Pulling 0-sized golang chan - go

Question

My use-case is the following: I need to send POST requests to 0...N subscribers, which are represented by a targetUrl. I want to limitate the max number of goroutine to let's say, 100. My code (simplified) is the following:
package main
import (
"fmt"
"log"
"net/http"
"errors"
)
const MAX_CONCURRENT_NOTIFICATIONS = 100
type Subscription struct {
TargetUrl string
}
func notifySubscribers(subs []Subscription) {
log.Println("notifySubscribers")
var buffer = make(chan Subscription, len(subs))
defer close(buffer)
for i := 0; i < MAX_CONCURRENT_NOTIFICATIONS; i++ {
go notifySubscriber(buffer)
}
for i := range subs {
buffer <- subs[i]
}
}
func notifySubscriber(buffer chan Subscription) {
log.Println("notifySubscriber")
for {
select {
case sub := <-buffer:
log.Println("sending notification to " + sub.TargetUrl)
resp, err := failPost()
if err != nil {
log.Println(fmt.Sprintf("failed to notify %s. error: %s", sub.TargetUrl, err.Error()))
} else {
resp.Body.Close()
if resp.StatusCode != http.StatusOK {
log.Println(fmt.Sprintf("%s responded with %d", sub.TargetUrl, resp.StatusCode))
}
}
}
log.Println(fmt.Sprintf("buffer size: %d", len(buffer)))
}
}
func failPost() (*http.Response, error) {
return &http.Response{
StatusCode: http.StatusBadRequest,
}, errors.New("some bad error")
}
func main() {
log.Println("main")
var subs []Subscription
subs = append(subs, Subscription{TargetUrl: "http://foo.bar"})
subs = append(subs, Subscription{TargetUrl: "http://fizz.buzz"})
notifySubscribers(subs)
select {}
}
The output is the following:
2018/01/24 10:52:48 failed to notify . error: some bad error
2018/01/24 10:52:48 buffer size: 1
2018/01/24 10:52:48 sending notification to
2018/01/24 10:52:48 failed to notify . error: some bad error
2018/01/24 10:52:48 buffer size: 0
2018/01/24 10:52:48 sending notification to
2018/01/24 10:52:48 failed to notify . error: some bad error
... and so on till I SIGINT the program
So basically it means that I've successfuly send the notifications to the right people, but I still continue to send to empty targetUrl because I read from an empty chan.
What is wrong ?
[EDIT] Workaround, but I don't like it
for {
select {
case sub, more := <-buffer:
if !more {
return
}
}
}

Answer 1

It's because you are closing the buffer but your notifySubscriber is still listening on the buffer. A closed channel always returns the default type value(in this case an empty Subscription with empty TargetURL). Hence, you are getting an empty string.
Scenarios:
If you want to keep the goroutines running, then don't close the buffer.
Stop the goroutines once the work is done and then close the buffer.

Answer 2

From the spec:
For a channel c, the built-in function close(c) records that no more
values will be sent on the channel. It is an error if c is a
receive-only channel. Sending to or closing a closed channel causes a
run-time panic. Closing the nil channel also causes a run-time panic.
After calling close, and after any previously sent values have been
received, receive operations will return the zero value for the
channel's type without blocking. The multi-valued receive operation
returns a received value along with an indication of whether the
channel is closed.
The last sentence means that sub, more := <-buffer, more will be false if buffer is closed.
However, in your case, the code can use some improvement.
First, it makes no sense to use a select statement where there are only one case. It would just act the same without the select.
Second, in cases that the recieving channel is guaranteed to return, range over channel can be used. So your code can be changed to:
func notifySubscriber(buffer chan Subscription) {
log.Println("notifySubscriber")
for sub:= range buffer {
//Code here...
}
}