What happens to goroutines that are already blocked on a channel when it is closed?

closechan sets the closed flag under the channel lock, removes every waiting sudog from both recvq and sendq, and makes all of those goroutines runnable:

• Blocked receivers wake up with the zero value and ok == false. On an unbuffered channel, or a buffered one that is empty, nothing is left to drain.
• Blocked senders wake up and panic with send on closed channel, even though their send started before the close.

package main

import (
	"fmt"
	"time"
)

func main() {
	ch := make(chan int)
	for i := range 3 {
		go func() {
			defer func() { fmt.Println(i, recover()) }()
			ch <- i // all three park here
		}()
	}
	time.Sleep(100 * time.Millisecond) // demo only: let them block
	close(ch)
	time.Sleep(100 * time.Millisecond)
}

This prints three lines like 1 send on closed channel, in any order. Without the recover, the first panic would crash the whole program.

Why interviewers ask: it shows why "the receiver closes to tell senders to stop" is wrong. Senders that are in flight at that moment crash. A close is only safe once no send can still happen, which is why the owner/single-closer rule exists and why stop signals go through a separate done channel or context.