Add inotify support

.gitignore

@@ -1,3 +1,4 @@
 supercronic
 vendor
 dist/*
+.idea

README.md

@@ -229,6 +229,27 @@ docker kill --signal=USR2 <container id>
 kill -USR2 <pid>
 ```
 
+If you are running Supercronic in an environment were sending `SIGUSR2` is a bit of a hassle, or you expect frequent updates to your crontab file, you may opt to run Supercronic with the `-inotify` flag. This will start a watch on the crontab file, reloading it on changes. An example use case would be a kubernetes pod runnning Supercronic that mounts its crontab file from a configMap. With the `-inotify` flag, any update to this configmap, provided it is not immutable, will trigger a reload in Supercronic, without you having to figure out a mechanism to send the `SIGUSR2` signal to the pod. The watch on the crontab file triggers on `Write` and `Remove` events, the latter ensures detection of kubernetes' atomic writes.
+
+```
+$ ./supercronic -inotify ./my-crontab
+...
+time="2024-09-11T09:23:18+02:00" level=debug msg="event: CHMOD         \"./my-crontab\", watch-list: []"
+time="2024-09-11T09:23:18+02:00" level=debug msg="event: REMOVE        \"./my-crontab\", watch-list: []"
+time="2024-09-11T09:23:18+02:00" level=debug msg="watched file changed"
+time="2024-09-11T09:23:18+02:00" level=info msg="received user defined signal 2, reloading crontab"
+time="2024-09-11T09:23:18+02:00" level=info msg="waiting for jobs to finish"
+time="2024-09-11T09:23:18+02:00" level=debug msg="shutting down" job.command="sleep 2" job.position=0 job.schedule="* * * * *"
+time="2024-09-11T09:23:18+02:00" level=info msg="read crontab: ./my-crontab"
+time="2024-09-11T09:23:18+02:00" level=debug msg="try parse (7 fields): '* * * * * sleep 5'"
+time="2024-09-11T09:23:18+02:00" level=debug msg="failed to parse (7 fields): '* * * * * sleep 5': failed: syntax error in day-of-week field: 'sleep'"
+time="2024-09-11T09:23:18+02:00" level=debug msg="try parse (6 fields): '* * * * * sleep'"
+time="2024-09-11T09:23:18+02:00" level=debug msg="failed to parse (6 fields): '* * * * * sleep': failed: syntax error in year field: 'sleep'"
+time="2024-09-11T09:23:18+02:00" level=debug msg="try parse (5 fields): '* * * * *'"
+time="2024-09-11T09:23:18+02:00" level=debug msg="job will run next at 2024-09-11 09:24:00 +0200 CEST" job.command="sleep 5" job.position=0 job.schedule="* * * * *"
+
+```
+
 ## Testing your crontab
 
 Use the `-test` flag to prompt Supercronic to verify your crontab, but not

go.mod

@@ -4,6 +4,7 @@ go 1.23.0
 
 require (
 	github.com/evalphobia/logrus_sentry v0.8.2
+	github.com/fsnotify/fsnotify v1.7.0
 	github.com/prometheus/client_golang v1.20.2
 	github.com/ramr/go-reaper v0.2.1
 	github.com/sirupsen/logrus v1.9.3

go.sum

@@ -10,6 +10,8 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/evalphobia/logrus_sentry v0.8.2 h1:dotxHq+YLZsT1Bb45bB5UQbfCh3gM/nFFetyN46VoDQ=
 github.com/evalphobia/logrus_sentry v0.8.2/go.mod h1:pKcp+vriitUqu9KiWj/VRFbRfFNUwz95/UkgG8a6MNc=
+github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nosvA=
+github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
 github.com/getsentry/raven-go v0.2.0 h1:no+xWJRb5ZI7eE8TWgIq1jLulQiIoLG0IfYxv5JYMGs=
 github.com/getsentry/raven-go v0.2.0/go.mod h1:KungGk8q33+aIAZUIVWZDr2OfAEBsO49PX4NzFV5kcQ=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=

integration/reload.bats

@@ -42,3 +42,46 @@ grep_test_file() {
   kill -s TERM "$PID"
   wait
 }
+
+@test "if inotify is enabled it reloads on file change when receiving a WRITE event" {
+  echo '* * * * * * * echo a > "$TEST_FILE"' > "$CRONTAB_FILE"
+
+  "${BATS_TEST_DIRNAME}/../supercronic" -inotify "$CRONTAB_FILE" 3>&- &
+  PID="$!"
+
