can read from a closed channel

[kbkpbot]

Describe the bug

After a channel was closed, following read can read from the closed channel.

Reproduction Steps

cc.v

module main

fn main() {
        ch := chan int{cap: 10}
        mut received := []int{}

        ch <- 1
        ch.close()    // close channel here
        mut done := false
        for !done {
                select {
                        val := <-ch {
                                // should not be here
                                dump('received ch')
                                received << val
                        }
                        else {
                                // channel closed
                                dump('closed')
                                done = true
                        }
                }
        }

        dump(received)
}

Expected Behavior

$ ./cc
[cc.v:19] 'closed': closed
[cc.v:25] received: []

Current Behavior

$ ./cc
[cc.v:14] 'received ch': received ch
^C

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 566d22ab66b14655f1b1f2f4aa8396264ebf0c26

Environment details (OS name and version, etc.)

V full version	V 0.4.10 566d22ab66b14655f1b1f2f4aa8396264ebf0c26
OS	linux, Ubuntu 24.04.2 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	7.54GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2025-06-02 00:07:12
V home dir	OK, value: /media/HD/github/kbkpbot/v
VMODULES	OK, value: /home/mars/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/mars/v/bug/channel
Git version	git version 2.43.0
V git status	weekly.2025.17-228-g8f1bf53f-dirty
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.4) 2.39

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[huly-for-github[bot]]

Connected to Huly®: V_0.6-23036

[jorgeluismireles]

I don't know if is useful a channel in a single thread, but seems next complete select form mentioned in the docs, registers the whole history channel had:

fn main() {
    ch := chan int{cap: 10}
    mut received := []int{}
    ch <- 1
    ch <- 2
    ch.close()
    mut done := false
    for !done {
        if select {
            val := <-ch {
                received << val
                println('received: ${received}')
            }
        } {
            println('not closed yet')
        } else {
            println('closed')
            done = true
        }
    }
}

received: [1]
not closed yet
received: [1, 2]
not closed yet
closed

[kbkpbot]

close channel in another thread, also has this problem:

module main

fn main() {
        ch := chan int{cap: 10}
        block := chan bool{}
        mut received := []int{}

        spawn fn [block, ch] () {
                ch <- 1
                ch.close()
                block <- true
        }()

        _ := <-block

        mut done := false
        for !done {
                select {
                        val := <-ch {
                                // should not be here
                                dump('received ch')
                                received << val
                        }
                        else {
                                // channel closed
                                dump('channel closed')
                                done = true
                        }
                }
        }

        dump(received)
}

$ ./ccc
[ccc.v:21] 'received ch': received ch
^C

[kbkpbot]

how about go handle this?

[jorgeluismireles]

In go this fails (with close or defaul is the same):

func main() {
	c := make(chan int)
	c <- 43
	//close(c)	
	for {
		select {
			case i := <- c:
				fmt.Println(i)
			//default:
			//	println("?")
		}
	}
}

fatal error: all goroutines are asleep - deadlock!

You need more code to communicate:

func read(c chan int, s chan bool) {
	i := <- c
	fmt.Println(i)
	s<- true
}

func write(c chan int) {
	time.Sleep(1 * time.Second)
	c <- 43
	close(c)
}

func main() {
	c := make(chan int)
	s := make(chan bool)
	go read(c, s)
	go write(c)
	<-s
}

43