From 140c528d6c82b6d771c378a84c140ad0772fcfdf Mon Sep 17 00:00:00 2001
From: Leonid Zharikov <leon.zharikov@gmail.com>
Date: Fri, 6 Nov 2020 00:20:04 +0300
Subject: [PATCH 1/2] feat: Add Singleton mode in job

This mode prevents double start.
---
 README.md         | 28 +++++++++++++++++-----------
 example_test.go   | 16 ++++++++++++++++
 executor.go       | 12 +++++++++++-
 go.mod            |  1 +
 go.sum            |  2 ++
 job.go            | 39 +++++++++++++++++++++++++++++++++------
 job_test.go       |  6 ++++--
 scheduler.go      | 10 ++++++++++
 scheduler_test.go | 27 +++++++++++++++++++++++++--
 9 files changed, 119 insertions(+), 22 deletions(-)

diff --git a/README.md b/README.md
index f41bc9ad..b77a75b0 100644
--- a/README.md
+++ b/README.md
@@ -15,26 +15,32 @@ If you want to chat, you can find us at Slack! [<img src="https://img.shields.io
 
 ## Examples
 
-Take a look in our [go docs](https://pkg.go.dev/github.com/go-co-op/gocron#pkg-examples)
+```golang
+s := gocron.NewScheduler(time.UTC)
+
+s.Every(5).Seconds().Do(func(){ ... })
+```
+
+For more examples, take a look in our [go docs](https://pkg.go.dev/github.com/go-co-op/gocron#pkg-examples)
 
 ## FAQ
 
-* Q: I'm running multiple pods on a distributed environment. How can I make a job not run once per pod causing duplication? 
+* Q: I'm running multiple pods on a distributed environment. How can I make a job not run once per pod causing duplication?
 * A: We recommend using your own lock solution within the jobs themselves (you could use [Redis](https://redis.io/topics/distlock), for example)
 
 * Q: I've removed my job from the scheduler, but how can I stop a long-running job that has already been triggered?
 * A: We recommend using a means of canceling your job, e.g. a `context.WithCancel()`.
-  
+
 --- 
 Looking to contribute? Try to follow these guidelines:
- * Use issues for everything
- * For a small change, just send a PR!
- * For bigger changes, please open an issue for discussion before sending a PR.
- * PRs should have: tests, documentation and examples (if it makes sense)
- * You can also contribute by:
-    * Reporting issues
-    * Suggesting new features or enhancements
-    * Improving/fixing documentation
+* Use issues for everything
+* For a small change, just send a PR!
+* For bigger changes, please open an issue for discussion before sending a PR.
+* PRs should have: tests, documentation and examples (if it makes sense)
+* You can also contribute by:
+   * Reporting issues
+   * Suggesting new features or enhancements
+   * Improving/fixing documentation
 ---
 
 [Jetbrains](https://www.jetbrains.com/?from=gocron) supports this project with GoLand licenses. We appreciate their support for free and open source software!
diff --git a/example_test.go b/example_test.go
index 3fa916d4..2307dd92 100644
--- a/example_test.go
+++ b/example_test.go
@@ -85,6 +85,12 @@ func ExampleJob_ScheduledTime() {
 	fmt.Println(job.ScheduledTime())
 }
 
+func ExampleJob_SingletonMode() {
+	s := gocron.NewScheduler(time.UTC)
+	job, _ := s.Every(1).Second().Do(task)
+	job.SingletonMode()
+}
+
 func ExampleJob_Tag() {
 	s := gocron.NewScheduler(time.UTC)
 	job, _ := s.Every("1s").Do(task)
@@ -427,6 +433,8 @@ func ExampleScheduler_Second() {
 	_, _ = s.Every(1).Do(task)
 	_, _ = s.Every(1).Second().Do(task)
 	_, _ = s.Every(1).Seconds().Do(task)
+	_, _ = s.Every("1s").Seconds().Do(task)
+	_, _ = s.Every(time.Second).Seconds().Do(task)
 }
 
 func ExampleScheduler_Seconds() {
@@ -437,6 +445,14 @@ func ExampleScheduler_Seconds() {
 	_, _ = s.Every(1).Do(task)
 	_, _ = s.Every(1).Second().Do(task)
 	_, _ = s.Every(1).Seconds().Do(task)
+	_, _ = s.Every("1s").Seconds().Do(task)
+	_, _ = s.Every(time.Second).Seconds().Do(task)
+
+}
+
+func ExampleScheduler_SingletonMode() {
+	s := gocron.NewScheduler(time.UTC)
+	_, _ = s.Every(1).Second().SingletonMode().Do(task)
 }
 
