Program hangs with channel - go

Question

I want to use goroutines to batch requests from different customers' with different date.
I mean 50 consumer goroutines to consume all customers from db, and 2 date consumer goroutines to consume date slice.
Main codes as below, but it hung and didn't exit as expected.
Why doesn't it exit as expected?
func Run(){
var syncWg sync.WaitGroup
syncWg.Add(1)
go SyncCustomerMetricsHistory(&syncWg)
syncWg.Wait()
}
func SyncCustomerMetricsHistory(wg *sync.WaitGroup){
defer wg.Done()
odb := orm.NewOrm()
start := time.Now()
logs.Info("start sync customer metrics, time:[%v]", start)
qs := odb.QueryTable("gg_customer")
var customers []*db.GgCustomer
if num, err := qs.All(&customers); err != nil || num == 0 {
logs.Error("Get customer error, rows:[%v], err:[%v]", num, err)
}
customersChan := make(chan *db.GgCustomer, 50)
var wgC sync.WaitGroup
wgC.Add(50)
for i := 0; i < 50; i++ {
go syncCustomerMetricsHistory(customersChan, &wgC)
}
go func() {
for _, customer := range customers {
customersChan <- customer
}
close(customersChan)
}()
wgC.Wait()
}
func syncCustomerMetricsHistory(customerChan <- chan *db.GgCustomer, wg *sync.WaitGroup){
defer wg.Done()
for customer := range customerChan{
dateChan := make(chan string, 2)
var wgD sync.WaitGroup
wgD.Add(2)
for i := 1; i < 2; i++{
go test(dateChan, customer, &wgD)
}
go func(){
for _, date := range GetAllYearDate(){
dateChan <- date
}
close(dateChan)
}()
wgD.Wait()
}
}
}
func test(dateChan <- chan string, customer *db.GgCustomer, wg *sync.WaitGroup){
defer wg.Done()
for date := range dateChan{
fmt.Println(date, customer)
}
}
func GetAllYearDate() []string{
return []string{"2019-10-01", "2019-10-02"}
}

Answer 1

I have not tried to run this (as it requires additional code) but believe your issue is:
wgD.Add(2)
for i := 1; i < 2; i++{
go test(dateChan, customer, &wgD)
}
That for loop will only iterate once but you called wgD.Add(2) (I think you probably meant the loop to iterate twice; try i <= 2).
One other bit of feedback; the way you are using waitgroups will work but is hard to follow (perhaps leading to you not spotting the issue); how about something like:
func Run(){
SyncCustomerMetricsHistory() // No wait group needed as this will not return before done
}
func SyncCustomerMetricsHistory(){
odb := orm.NewOrm()
start := time.Now()
logs.Info("start sync customer metrics, time:[%v]", start)
qs := odb.QueryTable("gg_customer")
var customers []*db.GgCustomer
if num, err := qs.All(&customers); err != nil || num == 0 {
logs.Error("Get customer error, rows:[%v], err:[%v]", num, err)
}
customersChan := make(chan *db.GgCustomer, 50)
var wgC sync.WaitGroup
wgC.Add(50)
for i := 0; i < 50; i++ {
go func() {
syncCustomerMetricsHistory(customersChan)
wgC.Done()
}()
}
go func() {
for _, customer := range customers {
customersChan <- customer
}
close(customersChan)
}()
wgC.Wait()
}
func syncCustomerMetricsHistory(customerChan <- chan *db.GgCustomer){
for customer := range customerChan{
dateChan := make(chan string, 2)
var wgD sync.WaitGroup
wgD.Add(2)
for i := 1; i < 2; i++{
go func() {
test(dateChan, customer)
wgD.Done()
}()
}
go func(){
for _, date := range GetAllYearDate(){
dateChan <- date
}
close(dateChan)
}()
wgD.Wait()
}
}
}
I think this is easier to follow because you can see where wg.Done() is being called. It's also really easy to stick some fmt.Println commands on either side which makes it simpler to debug this kind of issue.