Context cancellation and returned error

[Pierre Durand]

My code has a select with a case that waits for context cancellation.

Currently I'm doing this:

case <-ctx.Done():
return ctx.Err()

I noticed that some libraries and the stdlib are doing this:

case <-ctx.Done():
return context.Cause(ctx)

What is the best practice ?

[Axel Wagner]

The latter. ctx.Err() can only return the context-specific errors, while context.Cause can also return the error passed using WithCancelCause.

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[Pierre Durand]

Thank you for your answer.

Yes I understand the difference between the 2 functions.

Can we say it's a best practice to use return context.Cause(ctx) instead of return ctx.Err() ?

Is there a reason to use return ctx.Err() instead of return context.Cause(ctx) ?

[Axel Wagner]

On Sat, 4 Apr 2026 at 23:02, Pierre Durand <pierre...@gmail.com> wrote:

Thank you for your answer.

Yes I understand the difference between the 2 functions.

Can we say it's a best practice to use return context.Cause(ctx) instead of return ctx.Err() ?

That is what I said, yes.

 

Is there a reason to use return ctx.Err() instead of return context.Cause(ctx) ?

Probably. You might need to transfer an error over the wire and only have codes for "cancelled" and "timeout", for example. But in general, `context.Cause` has superseded `Context.Err`, the latter just can't be changed for compatibility reasons.

 

Le samedi 4 avril 2026 à 06:17:24 UTC+2, Axel Wagner a écrit :

The latter. ctx.Err() can only return the context-specific errors, while context.Cause can also return the error passed using WithCancelCause.

On Fri, 3 Apr 2026 at 19:43, Pierre Durand <pierre...@gmail.com> wrote:

My code has a select with a case that waits for context cancellation.

Currently I'm doing this:

case <-ctx.Done():
return ctx.Err()

I noticed that some libraries and the stdlib are doing this:

case <-ctx.Done():
return context.Cause(ctx)

What is the best practice ?

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