select in a for loop deadlock's - go

Question

https://play.golang.org/p/FyIUPkwq0R
Why does the following deadlock?
package main
import (
"fmt"
)
var quit chan bool
var buffer chan string
func main() {
buffer = make(chan string)
quit = make(chan bool)
go func() {
i:=0
for {
select {
case <- quit:
fmt.Println("Bye!")
return
default:
fmt.Println(<-buffer)
}
i++
fmt.Println(i)
}
}()
buffer <- "Go!"
quit <- true // This line dead locks
//buffer <- "Hello" // When I do this instead it works?
//quit <- true // Also when I don't quit it still exit's?
}

Answer 1

This code isn't guaranteed to work properly. You might get lucky, but apparently you've been getting unlucky. You might get lucky and the following might happen:
Let's say we have two goroutines, A and B, where A is the goroutine running main and B is the goroutine running the anonymous function. The following might happen:
B: Execute select; there's nobody writing on the quit channel, so execute the default case
B: Execute <-buffer, so begin blocking, waiting for somebody to write to buffer
A: Write "Go!" to buffer
B: Receive "Go!" and print it. Continue looping.
A: Write true to quit
B: Execute select; A is trying to write to quit, so execute that case. Print "Bye!" and return
A: Since write has finished, continue and return from main
However, this isn't guaranteed to happen. In particular, after reading from buffer, B might keep executing, execute the select, and fall into the default case before A has a chance to write to quit. That's what's probably happening, and would look like this:
B: Execute select; there's nobody writing on the quit channel, so execute the default case
B: Execute <-buffer, so begin blocking, waiting for somebody to write to buffer
A: Write "Go!" to buffer
B: Receive "Go!" and print it. Continue looping
B: Execute select; there's nobody writing on the quit channel, so execute the default case
B: Execute <-buffer, so begin blocking, waiting for somebody to write to buffer
A: Write true to quit, so begin blocking, waiting for somebody to read from quit
Now both A and B are blocked, and since there are no other goroutines in the system, no event can ever unblock either of them, so the system is stuck.
Solution
One way of fixing this would be to make it so that goroutine B reads from the buffer as one of the select cases instead of inside a select case. This way, the select will simply block until either channel becomes available for an action, and your code will behave as you probably wanted it to:
select {
case <-quit:
fmt.Println("Bye!")
return
case str := <-buffer:
fmt.Println(str)
}
See it here on the Go Playground.
Note, however, that since the main goroutine is returning as soon as it writes to the quit channel, and the entire Go program exits as soon as that happens, you may (and probably will) get unlucky and fmt.Println("Bye!") will not execute before the program quits.

Answer 2

Change the select default clause to
default:
fmt.Println("waiting on <-buffer")
fmt.Println(<-buffer)
to see what's going on.
The issue is that the goroutine executes the default branch in the select before main executes quit <- true.
The goroutine blocks at fmt.Println(<-buffer) and the main function blocks at quit <- true.
To prevent the deadlock, receive in the case statement:
select {
case <-quit:
fmt.Println("Bye!")
return
case msg := <-buffer:
fmt.Println(msg)
}
playground example

Answer 3

package main
import (
"fmt"
)
var quit chan bool
var buffer chan string
func main() {
buffer = make(chan string)
quit = make(chan bool)
go func() {
i := 0
for {
select {
case <-quit:
fmt.Println("Bye!")
return
case v := <-buffer:
fmt.Println(v)
default:
// 这里也可能 block
// fmt.Println(<-buffer)
}
i++
fmt.Println(i)
}
}()
buffer <- "Go!"
quit <- true // This line dead locks
//buffer <- "Hello" // When I do this instead it works?
//quit <- true // Also when I don't quit it still exit's?
}