webhooker/internal/datadir/lock.go

// Package datadir guards exclusive access to the directory holding
// every SQLite database webhooker writes.
//
// Two processes sharing a DATA_DIR each open the same per-webhook
// event databases and each run delivery recovery over the same rows,
// so every pending delivery goes out twice. SQLite's own locking does
// not prevent that: both writers are serialised correctly and both
// deliver. The only thing that prevents it is refusing to be the
// second process.
//
// The lock lives here rather than in the server's fx graph so that any
// entry point which touches DATA_DIR — the server, or a CLI
// subcommand that must not operate on a live deployment's data — takes
// it the same way.
package datadir

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"

	"github.com/gofrs/flock"
)

// LockFileName is the advisory lock file created inside DATA_DIR. Its
// contents are never read: the lock is the flock(2) held on the open
// descriptor, not the file's existence, so a leftover file from a
// process that was killed with SIGKILL blocks nothing.
const LockFileName = "webhooker.lock"

// dirPerm is the mode Acquire creates DATA_DIR with. It matches what
// internal/database uses, since whichever runs first creates it.
const dirPerm = 0o750

// ErrLocked reports that another live process holds the data
// directory. Callers that need to know whether a deployment is running
// — rather than merely failing to start — test for this with
// errors.Is.
var ErrLocked = errors.New(
	"data directory is already in use by another instance",
)

// ErrNoDir reports that Acquire was given an empty directory.
var ErrNoDir = errors.New("no data directory given")

// Lock is a held exclusive advisory lock on a data directory. It is
// valid only while the process that took it lives: the kernel drops it
// when the descriptor closes, whether that is Release, a normal exit,
// or a SIGKILL.
type Lock struct {
	dir  string
	file *flock.Flock
}

// Acquire takes the exclusive advisory lock on dir, creating dir if it
// does not exist. It never waits: if another process holds the lock it
// returns an error wrapping ErrLocked and naming dir.
//
// The returned Lock must be held for as long as the caller intends to
// use dir.
func Acquire(dir string) (*Lock, error) {
	if dir == "" {
		return nil, ErrNoDir
	}

	err := os.MkdirAll(dir, dirPerm)
	if err != nil {
		return nil, fmt.Errorf(
			"creating data directory %s: %w", dir, err,
		)
	}

	path := filepath.Join(dir, LockFileName)
	fl := flock.New(path)

	held, err := fl.TryLock()
	if err != nil {
		return nil, fmt.Errorf(
			"locking data directory %s: %w", dir, err,
		)
	}

	if !held {
		// A no-op on flock v0.13.0, which closes its own descriptor on
		// a failed TryLock; kept so no version can leak one.
		_ = fl.Close()

		return nil, fmt.Errorf(
			"%w: %s (%s). Only one webhooker may use a data "+
				"directory: two both run delivery recovery over the "+
				"same rows and both deliver",
			ErrLocked, dir, path,
		)
	}

	return &Lock{dir: dir, file: fl}, nil
}

// Dir returns the locked directory.
func (l *Lock) Dir() string {
	return l.dir
}

// Path returns the lock file backing the lock.
func (l *Lock) Path() string {
	return l.file.Path()
}

// Release drops the lock and closes the descriptor. It is safe to call
// more than once.
//
// The lock file is deliberately left on disk. Unlinking it would let
// the next process create and lock a fresh inode while a third still
// holds the old one, which is the one outcome this package exists to
// prevent.
func (l *Lock) Release() error {
	err := l.file.Unlock()
	if err != nil {
		return fmt.Errorf(
			"releasing lock on data directory %s: %w", l.dir, err,
		)
	}

	return nil
}