Update jobs only when a new container id is found

Rather than trashing the whole schedule, instead compare what is
currently scheduled with the newly found containers and then add or
remove jobs as needed.

Fixes #2

Makefile

@@ -1,3 +1,4 @@
+.PHONY: test all
 DOCKER_TAG ?= dockron-dev-${USER}
 GIT_TAG_NAME := $(shell git tag -l --contains HEAD)
 GIT_SHA := $(shell git rev-parse HEAD)
@@ -13,7 +14,7 @@ vendor:
 # Runs the application, useful while developing
 .PHONY: run
 run:
-	go run *.go
+	go run *.go -watch 10s -debug
 
 # Output target
 dockron:

main.go

@@ -22,6 +22,9 @@ var (
 
 	// version of dockron being run
 	version = "dev"
+
+	// Debug can be used to show or supress certain log messages
+	Debug = true
 )
 
 // ContainerStartJob represents a scheduled container task
@@ -39,17 +42,33 @@ type ContainerStartJob struct {
 // container
 func (job ContainerStartJob) Run() {
 	log.Println("Starting:", job.Name)
-	err := job.Client.ContainerStart(job.Context, job.ContainerID, types.ContainerStartOptions{})
+	err := job.Client.ContainerStart(
+		job.Context,
+		job.ContainerID,
+		types.ContainerStartOptions{},
+	)
 	if err != nil {
 		panic(err)
 	}
 }
 
+// UniqueName returns a unique identifier for a container start job
+func (job ContainerStartJob) UniqueName() string {
+	// ContainerID should be unique as a change in label will result in
+	// a new container as they are immutable
+	return job.ContainerID
+}
+
 // QueryScheduledJobs queries Docker for all containers with a schedule and
 // returns a list of ContainerStartJob records to be scheduled
 func QueryScheduledJobs(cli *client.Client) (jobs []ContainerStartJob) {
-	log.Println("Scanning containers for new schedules...")
-	containers, err := cli.ContainerList(context.Background(), types.ContainerListOptions{All: true})
+	if Debug {
+		log.Println("Scanning containers for new schedules...")
+	}
+	containers, err := cli.ContainerList(
+		context.Background(),
+		types.ContainerListOptions{All: true},
+	)
 	if err != nil {
 		panic(err)
 	}
@@ -73,16 +92,48 @@ func QueryScheduledJobs(cli *client.Client) (jobs []ContainerStartJob) {
 // ScheduleJobs accepts a Cron instance and a list of jobs to schedule.
 // It then schedules the provided jobs
 func ScheduleJobs(c *cron.Cron, jobs []ContainerStartJob) {
+	// Fetch existing jobs from the cron
+	existingJobs := map[string]cron.EntryID{}
+	for _, entry := range c.Entries() {
+		existingJobs[entry.Job.(ContainerStartJob).UniqueName()] = entry.ID
+	}
+
 	for _, job := range jobs {
-		// TODO: Do something with the entryId returned here
+		if _, ok := existingJobs[job.UniqueName()]; ok {
+			// Job already exists, remove it from existing jobs so we don't
+			// unschedule it later
+			if Debug {
+				log.Printf("Job %s is already scheduled. Skipping", job.Name)
+			}
+			delete(existingJobs, job.UniqueName())
+			continue
+		}
+
+		// Job doesn't exist yet, schedule it
 		_, err := c.AddJob(job.Schedule, job)
 		if err == nil {
-			log.Printf("Scheduled %s (%s) with schedule '%s'\n", job.Name, job.ContainerID[:10], job.Schedule)
+			log.Printf(
+				"Scheduled %s (%s) with schedule '%s'\n",
+				job.Name,
+				job.ContainerID[:10],
+				job.Schedule,
+			)
 		} else {
 			// TODO: Track something for a healthcheck here
-			log.Printf("Error scheduling %s (%s) with schedule '%s'. %v", job.Name, job.ContainerID[:10], job.Schedule, err)
+			log.Printf(
+				"Error scheduling %s (%s) with schedule '%s'. %v",
+				job.Name,
+				job.ContainerID[:10],
+				job.Schedule,
+				err,
+			)
 		}
 	}
+
+	// Remove remaining scheduled jobs that weren't in the new list
+	for _, entryID := range existingJobs {
+		c.Remove(entryID)
+	}
 }
 
 func main() {
@@ -96,6 +147,7 @@ func main() {
 	var watchInterval time.Duration
 	flag.DurationVar(&watchInterval, "watch", defaultWatchInterval, "Interval used to poll Docker for changes")
 	var showVersion = flag.Bool("version", false, "Display the version of dockron and exit")
+	flag.BoolVar(&Debug, "debug", false, "Show debug logs")
 	flag.Parse()
 
 	// Print version if asked
@@ -106,21 +158,13 @@ func main() {
 
 	// Create a Cron
 	c := cron.New()
+	c.Start()
 
 	// Start the loop
 	for {
-		// HACK: This is risky as it could fall on the same interval as a task and that task would get skipped
-		// It would be best to manage a ContainerID to Job mapping and then remove entries that are missing
-		// in the new list and add new entries. However, cron does not support this yet.
-
-		// Stop and create a new cron
-		c.Stop()
-		c = cron.New()
-
 		// Schedule jobs again
 		jobs := QueryScheduledJobs(cli)
 		ScheduleJobs(c, jobs)
-		c.Start()
 
 		// Sleep until the next query time
 		time.Sleep(watchInterval)