Is `make(chan _, _)` atomic? - go

Question

Is it thread-safe to modify the channel that a consumer is reading from?
Consider the following code:
func main(){
channel := make(chan int, 3)
channel_ptr := &channel
go supplier (channel_ptr)
go consumer (channel_ptr)
temp = *channel_ptr
// Important bit
*channel_ptr = make(chan int, 5)
more := true
for more{
select {
case msg := <-temp:
*channel_ptr <- msg
default:
more = false
}
}
// Block main indefinitely to keep the children alive
<-make(chan bool)
}
func consumer(c *chan int){
for true{
fmt.Println(<-(*c))
}
}
func supplier(c *chan int){
for i :=0; i < 5; i ++{
(*c)<-i
}
}
If channels and make work the way that I want them to, I should get the following properties:
The program always outputs 0 1 2 3 4
The program will never panic from trying to read from a non-initialized channel (IE, the part I labelled Important bit is atomic)
From several test runs, this seems to be true, but I can't find it anywhere in the documentation and I'm worried about subtle race conditions.
Update
Yeah, what I was doing doesn't work. This thread is probably buried at this point, but does anybody know how to dynamically resize a buffered channel?

Answer 1

It's not thread safe.
If you run with -race flag to use race detector, you'll see the bug:
$ run -race t.go
==================
WARNING: DATA RACE
Write at 0x00c420086018 by main goroutine:
main.main()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:14 +0x128
Previous read at 0x00c420086018 by goroutine 6:
main.supplier()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:37 +0x51
Goroutine 6 (running) created at:
main.main()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:9 +0xb4
0
==================
1
2
3
==================
WARNING: DATA RACE
Read at 0x00c420086018 by goroutine 6:
main.supplier()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:37 +0x51
Previous write at 0x00c420086018 by main goroutine:
main.main()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:14 +0x128
Goroutine 6 (running) created at:
main.main()
/Users/kjk/src/go/src/github.com/kjk/go-cookbook/start-mysql-in-docker-go/t.go:9 +0xb4
==================
4
As a rule of thumb, you should never pass channel as a pointer. Channel already is a pointer internally.
Stepping back a bit: I don't understand what you're trying to achieve.
I guess there's a reason you're trying to pass a channel as a pointer. The pattern of using channels in Go is: you create it once and you pass it around as value. You don't pass a pointer to it and you never modify it after creation.
In your example the problem is that you have a shared piece of memory (memory address pointed to by channel_ptr) and you write to that memory in one thread while some other thread reads it. That's data race.
It's not specific to a channel, you would have the same issue if it was pointer to an int and two threads were modifying the value of an int.