Prevent "send on closed channel" in purger goroutine

pond.go

@@ -88,11 +88,11 @@ type WorkerPool struct {
 	successfulTaskCount uint64
 	failedTaskCount     uint64
 	// Private properties
-	tasks     chan func()
-	stopOnce  sync.Once
-	waitGroup sync.WaitGroup
-	mutex     sync.Mutex
-	stopped   bool
+	tasks            chan func()
+	workersWaitGroup sync.WaitGroup
+	tasksWaitGroup   sync.WaitGroup
+	mutex            sync.Mutex
+	stopped          bool
 }
 
 // New creates a worker pool with that can scale up to the given maximum number of workers (maxWorkers).
@@ -242,12 +242,14 @@ func (p *WorkerPool) submit(task func(), mustSubmit bool) (submitted bool) {
 	// Increment submitted and waiting task counters as soon as we receive a task
 	atomic.AddUint64(&p.submittedTaskCount, 1)
 	atomic.AddUint64(&p.waitingTaskCount, 1)
+	p.tasksWaitGroup.Add(1)
 
 	defer func() {
 		if !submitted {
 			// Task was not sumitted to the pool, decrement submitted and waiting task counters
 			atomic.AddUint64(&p.submittedTaskCount, ^uint64(0))
 			atomic.AddUint64(&p.waitingTaskCount, ^uint64(0))
+			p.tasksWaitGroup.Done()
 		}
 	}()
 
@@ -328,46 +330,24 @@ func (p *WorkerPool) SubmitBefore(task func(), deadline time.Duration) {
 
 // Stop causes this pool to stop accepting new tasks and signals all workers to stop processing new tasks.
 // Tasks being processed by workers will continue until completion unless the process is terminated.
-// This method can only be called once.
 func (p *WorkerPool) Stop() {
-	p.stopOnce.Do(func() {
-		// Mark pool as stopped
-		p.stopped = true
-
-		// Stop accepting new tasks
-		close(p.tasks)
-
-		// Terminate all workers & purger goroutine
-		p.contextCancel()
-	})
+	p.stop(false)
 }
 
 // StopAndWait causes this pool to stop accepting new tasks and then waits for all tasks in the queue
-// to complete before returning. This method can only be called once.
+// to complete before returning.
 func (p *WorkerPool) StopAndWait() {
-	p.stopOnce.Do(func() {
-		// Mark pool as stopped
-		p.stopped = true
-
-		// Stop accepting new tasks
-		close(p.tasks)
-
-		// Wait for all workers to exit
-		p.waitGroup.Wait()
-
-		// Terminate all workers & purger goroutine
-		p.contextCancel()
-	})
+	p.stop(true)
 }
 
 // StopAndWaitFor stops this pool and waits for all tasks in the queue to complete before returning
-// or until the given deadline is reached, whichever comes first. This method can only be called once.
+// or until the given deadline is reached, whichever comes first.
 func (p *WorkerPool) StopAndWaitFor(deadline time.Duration) {
 
-	// Detect if worker pool is already stopped
+	// Launch goroutine to detect when worker pool has stopped gracefully
 	workersDone := make(chan struct{})
 	go func() {
-		p.StopAndWait()
+		p.stop(true)
 		workersDone <- struct{}{}
 	}()
 
@@ -381,6 +361,25 @@ func (p *WorkerPool) StopAndWaitFor(deadline time.Duration) {
 	}
 }
 
+func (p *WorkerPool) stop(waitForCompletion bool) {
+
+	// Mark pool as stopped
+	p.stopped = true
+
+	// Wait for all queued tasks to complete
+	if waitForCompletion {
+		p.tasksWaitGroup.Wait()
+	}
+
+	// Terminate all workers & purger goroutine
+	p.contextCancel()
+
+	// Wait for all workers to exit
+	if waitForCompletion {
+		p.workersWaitGroup.Wait()
+	}
+}
+
 // purge represents the work done by the purger goroutine
 func (p *WorkerPool) purge() {
 
@@ -389,11 +388,9 @@ func (p *WorkerPool) purge() {
 
 	for {
 		select {
-		// Timed out waiting for any activity to happen, attempt to kill an idle worker
+		// Timed out waiting for any activity to happen, attempt to stop an idle worker
 		case <-idleTicker.C:
-			if p.IdleWorkers() > 0 && p.RunningWorkers() > p.minWorkers {
-				p.tasks <- nil
-			}
+			p.stopIdleWorker()
 		// Pool context was cancelled, exit
 		case <-p.context.Done():
 			return
@@ -401,6 +398,13 @@ func (p *WorkerPool) purge() {
 	}
 }
 
+// stopIdleWorker attempts to stop an idle worker by sending it a nil task
+func (p *WorkerPool) stopIdleWorker() {
+	if p.IdleWorkers() > 0 && p.RunningWorkers() > p.minWorkers && !p.Stopped() {
+		p.tasks <- nil
+	}
+}
+
 // startWorkers creates new worker goroutines to run the given tasks
 func (p *WorkerPool) maybeStartWorker(firstTask func()) bool {
 
@@ -426,6 +430,7 @@ func (p *WorkerPool) executeTask(task func()) {
 			// Invoke panic handler
 			p.panicHandler(panic)
 		}
+		p.tasksWaitGroup.Done()
 	}()
 
 	// Decrement waiting task count
@@ -451,7 +456,7 @@ func (p *WorkerPool) incrementWorkerCount() bool {
 	p.mutex.Unlock()
 
 	// Increment waiting group semaphore
-	p.waitGroup.Add(1)
+	p.workersWaitGroup.Add(1)
 
 	return true
 }
@@ -464,7 +469,7 @@ func (p *WorkerPool) decrementWorkerCount() {
 	p.mutex.Unlock()
 
 	// Decrement waiting group semaphore
-	p.waitGroup.Done()
+	p.workersWaitGroup.Done()
 }
 
 // Group creates a new task group

pond_test.go

@@ -1,6 +1,7 @@
 package pond
 
 import (
+	"sync/atomic"
 	"testing"
 	"time"
 )
@@ -29,3 +30,19 @@ func TestNewWithInconsistentOptions(t *testing.T) {
 	assertEqual(t, 1, pool.minWorkers)
 	assertEqual(t, defaultIdleTimeout, pool.idleTimeout)
 }
+
+func TestPurgeAfterPoolStopped(t *testing.T) {
+
+	pool := New(1, 1)
+
+	var doneCount int32
+	pool.SubmitAndWait(func() {
+		atomic.AddInt32(&doneCount, 1)
+	})
+	assertEqual(t, int32(1), atomic.LoadInt32(&doneCount))
+	assertEqual(t, 1, pool.RunningWorkers())
+
+	// Simulate purger goroutine attempting to stop a worker after tasks channel is closed
+	pool.stopped = true
+	pool.stopIdleWorker()
+}