unlock: %w", err)
+	}
+	return l.file.Close()
+}
+
+// CheckLock attempts to detect if the file is locked by another process.
+func CheckLock(path string) (locked bool, holderPID int, err error) {
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		return false, 0, fmt.Errorf("open file: %w", err)
+	}
+	defer func() {
+		_ = f.Close()
+	}()
+
+	check := unix.Flock_t{
+		Type:   unix.F_WRLCK,
+		Whence: int16(io.SeekStart),
+		Start:  0,
+		Len:    0,
+	}
+
+	if err := unix.FcntlFlock(f.Fd(), unix.F_GETLK, &check); err != nil {
+		return false, 0, fmt.Errorf("get lock: %w", err)
+	}
+
+	if check.Type == unix.F_UNLCK {
+		return false, 0, nil
+	}
+	return true, int(check.Pid), nil
+}
+
+// WaitUnlocked waits until the file is unlocked by another process, and uses it for reading but not writing.
+func WaitUnlocked(path string) error {
+	f, err := os.OpenFile(path, os.O_RDONLY, 0644)
+	if err != nil {
+		return fmt.Errorf("open file: %w", err)
+	}
+	defer func() {
+		_ = f.Close()
+	}()
+
+	flock := unix.Flock_t{
+		Type:   unix.F_RDLCK,
+		Whence: int16(io.SeekStart),
+		Start:  0,
+		Len:    0,
+	}
+
+	if err := unix.FcntlFlock(f.Fd(), unix.F_SETLKW, &flock); err != nil {
+		_ = f.Close()
+		return fmt.Errorf("set lock: %w", err)
+	}
+	fileRef := &FileLock{file: f}
+	_ = fileRef.Unlock()
+	return nil
+}