Move cron scheduling inside application (#338)

* Move cron scheduling inside application

* Make envvar a fallback and check for errors

* Panic significantly less

* propagate error out of runBackup

* Add structured logging

* FIx error propagation to exit

* Enable the new scheduler by default

* Review fixes

* Added docs and better error propagation

cmd/backup/main.go

@@ -4,21 +4,33 @@
 package main
 
 import (
+	"flag"
 	"fmt"
+	"log/slog"
 	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/robfig/cron/v3"
 )
 
-func main() {
-	s, err := newScript()
+func runScript(c *Config) (ret error) {
+	s, err := newScript(c)
 	if err != nil {
-		panic(err)
+		return err
 	}
 
 	unlock, err := s.lock("/var/lock/dockervolumebackup.lock")
+	if err != nil {
+		return err
+	}
+
 	defer func() {
-		s.must(unlock())
+		err = unlock()
+		if err != nil {
+			ret = err
+		}
 	}()
-	s.must(err)
 
 	defer func() {
 		if pArg := recover(); pArg != nil {
@@ -31,9 +43,14 @@ func main() {
 						fmt.Sprintf("An error occurred calling the registered hooks: %s", hookErr),
 					)
 				}
-				os.Exit(1)
+				ret = err
+			} else {
+				s.logger.Error(
+					fmt.Sprintf("Executing the script encountered an unrecoverable panic: %v", err),
+				)
+
+				panic(pArg)
 			}
-			panic(pArg)
 		}
 
 		if err := s.runHooks(nil); err != nil {
@@ -43,7 +60,7 @@ func main() {
 					err,
 				),
 			)
-			os.Exit(1)
+			ret = err
 		}
 		s.logger.Info("Finished running backup tasks.")
 	}()
@@ -65,4 +82,90 @@ func main() {
 	s.must(s.withLabeledCommands(lifecyclePhaseProcess, s.encryptArchive)())
 	s.must(s.withLabeledCommands(lifecyclePhaseCopy, s.copyArchive)())
 	s.must(s.withLabeledCommands(lifecyclePhasePrune, s.pruneBackups)())
+
+	return nil
+}
+
+func runInForeground() error {
+	cr := cron.New(
+		cron.WithParser(
+			cron.NewParser(
+				cron.SecondOptional | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor,
+			),
+		),
+	)
+
+	addJob := func(c *Config) error {
+		_, err := cr.AddFunc(c.BackupCronExpression, func() {
+			err := runScript(c)
+			if err != nil {
+				slog.Error("unexpected error during backup", "error", err)
+			}
+		})
+		return err
+	}
+
+	cs, err := loadEnvFiles("/etc/dockervolumebackup/conf.d")
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return fmt.Errorf("could not load config from environment files, error: %w", err)
+		}
+
+		c, err := loadEnvVars()
+		if err != nil {
+			return fmt.Errorf("could not load config from environment variables")
+		} else {
+			err = addJob(c)
+			if err != nil {
+				return fmt.Errorf("could not add cron job, error: %w", err)
+			}
+		}
+	} else {
+		for _, c := range cs {
+			err = addJob(c)
+			if err != nil {
+				return fmt.Errorf("could not add cron job, error: %w", err)
+			}
+		}
+	}
+
+	var quit = make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGTERM, syscall.SIGINT)
+	cr.Start()
+	<-quit
+	ctx := cr.Stop()
+	<-ctx.Done()
+
+	return nil
+}
+
+func runAsCommand() error {
+	c, err := loadEnvVars()
+	if err != nil {
+		return fmt.Errorf("could not load config from environment variables, error: %w", err)
+	}
+
+	err = runScript(c)
+	if err != nil {
+		return fmt.Errorf("unexpected error during backup, error: %w", err)
+	}
+
+	return nil
+}
+
+func main() {
+	serve := flag.Bool("foreground", false, "run the tool in the foreground")
+	flag.Parse()
+
+	var err error
+	if *serve {
+		err = runInForeground()
+	} else {
+		err = runAsCommand()
+	}
+
+	if err != nil {
+		slog.Error("ran into an issue during execution", "error", err)
+		os.Exit(1)
+	}
 }