Source File
	executor.go

Belonging Package
	github.com/go-co-op/gocron

package gocron

import (
	"context"
	"sync"
	"time"

	"go.uber.org/atomic"
)

const (
	// RescheduleMode - the default is that if a limit on maximum
	// concurrent jobs is set and the limit is reached, a job will
	// skip it's run and try again on the next occurrence in the schedule
	RescheduleMode limitMode = iota

	// WaitMode - if a limit on maximum concurrent jobs is set
	// and the limit is reached, a job will wait to try and run
	// until a spot in the limit is freed up.
	//
	// Note: this mode can produce unpredictable results as
	// job execution order isn't guaranteed. For example, a job that
	// executes frequently may pile up in the wait queue and be executed
	// many times back to back when the queue opens.
	//
	// Warning: do not use this mode if your jobs will continue to stack
	// up beyond the ability of the limit workers to keep up. An example of
	// what NOT to do:
	//
	//     s.Every("1s").Do(func() {
	//         // this will result in an ever-growing number of goroutines
	//    	   // blocked trying to send to the buffered channel
	//         time.Sleep(10 * time.Minute)
	//     })
	WaitMode
)

type executor struct {
	jobFunctions  chan jobFunction   // the chan upon which the jobFunctions are passed in from the scheduler
	ctx           context.Context    // used to tell the executor to stop
	cancel        context.CancelFunc // used to tell the executor to stop
	wg            *sync.WaitGroup    // used by the scheduler to wait for the executor to stop
	jobsWg        *sync.WaitGroup    // used by the executor to wait for all jobs to finish
	singletonWgs  *sync.Map          // used by the executor to wait for the singleton runners to complete
	skipExecution *atomic.Bool       // used to pause the execution of jobs

	limitMode               limitMode        // when SetMaxConcurrentJobs() is set upon the scheduler
	limitModeMaxRunningJobs int              // stores the maximum number of concurrently running jobs
	limitModeFuncsRunning   *atomic.Int64    // tracks the count of limited mode funcs running
	limitModeFuncWg         *sync.WaitGroup  // allow the executor to wait for limit mode functions to wrap up
	limitModeQueue          chan jobFunction // pass job functions to the limit mode workers
	limitModeQueueMu        *sync.Mutex      // for protecting the limitModeQueue
	limitModeRunningJobs    *atomic.Int64    // tracks the count of running jobs to check against the max
	stopped                 *atomic.Bool     // allow workers to drain the buffered limitModeQueue

	distributedLocker  Locker  // support running jobs across multiple instances
	distributedElector Elector // support running jobs across multiple instances
}

func newExecutor() executor {
	e := executor{
		jobFunctions:          make(chan jobFunction, 1),
		singletonWgs:          &sync.Map{},
		limitModeFuncsRunning: atomic.NewInt64(0),
		limitModeFuncWg:       &sync.WaitGroup{},
		limitModeRunningJobs:  atomic.NewInt64(0),
		limitModeQueueMu:      &sync.Mutex{},
	}
	return e
}

func runJob(f jobFunction) {
	panicHandlerMutex.RLock()
	defer panicHandlerMutex.RUnlock()

	if panicHandler != nil {
		defer func() {
			if r := recover(); r != nil {
				panicHandler(f.funcName, r)
			}
		}()
	}
	f.runStartCount.Add(1)
	f.isRunning.Store(true)
	callJobFunc(f.eventListeners.onBeforeJobExecution)
	_ = callJobFuncWithParams(f.eventListeners.beforeJobRuns, []interface{}{f.getName()})
	err := callJobFuncWithParams(f.function, f.parameters)
	if err != nil {
		_ = callJobFuncWithParams(f.eventListeners.onError, []interface{}{f.getName(), err})
	} else {
		_ = callJobFuncWithParams(f.eventListeners.noError, []interface{}{f.getName()})
	}
	_ = callJobFuncWithParams(f.eventListeners.afterJobRuns, []interface{}{f.getName()})
	callJobFunc(f.eventListeners.onAfterJobExecution)
	f.isRunning.Store(false)
	f.runFinishCount.Add(1)
}

func (jf *jobFunction) singletonRunner() {
	jf.singletonRunnerOn.Store(true)
	jf.singletonWgMu.Lock()
	jf.singletonWg.Add(1)
	jf.singletonWgMu.Unlock()
	for {
		select {
		case <-jf.ctx.Done():
			jf.singletonWg.Done()
			jf.singletonRunnerOn.Store(false)
			jf.singletonQueueMu.Lock()
			jf.singletonQueue = make(chan struct{}, 1000)
			jf.singletonQueueMu.Unlock()
			jf.stopped.Store(false)
			return
		case <-jf.singletonQueue:
			if !jf.stopped.Load() {
				runJob(*jf)
			}
		}
	}
}

