8.9 Cancellation
Sometimes we need to instruct a goroutine to stop what it is doing, for example, in a web server performing a computation on behalf of a client that has disconnected.
There is no way for one goroutine to terminate another directly, since
that would leave all its shared variables in undefined states.
In the rocket launch program (§8.7)
we sent a single value on a channel named abort, which the
countdown goroutine interpreted as a request to stop itself.
But what if we need to cancel two goroutines, or an arbitrary number?
One possibility might be to send as many events on the abort
channel as there are goroutines to cancel.
If some of the goroutines have already terminated themselves, however, our
count will be too large, and our sends will get stuck.
On the other hand, if those goroutines have spawned other
goroutines, our count will be too small, and some goroutines will
remain unaware of the cancellation.
In general, it’s hard to know how many goroutines are working on
our behalf at any given moment.
Moreover, when a goroutine receives a value from the abort
channel, it consumes that value so that other goroutines won’t see it.
For cancellation, what we need is a reliable mechanism to
broadcast
an event over a channel so that many goroutines can see
it as it occurs and can later see that it has occurred.
Recall that after a channel has been closed and drained of all sent values, subsequent receive operations proceed immediately, yielding zero values. We can exploit this to create a broadcast mechanism: don’t send a value on the channel, close it.
We can add cancellation to the du program from the previous
section with a few simple changes.
First, we create a cancellation channel on which no values are ever
sent, but whose closure indicates that it is time for the program to
stop what it is doing.
We also define a utility function, cancelled, that
checks or polls the cancellation state at the instant it is called.
var done = make(chan struct{})
func cancelled() bool {
select {
case <-done:
return true
default:
return false
}
}
Next, we create a goroutine that will read from the standard
input, which is typically connected to the terminal.
As soon as any input is read (for instance, the user presses the return key),
this goroutine broadcasts the cancellation by closing the
done channel.
// Cancel traversal when input is detected.
go func() {
os.Stdin.Read(make([]byte, 1)) // read a single byte
close(done)
}()
Now we need to make our goroutines respond to the cancellation.
In the main goroutine, we add a third case to the select statement
that tries to receive from the done channel.
The function returns if this case is ever selected, but before it
returns it must first drain the fileSizes channel, discarding
all values until the channel is closed.
It does this to ensure that any active calls to walkDir can run
to completion without getting stuck sending to fileSizes.
for {
select {
case <-done:
// Drain fileSizes to allow existing goroutines to finish.
for range fileSizes {
// Do nothing.
}
return
case size, ok := <-fileSizes:
// ...
}
}
The walkDir goroutine polls the cancellation status when it
begins, and returns without doing anything if the status is set.
This turns all goroutines created after cancellation into no-ops:
func walkDir(dir string, n *sync.WaitGroup, fileSizes chan<- int64) {
defer n.Done()
if cancelled() {
return
}
for _, entry := range dirents(dir) {
// ...
}
}
It might be profitable to poll the cancellation status again within
walkDir’s loop, to avoid creating goroutines after the
cancellation event.
Cancellation involves a trade-off; a quicker response often requires
more intrusive changes to program logic.
Ensuring that no expensive operations ever occur after the
cancellation event may require updating many places in your code, but
often most of the benefit can be obtained by checking for cancellation in
a few important places.
A little profiling of this program revealed that the bottleneck was
the acquisition of a semaphore token in dirents.
The select below makes this operation cancellable
and reduces the typical cancellation latency of the program from
hundreds of milliseconds to tens:
func dirents(dir string) []os.FileInfo {
select {
case sema <- struct{}{}: // acquire token
case <-done:
return nil // cancelled
}
defer func() { <-sema }() // release token
// ...read directory...
}
Now, when cancellation occurs, all the background goroutines quickly
stop and the main function returns.
Of course, when main returns, a program exits, so it can be
hard to tell a main function that cleans up after itself from
one that does not.
There’s a handy trick we can use during testing: if instead of
returning from main in the event of cancellation, we execute a
call to panic, then the runtime will dump the stack of
every goroutine in the program.
If the main goroutine is the only one left, then it has cleaned up
after itself.
But if other goroutines remain, they may not have been properly
cancelled, or perhaps they have been cancelled but the cancellation
takes time; a little investigation may be worthwhile.
The panic dump often contains sufficient
information to distinguish these cases.
Exercise 8.10:
HTTP requests may be cancelled by closing the optional Cancel
channel in the http.Request struct.
Modify the web crawler of Section 8.6 to
support cancellation.
Hint: the http.Get convenience function does not give you an
opportunity to customize a Request.
Instead, create the request using http.NewRequest, set
its Cancel field, then perform the request by calling
http.DefaultClient.Do(req).
Exercise 8.11:
Following the approach of mirroredQuery in Section 8.4.4, implement a variant of fetch that
requests several URLs concurrently.
As soon as the first response arrives, cancel the other requests.