From 88cda2aeaea01ab3d482c3dcdea422f502f3e30b Mon Sep 17 00:00:00 2001
From: Lucas Godoy <godoy.lucas.e@gmail.com>
Date: Thu, 1 Jul 2021 09:42:56 -0300
Subject: [PATCH] Replace WaitGroup using semphore instead

---
 wpool/exec.go      | 56 +++++++++++++++++++++++++++++++++++++---------
 wpool/exec_test.go | 20 +++++++++--------
 2 files changed, 56 insertions(+), 20 deletions(-)

diff --git a/wpool/exec.go b/wpool/exec.go
index f9057a6..2efdfd2 100644
--- a/wpool/exec.go
+++ b/wpool/exec.go
@@ -3,11 +3,10 @@ package wpool
 import (
 	"context"
 	"fmt"
-	"sync"
 )
 
-func worker(ctx context.Context, wg *sync.WaitGroup, jobs <-chan Job, results chan<- Result) {
-	defer wg.Done()
+func worker(ctx context.Context, jobs <-chan Job, results chan<- Result, sem semaphore) {
+	defer sem.release()
 	for {
 		select {
 		case job, ok := <-jobs:
@@ -26,11 +25,47 @@ func worker(ctx context.Context, wg *sync.WaitGroup, jobs <-chan Job, results ch
 	}
 }
 
+type semaphore interface {
+	acquire()
+	release()
+	wait()
+	close()
+}
+
+type slot struct{}
+type slots chan slot
+
+type execution struct {
+	slots slots
+}
+
+func newExecutionSlots(capacity int) execution {
+	slots := make(chan slot, capacity)
+	return execution{slots: slots}
+}
+
+func (e execution) acquire() {
+	e.slots <- slot{}
+}
+
+func (e execution) release() {
+	<-e.slots
+}
+
+func (e execution) wait() {
+	for i := 0; i < cap(e.slots); i++ {
+		e.slots <- slot{}
+	}
+}
+
+func (e execution) close() {
+	close(e.slots)
+}
+
 type WorkerPool struct {
 	workersCount int
 	jobs         chan Job
 	results      chan Result
-	Done         chan struct{}
 }
 
 func New(wcount int) WorkerPool {
@@ -38,23 +73,22 @@ func New(wcount int) WorkerPool {
 		workersCount: wcount,
 		jobs:         make(chan Job, wcount),
 		results:      make(chan Result, wcount),
-		Done:         make(chan struct{}),
 	}
 }
 
 func (wp WorkerPool) Run(ctx context.Context) {
-	var wg sync.WaitGroup
+	eSlots := newExecutionSlots(wp.workersCount)
+	defer eSlots.close()
 
 	for i := 0; i < wp.workersCount; i++ {
-		wg.Add(1)
+		eSlots.acquire()
 		// fan out worker goroutines
 		//reading from jobs channel and
 		//pushing calcs into results channel
-		go worker(ctx, &wg, wp.jobs, wp.results)
+		go worker(ctx, wp.jobs, wp.results, eSlots)
 	}
 
-	wg.Wait()
-	close(wp.Done)
+	eSlots.wait()
 	close(wp.results)
 }
 
@@ -63,7 +97,7 @@ func (wp WorkerPool) Results() <-chan Result {
 }
 
 func (wp WorkerPool) GenerateFrom(jobsBulk []Job) {
-	for i, _ := range jobsBulk {
+	for i := range jobsBulk {
 		wp.jobs <- jobsBulk[i]
 	}
 	close(wp.jobs)
diff --git a/wpool/exec_test.go b/wpool/exec_test.go
index 617c58b..b475cc9 100644
--- a/wpool/exec_test.go
+++ b/wpool/exec_test.go
@@ -27,7 +27,7 @@ func TestWorkerPool(t *testing.T) {
 		select {
 		case r, ok := <-wp.Results():
 			if !ok {
-				continue
+				return
 			}
 
 			i, err := strconv.ParseInt(string(r.Descriptor.ID), 10, 64)
@@ -39,8 +39,6 @@ func TestWorkerPool(t *testing.T) {
 			if val != int(i)*2 {
 				t.Fatalf("wrong value %v; expected %v", val, int(i)*2)
 			}
-		case <-wp.Done:
-			return
 		default:
 		}
 	}
@@ -56,12 +54,14 @@ func TestWorkerPool_TimeOut(t *testing.T) {
 
 	for {
 		select {
-		case r := <-wp.Results():
+		case r, ok := <-wp.Results():
+			if !ok {
+				return
+			}
+
 			if r.Err != nil && r.Err != context.DeadlineExceeded {
 				t.Fatalf("expected error: %v; got: %v", context.DeadlineExceeded, r.Err)
 			}
-		case <-wp.Done:
-			return
 		default:
 		}
 	}
@@ -77,12 +77,14 @@ func TestWorkerPool_Cancel(t *testing.T) {
 
 	for {
 		select {
-		case r := <-wp.Results():
+		case r, ok := <-wp.Results():
+			if !ok {
+				return
+			}
+
 			if r.Err != nil && r.Err != context.Canceled {
 				t.Fatalf("expected error: %v; got: %v", context.Canceled, r.Err)
 			}
-		case <-wp.Done:
-			return
 		default:
 		}
 	}