func (e *executor) limitModeRunner() {
	for {
		select {
		case <-e.ctx.Done():
			e.limitModeFuncsRunning.Inc()
			e.limitModeFuncWg.Done()
			return
		case jf := <-e.limitModeQueue:
			if !e.stopped.Load() {
				select {
				case <-jf.ctx.Done():
				default:
					e.runJob(jf)
				}
			}
		}
	}
}

func (e *executor) start() {
	e.wg = &sync.WaitGroup{}
	e.wg.Add(1)

	stopCtx, cancel := context.WithCancel(context.Background())
	e.ctx = stopCtx
	e.cancel = cancel

	e.jobsWg = &sync.WaitGroup{}

	e.stopped = atomic.NewBool(false)
	e.skipExecution = atomic.NewBool(false)

	e.limitModeQueueMu.Lock()
	e.limitModeQueue = make(chan jobFunction, 1000)
	e.limitModeQueueMu.Unlock()
	go e.run()
}

func (e *executor) runJob(f jobFunction) {
	defer func() {
		if e.limitMode == RescheduleMode && e.limitModeMaxRunningJobs > 0 {
			e.limitModeRunningJobs.Add(-1)
		}
	}()
	switch f.runConfig.mode {
	case defaultMode:
		lockKey := f.jobName
		if lockKey == "" {
			lockKey = f.funcName
		}
		if e.distributedElector != nil {
			err := e.distributedElector.IsLeader(e.ctx)
			if err != nil {
				return
			}
			runJob(f)
			return
		}
		if e.distributedLocker != nil {
			l, err := e.distributedLocker.Lock(f.ctx, lockKey)
			if err != nil || l == nil {
				return
			}
			defer func() {
				durationToNextRun := time.Until(f.jobFuncNextRun)
				if durationToNextRun > time.Second*5 {
					durationToNextRun = time.Second * 5
				}

				delay := time.Duration(float64(durationToNextRun) * 0.9)
				if e.limitModeMaxRunningJobs > 0 {
					time.AfterFunc(delay, func() {
						_ = l.Unlock(f.ctx)
					})
					return
				}

				if durationToNextRun > time.Millisecond*100 {
					timer := time.NewTimer(delay)
					defer timer.Stop()

					select {
					case <-e.ctx.Done():
					case <-timer.C:
					}
				}
				_ = l.Unlock(f.ctx)
			}()
			runJob(f)
			return
		}
		runJob(f)
	case singletonMode:
		e.singletonWgs.Store(f.singletonWg, f.singletonWgMu)

		if !f.singletonRunnerOn.Load() {
			go f.singletonRunner()
		}
		f.singletonQueueMu.Lock()
		f.singletonQueue <- struct{}{}
		f.singletonQueueMu.Unlock()
	}
}

func (e *executor) run() {
	for {
		select {
		case f := <-e.jobFunctions:
			if e.stopped.Load() || e.skipExecution.Load() {
				continue
			}

			if e.limitModeMaxRunningJobs > 0 {
				countRunning := e.limitModeFuncsRunning.Load()
				if countRunning < int64(e.limitModeMaxRunningJobs) {
					diff := int64(e.limitModeMaxRunningJobs) - countRunning
					for i := int64(0); i < diff; i++ {
						e.limitModeFuncWg.Add(1)
						go e.limitModeRunner()
						e.limitModeFuncsRunning.Inc()
					}
				}
			}

			e.jobsWg.Add(1)
			go func() {
				defer e.jobsWg.Done()

				if e.limitModeMaxRunningJobs > 0 {
					switch e.limitMode {
					case RescheduleMode:
						if e.limitModeRunningJobs.Load() < int64(e.limitModeMaxRunningJobs) {
							select {
							case e.limitModeQueue <- f:
								e.limitModeRunningJobs.Inc()
							case <-e.ctx.Done():
							}
						}
					case WaitMode:
						select {
						case e.limitModeQueue <- f:
						case <-e.ctx.Done():
						}
					}
					return
				}

				e.runJob(f)
			}()
		case <-e.ctx.Done():
			e.jobsWg.Wait()
			e.wg.Done()
			return
		}
	}
}

func (e *executor) stop() {
	e.stopped.Store(true)
	e.cancel()
	e.wg.Wait()
	if e.singletonWgs != nil {
		e.singletonWgs.Range(func(key, value interface{}) bool {
			wg, wgOk := key.(*sync.WaitGroup)
			mu, muOk := value.(*sync.Mutex)
			if wgOk && muOk {
				mu.Lock()
				wg.Wait()
				mu.Unlock()
			}
			return true
		})
	}
	if e.limitModeMaxRunningJobs > 0 {
		e.limitModeFuncWg.Wait()
		e.limitModeQueueMu.Lock()
		e.limitModeQueue = nil
		e.limitModeQueueMu.Unlock()
	}
}

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