+  wait_for grep_test_file a
+
+  echo '* * * * * * * echo b > "$TEST_FILE"' > "$CRONTAB_FILE"
+
+  wait_for grep_test_file b
+
+  kill -s TERM "$PID"
+  wait
+}
+
+@test "if inotify is enabled it handles kubernetes like atomic writes using updated symlinks and folder deletion" {
+  CRONTAB_FILE_NAME="$(basename $(mktemp --dry-run --tmpdir))"
+
+  WORK_DIR="$(mktemp -d)"
+  CRONTAB_PRE_DIR="$(mktemp -d)"
+  CRONTAB_POST_DIR="$(mktemp -d)"
+
+  echo '* * * * * * * echo a > "$TEST_FILE"' > "$CRONTAB_PRE_DIR"/"$CRONTAB_FILE_NAME"
+  echo '* * * * * * * echo b > "$TEST_FILE"' > "$CRONTAB_POST_DIR"/"$CRONTAB_FILE_NAME"
+
+  ln -s "$CRONTAB_PRE_DIR"/"$CRONTAB_FILE_NAME" "$WORK_DIR"/"$CRONTAB_FILE_NAME"
+
+  "${BATS_TEST_DIRNAME}/../supercronic" -inotify -debug "$WORK_DIR"/"$CRONTAB_FILE_NAME" 3>&- &
+  PID="$!"
+
+  wait_for grep_test_file a
+
+  ln -sf "$CRONTAB_POST_DIR"/"$CRONTAB_FILE_NAME"  "$WORK_DIR"/"$CRONTAB_FILE_NAME"
+
+  rm -r "$CRONTAB_PRE_DIR"
+
+  wait_for grep_test_file b
+
+  kill -s TERM "$PID"
+  wait
+}

main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/aptible/supercronic/log/hook"
 	"github.com/aptible/supercronic/prometheus_metrics"
 	"github.com/evalphobia/logrus_sentry"
+	"github.com/fsnotify/fsnotify"
 	reaper "github.com/ramr/go-reaper"
 	"github.com/sirupsen/logrus"
 )
@@ -29,6 +30,7 @@ func main() {
 	quiet := flag.Bool("quiet", false, "do not log informational messages (takes precedence over debug)")
 	json := flag.Bool("json", false, "enable JSON logging")
 	test := flag.Bool("test", false, "test crontab (does not run jobs)")
+	inotify := flag.Bool("inotify", false, "use inotify to detect crontab file changes")
 	prometheusListen := flag.String(
 		"prometheus-listen-address",
 		"",
@@ -102,6 +104,24 @@ func main() {
 
 	crontabFileName := flag.Args()[0]
 
+	var watcher *fsnotify.Watcher
+	if *inotify {
+		logrus.Info("using inotify to detect crontab file changes")
+		var err error
+		watcher, err = fsnotify.NewWatcher()
+		if err != nil {
+			logrus.Fatal(err)
+			return
+		}
+		defer watcher.Close()
+
+		logrus.Infof("adding file watch for '%s'", crontabFileName)
+		if err := watcher.Add(crontabFileName); err != nil {
+			logrus.Fatal(err)
+			return
+		}
+	}
+
 	var sentryHook *logrus_sentry.SentryHook
 	if sentryDsn != "" {
 		sentryLevels := []logrus.Level{
@@ -149,6 +169,44 @@ func main() {
 	go reaper.Reap()
 	// _ = reaper.Reap
 
+	termChan := make(chan os.Signal, 1)
+	signal.Notify(termChan, syscall.SIGINT, syscall.SIGTERM, syscall.SIGQUIT, syscall.SIGUSR2)
+
+	if *inotify {
+		go func() {
+			for {
+				select {
+				case event, ok := <-watcher.Events:
+					if !ok {
+						return
+					}
+					logrus.Debugf("event: %v, watch-list: %v", event, watcher.WatchList())
+
+					switch event.Op {
+					case event.Op & fsnotify.Write:
+						logrus.Debug("watched file changed")
+						termChan <- syscall.SIGUSR2
+
+					// workaround for k8s configmap and secret mounts
+					case event.Op & fsnotify.Remove:
+						logrus.Debug("watched file changed")
+						if err := watcher.Add(crontabFileName); err != nil {
+							logrus.Fatal(err)
+							return
+						}
+						termChan <- syscall.SIGUSR2
+					}
+
+				case err, ok := <-watcher.Errors:
+					if !ok {
+						return
+					}
+					logrus.Error("error:", err)
+				}
+			}
+		}()
+	}
+
 	for {
 		promMetrics.Reset()
 
@@ -179,9 +237,6 @@ func main() {
 			cron.StartJob(&wg, tab.Context, job, exitCtx, cronLogger, *overlapping, *passthroughLogs, &promMetrics)
 		}
 
-		termChan := make(chan os.Signal, 1)
-		signal.Notify(termChan, syscall.SIGINT, syscall.SIGTERM, syscall.SIGQUIT, syscall.SIGUSR2)
-
 		termSig := <-termChan
 
 		if termSig == syscall.SIGUSR2 {