util: Add an flock utility for lock file type things

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

util/flock_test.go

@@ -0,0 +1,65 @@
+// 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.
+
+//go:build !root
+
+package util
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+func ExampleFlock() {
+	lockFile := "/tmp/flock.lock"
+	fmt.Printf("hello\n")
+	defer fmt.Printf("goodbye\n")
+	defer os.Remove(lockFile)
+
+	m := NewFlock(lockFile)
+	unlock, err := m.TryLock()
+	if err != nil {
+		fmt.Printf("err: %+v\n", err)
+		return
+	}
+	if unlock == nil {
+		fmt.Printf("already locked\n")
+		return
+	}
+	fmt.Printf("locked...\n")
+	time.Sleep(1 * time.Second)
+	unlock()
+	fmt.Printf("unlocked...\n")
+
+	// Output: hello
+	// locked...
+	// unlocked...
+	// goodbye
+}