context: Err can return non-nil before Done channel is closed

[neild]

Originally reported by @marktheunissen here: #72040 (comment). Creating a new issue to track.

CL 653795 introduced a race condition in Context.Err, in which it can return a non-nil result before the context's done channel is closed. This violates the Err documentation:

// If Done is not yet closed, Err returns nil.

[gopherbot]

Change https://go.dev/cl/705235 mentions this issue: context: don't return a non-nil from Err before Done is closed

[neild]

@gopherbot please backport to go1.25. This is a regression.

[gopherbot]

Backport issue(s) opened: #75537 (for 1.25).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/705375 mentions this issue: [release-branch.go1.25] context: don't return a non-nil from Err before Done is closed