How do you implement and use Context.Done?

[so.q...@gmail.com]

I'm not sure how to implement and use the Done function and its returned channel for contexts.
https://golang.org/pkg/context/#Context

The comments say to refer to https://blog.golang.org/pipelines, but I didn't see any example there.

Can you provide an example of how to implement a context that supports cancellation with Done?

[Ian Lance Taylor]

You aren't really expected to write your own implementations of the
Context interface. You are expected to start with context.Background
and modify that context using WithCancel, WithTimeout, WithValue, etc.

For example, if you write
ctx, cancel := ctx.WithCancel(context.Background())
Then ctx.Done() will be closed when you call cancel().

Ian

[so.q...@gmail.com]

Thanks, I see you build it up with decorators on a standard prototype.
For example: https://play.golang.org/p/PJy5lE9QqF

So context.Done is a convenience function for those that require it? Otherwise a context will expire after it leaves scope, so Done does not need to be called?

[Matt Harden]

Done does not need to be called unless you want to detect when the context is either canceled or times out. It doesn't have any effect on the context itself.

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[Sina Siadat]

Done function returns a channel which is closed when our context is done, i.e., it is either timed out, its deadline is exceeded, or explicitly canceled.

This channel closing is a pattern used for broadcasting a signal to multiple goroutines that are receiving from it. Let's say we have 2 goroutines that are receiving from the channel returned by Done. They are both blocked. When our context is done, both goroutines will unblock. They should interpret this channel closing as a notification that the context is done.

The reason it is designed like this, is that with the alternative normal send/receive to a channel, only one receiver receives the value sent to a channel. But with this pattern, all goroutines that are receiving from that channel will be notified.

Sina Siadat

[Ian Lance Taylor]

On Sun, Feb 19, 2017 at 2:57 PM, <so.q...@gmail.com> wrote:
> Thanks, I see you build it up with decorators on a standard prototype.
> For example: https://play.golang.org/p/PJy5lE9QqF
>
> So context.Done is a convenience function for those that require it?
> Otherwise a context will expire after it leaves scope, so Done does not need
> to be called?

Cancelling the context just marks the context as cancelled. It does
not actually stop any goroutines using the context. Those goroutines
must themselves periodically check the context to see whether it has
been cancelled, and, if so, stop working. They do that by calling
either the Done or Err method; it's much more common to call the Done
method (and check whether the channel is closed).

Ian

[so.q...@gmail.com]

I see thanks for the additional detail.

[Thomas Bushnell, BSG]

Calling the Err() method to see if the context has been cancelled is incorrect.  See https://godoc.corp.google.com/pkg/google3/go/context/context#Context.Err: "Err's return value is unspecified before Done is closed."

Thomas

[Axel Wagner]

Wrong list :) You meant to link here:

https://godoc.org/context#Context.Err

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[Ian Lance Taylor]

On Tue, Feb 21, 2017 at 11:15 AM, Thomas Bushnell, BSG
<tbus...@google.com> wrote:
> On Mon, Feb 20, 2017 at 11:42 AM Ian Lance Taylor <ia...@golang.org> wrote:
>>
>> On Sun, Feb 19, 2017 at 2:57 PM, <so.q...@gmail.com> wrote:
>> > Thanks, I see you build it up with decorators on a standard prototype.
>> > For example: https://play.golang.org/p/PJy5lE9QqF
>> >
>> > So context.Done is a convenience function for those that require it?
>> > Otherwise a context will expire after it leaves scope, so Done does not
>> > need
>> > to be called?
>>
>> Cancelling the context just marks the context as cancelled. It does
>> not actually stop any goroutines using the context. Those goroutines
>> must themselves periodically check the context to see whether it has
>> been cancelled, and, if so, stop working. They do that by calling
>> either the Done or Err method; it's much more common to call the Done
>> method (and check whether the channel is closed).
>
>
> Calling the Err() method to see if the context has been cancelled is
> incorrect.

...

> "Err's return value is unspecified before Done is closed."

Hmmm, that text is not in the version of the context package included
in the Go standard library. Perhaps it should be.

Ian

[Ian Lance Taylor]

Sent https://golang.org/cl/37375 .

Ian

[Thomas Bushnell, BSG]

Oops! ::blush::

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