 func ExampleScheduler_StartBlocking() {
diff --git a/executor.go b/executor.go
index c3d3950b..e0b5af83 100644
--- a/executor.go
+++ b/executor.go
@@ -24,7 +24,17 @@ func (e *executor) start() {
 			wg.Add(1)
 			go func() {
 				defer wg.Done()
-				callJobFuncWithParams(f.functions[f.name], f.params[f.name])
+
+				switch f.runConfig.mode {
+				case defaultMode:
+					callJobFuncWithParams(f.functions[f.name], f.params[f.name])
+				case singletonMode:
+					_, _, _ = f.limiter.Do("main", func() (interface{}, error) {
+						callJobFuncWithParams(f.functions[f.name], f.params[f.name])
+						return nil, nil
+					})
+				}
+
 			}()
 		case <-e.stop:
 			wg.Wait()
diff --git a/go.mod b/go.mod
index d99cf491..a2739d33 100644
--- a/go.mod
+++ b/go.mod
@@ -5,5 +5,6 @@ go 1.15
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/stretchr/testify v1.7.0
+	golang.org/x/sync v0.0.0-20201207232520-09787c993a3a
 	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
 )
diff --git a/go.sum b/go.sum
index 06a1f6ee..4aa5a3b6 100644
--- a/go.sum
+++ b/go.sum
@@ -8,6 +8,8 @@ github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+golang.org/x/sync v0.0.0-20201207232520-09787c993a3a h1:DcqTD9SDLc+1P/r1EmRBwnVsrOwW+kk2vWf9n+1sGhs=
+golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
diff --git a/job.go b/job.go
index c6a9b875..b2d18d86 100644
--- a/job.go
+++ b/job.go
@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"sync"
 	"time"
+
+	"golang.org/x/sync/singleflight"
 )
 
 // Job struct stores the information necessary to run a Job
@@ -22,7 +24,6 @@ type Job struct {
 	scheduledWeekday  *time.Weekday // Specific day of the week to start on
 	dayOfTheMonth     int           // Specific day of the month to run the job
 	tags              []string      // allow the user to tag Jobs with certain labels
-	runConfig         runConfig     // configuration for how many times to run the job
 	runCount          int           // number of times the job ran
 	timer             *time.Timer
 }
@@ -31,14 +32,28 @@ type jobFunction struct {
 	functions map[string]interface{}   // Map for the function task store
 	params    map[string][]interface{} // Map for function and params of function
 	name      string                   // the Job name to run, func[jobFunc]
+	runConfig runConfig                // configuration for how many times to run the job
+	limiter   *singleflight.Group      // limits inflight runs of job to one
 }
 
 type runConfig struct {
 	finiteRuns         bool
 	maxRuns            int
+	mode               mode
 	removeAfterLastRun bool
 }
 
+// mode is the Job's running mode
+type mode int8
+
+const (
+	// defaultMode disable any mode
+	defaultMode mode = iota
+
+	// singletonMode switch to single job mode
+	singletonMode
+)
+
 // NewJob creates a new Job with the provided interval
 func NewJob(interval int) *Job {
 	return &Job{
@@ -137,7 +152,7 @@ func (j *Job) Weekday() (time.Weekday, error) {
 
 // LimitRunsTo limits the number of executions of this job to n.
 // The job will remain in the scheduler.
-// Note: If a job is added to a running scheduler and this method is used
+// Note: If a job is added to a running scheduler and this method is then used
 // you may see the job run more than the set limit as job is scheduled immediately
 // by default upon being added to the scheduler. It is recommended to use the
 // LimitRunsTo() func on the scheduler chain when scheduling the job.
@@ -145,10 +160,22 @@ func (j *Job) Weekday() (time.Weekday, error) {
 func (j *Job) LimitRunsTo(n int) {
 	j.Lock()
 	defer j.Unlock()
-	j.runConfig = runConfig{
-		finiteRuns: true,
-		maxRuns:    n,
-	}
+	j.runConfig.finiteRuns = true
+	j.runConfig.maxRuns = n
+}
+
+// SingletonMode prevents a new job from starting if the prior job has not yet
+// completed it's run
+// Note: If a job is added to a running scheduler and this method is then used
+// you may see the job run overrun itself as job is scheduled immediately
+// by default upon being added to the scheduler. It is recommended to use the
+// SingletonMode() func on the scheduler chain when scheduling the job.
+func (j *Job) SingletonMode() {
+	j.Lock()
+	defer j.Unlock()
+	j.runConfig.mode = singletonMode
+	j.jobFunction.limiter = &singleflight.Group{}
+
 }
 
