context: don't return a non-nil from Err before Done is closed
The Context.Err documentation states that it returns nil if the
context's done channel is not closed. Fix a race condition introduced
by CL 653795 where Err could return a non-nil error slightly before
the Done channel is closed.
No impact on Err performance when returning nil.
Slows down Err when returning non-nil by about 3x,
but that's still almost 2x faster than before CL 653795
and the performance of this path is less important.
(A tight loop checking Err for doneness will be terminated
by the first Err call to return a non-nil result.)
goos: darwin
goarch: arm64
pkg: context
cpu: Apple M4 Pro
│ /tmp/bench.0 │ /tmp/bench.1 │
│ sec/op │ sec/op vs base │
ErrOK-14 1.806n ± 1% 1.774n ± 0% -1.77% (p=0.000 n=8)
ErrCanceled-14 1.821n ± 1% 7.525n ± 3% +313.23% (p=0.000 n=8)
geomean 1.813n 3.654n +101.47%
Fixes #75533
Change-Id: Iea22781a199ace7e7f70cf65168c36e090cd2e2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/705235
TryBot-Bypass: Damien Neil <dneil@google.com> Reviewed-by: Nicholas Husin <husin@google.com> Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>