internal/app/state: use internal/lockedfile

This is a pretty solid implementation backed by robust tests, with a much cleaner interface.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/lockedfile/internal/filelock/filelock_unix.go

@@ -0,0 +1,40 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || dragonfly || freebsd || illumos || linux || netbsd || openbsd
+
+package filelock
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+type lockType int16
+
+const (
+	readLock  lockType = syscall.LOCK_SH
+	writeLock lockType = syscall.LOCK_EX
+)
+
+func lock(f File, lt lockType) (err error) {
+	for {
+		err = syscall.Flock(int(f.Fd()), int(lt))
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	if err != nil {
+		return &fs.PathError{
+			Op:   lt.String(),
+			Path: f.Name(),
+			Err:  err,
+		}
+	}
+	return nil
+}
+
+func unlock(f File) error {
+	return lock(f, syscall.LOCK_UN)
+}