Remove mutex from ReusableGoroutinesPool (#610)

I find it unsettling that we need to acquire a mutex (a concurrency
primitive) to send data to a channel (another concurrency primitive).

I think this achieves the same effect (we can stop the worker
goroutines in a safe way) without having to mix them and leaving the
main code path clear (just try to send a job, or run a new goroutine).

Signed-off-by: Oleg Zaytsev <[email protected]>

concurrency/worker.go

@@ -1,44 +1,36 @@
 package concurrency
 
-import (
-	"sync"
-)
-
 // NewReusableGoroutinesPool creates a new worker pool with the given size.
 // These workers will run the workloads passed through Go() calls.
 // If all workers are busy, Go() will spawn a new goroutine to run the workload.
 func NewReusableGoroutinesPool(size int) *ReusableGoroutinesPool {
 	p := &ReusableGoroutinesPool{
-		jobs: make(chan func()),
+		jobs:   make(chan func()),
+		closed: make(chan struct{}),
 	}
 	for i := 0; i < size; i++ {
 		go func() {
-			for f := range p.jobs {
-				f()
+			for {
+				select {
+				case f := <-p.jobs:
+					f()
+				case <-p.closed:
+					return
+				}
 			}
 		}()
 	}
 	return p
 }
 
 type ReusableGoroutinesPool struct {
-	jobsMu sync.RWMutex
-	closed bool
 	jobs   chan func()
+	closed chan struct{}
 }
 
 // Go will run the given function in a worker of the pool.
 // If all workers are busy, Go() will spawn a new goroutine to run the workload.
 func (p *ReusableGoroutinesPool) Go(f func()) {
-	p.jobsMu.RLock()
-	defer p.jobsMu.RUnlock()
-
-	// If the pool is closed, run the function in a new goroutine.
-	if p.closed {
-		go f()
-		return
-	}
-
 	select {
 	case p.jobs <- f:
 	default:
@@ -51,8 +43,5 @@ func (p *ReusableGoroutinesPool) Go(f func()) {
 // Close does NOT wait for all jobs to finish, it is the caller's responsibility to ensure that in the provided workloads.
 // Close is intended to be used in tests to ensure that no goroutines are leaked.
 func (p *ReusableGoroutinesPool) Close() {
-	p.jobsMu.Lock()
-	defer p.jobsMu.Unlock()
-	p.closed = true
-	close(p.jobs)
+	close(p.closed)
 }