Support identical cron schedule (#87)

* Retry on lock being unavailable

* Refactor locking to return plain error

* Collect LockedTime in stats

* Add test case

* Add documentation for LOCK_TIMEOUT

* Log in case lock needs to be awaited

* Release resources created for awaiting lock

cmd/backup/config.go

@@ -41,4 +41,5 @@ type Config struct {
 	WebdavPassword             string        `split_words:"true"`
 	ExecLabel                  string        `split_words:"true"`
 	ExecForwardOutput          bool          `split_words:"true"`
+	LockTimeout                time.Duration `split_words:"true" default:"60m"`
 }

cmd/backup/lock.go

@@ -0,0 +1,58 @@
+// Copyright 2022 - Offen Authors <hioffen@posteo.de>
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/gofrs/flock"
+)
+
+// lock opens a lockfile at the given location, keeping it locked until the
+// caller invokes the returned release func. In case the lock is currently blocked
+// by another execution, it will repeatedly retry until the lock is available
+// or the given timeout is exceeded.
+func (s *script) lock(lockfile string) (func() error, error) {
+	start := time.Now()
+	defer func() {
+		s.stats.LockedTime = time.Now().Sub(start)
+	}()
+
+	retry := time.NewTicker(5 * time.Second)
+	defer retry.Stop()
+	deadline := time.NewTimer(s.c.LockTimeout)
+	defer deadline.Stop()
+
+	fileLock := flock.New(lockfile)
+
+	for {
+		acquired, err := fileLock.TryLock()
+		if err != nil {
+			return noop, fmt.Errorf("lock: error trying lock: %w", err)
+		}
+		if acquired {
+			if s.encounteredLock {
+				s.logger.Info("Acquired exclusive lock on subsequent attempt, ready to continue.")
+			}
+			return fileLock.Unlock, nil
+		}
+
+		if !s.encounteredLock {
+			s.logger.Infof(
+				"Exclusive lock was not available on first attempt. Will retry until it becomes available or the timeout of %s is exceeded.",
+				s.c.LockTimeout,
+			)
+			s.encounteredLock = true
+		}
+
+		select {
+		case <-retry.C:
+			continue
+		case <-deadline.C:
+			return noop, errors.New("lock: timed out waiting for lockfile to become available")
+		}
+	}
+}

cmd/backup/main.go

@@ -8,14 +8,15 @@ import (
 )
 
 func main() {
-	unlock := lock("/var/lock/dockervolumebackup.lock")
-	defer unlock()
-
 	s, err := newScript()
 	if err != nil {
 		panic(err)
 	}
 
+	unlock, err := s.lock("/var/lock/dockervolumebackup.lock")
+	defer unlock()
+	s.must(err)
+
 	defer func() {
 		if pArg := recover(); pArg != nil {
 			if err, ok := pArg.(error); ok {

cmd/backup/script.go

@@ -47,6 +47,8 @@ type script struct {
 	file  string
 	stats *Stats
 
+	encounteredLock bool
+
 	c *Config
 }
 

cmd/backup/stats.go

@@ -42,6 +42,7 @@ type Stats struct {
 	StartTime  time.Time
 	EndTime    time.Time
 	TookTime   time.Duration
+	LockedTime time.Duration
 	LogOutput  *bytes.Buffer
 	Containers ContainersStats
 	BackupFile BackupFileStats

cmd/backup/util.go

@@ -10,27 +10,10 @@ import (
 	"io"
 	"os"
 	"strings"
-
-	"github.com/gofrs/flock"
 )
 
 var noop = func() error { return nil }
 
-// lock opens a lockfile at the given location, keeping it locked until the
-// caller invokes the returned release func. When invoked while the file is
-// still locked the function panics.
-func lock(lockfile string) func() error {
-	fileLock := flock.New(lockfile)
-	acquired, err := fileLock.TryLock()
-	if err != nil {
-		panic(err)
-	}
-	if !acquired {
-		panic("unable to acquire file lock")
-	}
-	return fileLock.Unlock
-}
-
 // copy creates a copy of the file located at `dst` at `src`.
 func copyFile(src, dst string) error {
 	in, err := os.Open(src)