Channels and Graceful shutdown deadlock - go

Question

Run the below program and run CTRL + C, the handle routine gets blocked as it is trying to send to a channel but the process routine has shutdown. What is a better concurrency design to solve this?
Edited the program to describe the problem applying the rules suggested here https://stackoverflow.com/a/66708290/4106031
package main
import (
"context"
"fmt"
"os"
"os/signal"
"sync"
"syscall"
"time"
)
func process(ctx context.Context, c chan string) {
fmt.Println("process: processing (select)")
for {
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case i := <-c:
fmt.Printf("process: received i: %v\n", i)
}
}
}
func handle(ctx context.Context, readChan <-chan string) {
c := make(chan string, 1)
wg := &sync.WaitGroup{}
wg.Add(1)
go func() {
process(ctx, c)
wg.Done()
}()
defer wg.Wait()
for i := 0; ; i++ {
select {
case <-ctx.Done():
fmt.Printf("handle: ctx done bye\n")
return
case i := <-readChan:
fmt.Printf("handle: received: %v\n", i)
fmt.Printf("handle: sending for processing: %v\n", i)
// suppose huge time passes here
// to cause the issue we want to happen
// we want the process() to exit due to ctx
// cancellation before send to it happens, this creates deadlock
time.Sleep(5 * time.Second)
// deadlock
c <- i
}
}
}
func main() {
wg := &sync.WaitGroup{}
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
readChan := make(chan string, 10)
wg.Add(1)
go func() {
defer wg.Done()
for i := 0; ; i++ {
select {
case <-ctx.Done():
fmt.Printf("read: ctx done bye\n")
return
case readChan <- fmt.Sprintf("%d", i):
fmt.Printf("read: sent msg: %v\n", i)
}
}
}()
wg.Add(1)
go func() {
handle(ctx, readChan)
wg.Done()
}()
go func() {
sigterm := make(chan os.Signal, 1)
signal.Notify(sigterm, syscall.SIGINT, syscall.SIGTERM)
select {
case <-sigterm:
fmt.Printf("SIGTERM signal received\n")
cancel()
}
}()
wg.Wait()
}
Output
$ go run chan-shared.go
read: sent msg: 0
read: sent msg: 1
read: sent msg: 2
read: sent msg: 3
process: processing (select)
read: sent msg: 4
read: sent msg: 5
read: sent msg: 6
handle: received: 0
handle: sending for processing: 0
read: sent msg: 7
read: sent msg: 8
read: sent msg: 9
read: sent msg: 10
handle: received: 1
handle: sending for processing: 1
read: sent msg: 11
process: received i: 0
process: received i: 1
read: sent msg: 12
handle: received: 2
handle: sending for processing: 2
^CSIGTERM signal received
process: ctx done bye
read: ctx done bye
handle: received: 3
handle: sending for processing: 3
Killed: 9

Answer 1

the step by step review
Always cancel context, whatever you think.
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
Dont wd.Add after starting a routine
wg.Add(1)
go handle(ctx, wg)
Dont sparsely consume waitgroups
wg.Add(1)
go func() {
handle(ctx)
wg.Done()
}()
dont for loop on a channel with a default case. Just read from it and let it unblocks
<-sigterm
fmt.Printf("SIGTERM signal received\n")
main never block on signals, main blocks on the processing routines. Signaling should just do signaling, ie cancel the context.
go func() {
sigterm := make(chan os.Signal, 1)
signal.Notify(sigterm, syscall.SIGINT, syscall.SIGTERM)
<-sigterm
fmt.Printf("SIGTERM signal received\n")
cancel()
}()
It is possible to check for context cancellation on channel writes.
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case c <- fmt.Sprintf("%d", i):
fmt.Printf("handled: sent to channel: %v\n", i)
}
Dont time.Sleep, you can t test for context cancellation with it.
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case <-time.After(time.Second * 5):
}
So a complete revised version of the code with those various rules applied gives us
package main
import (
"context"
"fmt"
"os"
"os/signal"
"sync"
"syscall"
"time"
)
func process(ctx context.Context, c chan string) {
fmt.Println("process: processing (select)")
for {
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case msg := <-c:
fmt.Printf("process: got msg: %v\n", msg)
}
}
}
func handle(ctx context.Context) {
c := make(chan string, 3)
wg := &sync.WaitGroup{}
wg.Add(1)
go func() {
process(ctx, c)
wg.Done()
}()
defer wg.Wait()
for i := 0; ; i++ {
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case <-time.After(time.Second * 5):
}
select {
case <-ctx.Done():
fmt.Printf("process: ctx done bye\n")
return
case c <- fmt.Sprintf("%d", i):
fmt.Printf("handled: sent to channel: %v\n", i)
}
}
}
func main() {
wg := &sync.WaitGroup{}
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
wg.Add(1)
go func() {
handle(ctx)
wg.Done()
}()
go func() {
sigterm := make(chan os.Signal, 1)
signal.Notify(sigterm, syscall.SIGINT, syscall.SIGTERM)
<-sigterm
fmt.Printf("SIGTERM signal received\n")
cancel()
}()
wg.Wait()
}
There is more to tell about exit conditions, but this is dependent on the requirements.

Answer 2

As mentioned https://stackoverflow.com/a/66708290/4106031 this change has fixed the issue for me. Thanks mh-cbon for the rules too!