 // RemoveAfterLastRun sets the job to be removed after it's last run (when limited)
diff --git a/job_test.go b/job_test.go
index 849ae664..3fd888bd 100644
--- a/job_test.go
+++ b/job_test.go
@@ -88,8 +88,10 @@ func TestJob_shouldRunAgain(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			j := &Job{
-				runConfig: tt.runConfig,
-				runCount:  tt.runCount,
+				jobFunction: jobFunction{
+					runConfig: tt.runConfig,
+				},
+				runCount: tt.runCount,
 			}
 			if got := j.shouldRun(); got != tt.want {
 				t.Errorf("Job.shouldRunAgain() = %v, want %v", got, tt.want)
diff --git a/scheduler.go b/scheduler.go
index 7d41e918..c2e4f201 100644
--- a/scheduler.go
+++ b/scheduler.go
@@ -346,6 +346,8 @@ func (s *Scheduler) run(job *Job) {
 		return
 	}
 
+	job.Lock()
+	defer job.Unlock()
 	job.setLastRun(s.now())
 	job.runCount++
 	s.executor.jobFunctions <- job.jobFunction
@@ -436,6 +438,14 @@ func (s *Scheduler) LimitRunsTo(i int) *Scheduler {
 	return s
 }
 
+// SingletonMode prevents a new job from starting if the prior job has not yet
+// completed it's run
+func (s *Scheduler) SingletonMode() *Scheduler {
+	job := s.getCurrentJob()
+	job.SingletonMode()
+	return s
+}
+
 // RemoveAfterLastRun sets the job to be removed after it's last run (when limited)
 func (s *Scheduler) RemoveAfterLastRun() *Scheduler {
 	job := s.getCurrentJob()
diff --git a/scheduler_test.go b/scheduler_test.go
index e66f12cf..edba3be2 100644
--- a/scheduler_test.go
+++ b/scheduler_test.go
@@ -4,12 +4,12 @@ import (
 	"fmt"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
-	"github.com/stretchr/testify/require"
-
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 var _ timeWrapper = (*fakeTime)(nil)
@@ -1144,6 +1144,29 @@ func TestCalculateMonths(t *testing.T) {
 	assert.Equal(t, s.time.Now(s.location).AddDate(0, 1, 0).Month(), job.nextRun.Month())
 }
 
+func TestScheduler_SingletonMode(t *testing.T) {
+	t.Run("next run of long running job doesn't overrun", func(t *testing.T) {
+		//semaphore := make(chan bool)
+
+		s := NewScheduler(time.UTC)
+		var trigger int32
+
+		_, err := s.Every(1).Second().SingletonMode().Do(func() {
+			if atomic.LoadInt32(&trigger) == 1 {
+				t.Fatal("Restart should not occur")
+			}
+			atomic.AddInt32(&trigger, 1)
+			fmt.Println("I am a long task")
+			time.Sleep(3 * time.Second)
+		})
+		require.NoError(t, err)
+
+		s.StartAsync()
+		time.Sleep(2 * time.Second)
+		s.Stop()
+	})
+}
+
 func TestScheduler_LimitRunsTo(t *testing.T) {
 	t.Run("job added before starting scheduler", func(t *testing.T) {
 		semaphore := make(chan bool)

From 22dc2b6731e1849d018f2c6dbb563c5fc6ce97ae Mon Sep 17 00:00:00 2001
From: John Roesler <johnrroesler@gmail.com>
Date: Fri, 19 Feb 2021 16:49:11 -0600
Subject: [PATCH 2/2] update readme

---
 README.md | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index b77a75b0..5c78502f 100644
--- a/README.md
+++ b/README.md
@@ -38,9 +38,9 @@ Looking to contribute? Try to follow these guidelines:
 * For bigger changes, please open an issue for discussion before sending a PR.
 * PRs should have: tests, documentation and examples (if it makes sense)
 * You can also contribute by:
-   * Reporting issues
-   * Suggesting new features or enhancements
-   * Improving/fixing documentation
+  * Reporting issues
+  * Suggesting new features or enhancements
+  * Improving/fixing documentation
 ---
 
 [Jetbrains](https://www.jetbrains.com/?from=gocron) supports this project with GoLand licenses. We appreciate their support for free and open source software!