util: Add an flock utility for lock file type things

Useful for ensuring only one binary runs at the same time.

util/flock.go

@@ -0,0 +1,75 @@
+// Mgmt
+// Copyright (C) 2013-2024+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package util
+
+import (
+	"os"
+	"syscall"
+)
+
+// TODO: Consider replacing this with: github.com/gofrs/flock if it's better.
+
+// NewFlock creates a new flock. Details available in the struct documentation.
+func NewFlock(path string) *Flock {
+	return &Flock{
+		Path: path,
+	}
+}
+
+// Flock is a structure for building a lock file. This can be used to prevent a
+// binary from running more than one copy at a time on a single machine.
+type Flock struct {
+	Path string
+
+	file *os.File
+}
+
+// TryLock attempts to take the lock. If it errors, it means we couldn't build a
+// lock today. If it returns (nil, nil) it means someone is already locked. If
+// it returns a non-nil function, then you are now locked. Call that function to
+// unlock. Note that it can error if it fails to unlock things.
+func (obj *Flock) TryLock() (func() error, error) {
+	var err error
+	obj.file, err = os.Create(obj.Path)
+	if err != nil {
+		return nil, err // actual error, can't continue
+	}
+
+	// Exclusive lock in non-blocking mode, we add a shared lock.
+	if err := syscall.Flock(int(obj.file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+		return nil, nil // can't lock
+	}
+
+	// We're locked!
+	return func() error {
+		defer obj.file.Close()
+		return syscall.Flock(int(obj.file.Fd()), syscall.LOCK_UN)
+	}, nil
+}