Re: [go-nuts] Context cancellation: Is it sufficient to make long-running things interruptible?
The precise ordering of things is a bit non-deterministic at the fringe. If
precise ordering really matters (maybe for cancelling a stream like this, it
doesn’t, sometimes it does), a default case in the select statement is really
useful, and it also matters what DoSomething does.
From: golang-nuts@googlegroups.com on behalf of
Torsten Bronger
Sent: Monday, December 19, 2022 1:52:36 AM
To: golang-nuts@googlegroups.com
Subject: [go-nuts] Context cancellation: Is it sufficient to make long-running
things interruptible?
Hallöchen!
The context documentation gives this example:
// Stream generates values with DoSomething and sends them to out
// until DoSomething returns an error or ctx.Done is closed.
func Stream(ctx context.Context, out chan<- Value) error {
for {
v, err := DoSomething(ctx)
if err != nil {
return err
}
select {
case <-ctx.Done():
return ctx.Err()
case out <- v:
}
}
}
What if the channel “out” is drained very efficiently? Then, an
arbitrary number of loop iterations could happen before a
cancellation is detected, couldn’t it?
I would additionally check for ctx.Err() != nil somewhere in the
loop. Or is there a reason why this is not necessary?
Regards,
Torsten.
--
Torsten Bronger
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Re: [go-nuts] Context cancellation: Is it sufficient to make long-running things interruptible?
On Mon, Dec 19, 2022 at 11:01 AM Torsten Bronger
wrote:
> The context documentation gives this example:
>
> // Stream generates values with DoSomething and sends them to out
> // until DoSomething returns an error or ctx.Done is closed.
> func Stream(ctx context.Context, out chan<- Value) error {
> for {
> v, err := DoSomething(ctx)
> if err != nil {
> return err
> }
> select {
> case <-ctx.Done():
> return ctx.Err()
> case out <- v:
> }
> }
> }
>
> What if the channel “out” is drained very efficiently? Then, an
> arbitrary number of loop iterations could happen before a
> cancellation is detected, couldn’t it?
>From https://go.dev/ref/spec#Select_statements
2. If one or more of the communications can proceed, a single one that
can proceed is chosen via a uniform pseudo-random selection.
Selecting the send case in the above code, when both cases can
proceed, for N times in a row, should have on average a probability of
2^(-N).
-j
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[go-nuts] Context cancellation: Is it sufficient to make long-running things interruptible?
Hallöchen!
The context documentation gives this example:
// Stream generates values with DoSomething and sends them to out
// until DoSomething returns an error or ctx.Done is closed.
func Stream(ctx context.Context, out chan<- Value) error {
for {
v, err := DoSomething(ctx)
if err != nil {
return err
}
select {
case <-ctx.Done():
return ctx.Err()
case out <- v:
}
}
}
What if the channel “out” is drained very efficiently? Then, an
arbitrary number of loop iterations could happen before a
cancellation is detected, couldn’t it?
I would additionally check for ctx.Err() != nil somewhere in the
loop. Or is there a reason why this is not necessary?
Regards,
Torsten.
--
Torsten Bronger
--
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