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context: skip the Done receive in Err when the channel is closedchan - #80915

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context: skip the Done receive in Err when the channel is closedchan#80915
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@davidteather davidteather commented Aug 17, 2026 •
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Err receives from the Done channel so that a non-nil error implies the
channel is closed. A blocking receive takes hchan.lock even when the
channel is already closed, and Done is lazy. Until something calls it,
cancel stores the package-level closedchan. Every canceled context in
the program shares one lock.

BenchmarkErrCanceledParallelDistinct gives each goroutine its own
context and is just as slow as the shared-context case, which is what
identifies the shared global rather than any per-context state.

closedchan is closed at init and cannot reopen, so observing it needs
no synchronization. Skip the receive in that case. Done still supplies
the channel, so a cancel in flight blocks on c.mu as before.

goos: darwin
goarch: arm64
pkg: context
cpu: Apple M2 Pro
│ old │ new │
│ sec/op │ sec/op vs base │
ErrOK-10 2.017n ± 2% 1.979n ± 2% ~ (p=0.427 n=25)
ErrCanceled-10 14.000n ± 2% 3.973n ± 2% -71.62% (p=0.000 n=25)
ErrOKParallel-10 0.3072n ± 7% 0.3191n ± 3% ~ (p=0.184 n=25)
ErrCanceledParallel-10 36.1900n ± 1% 0.7030n ± 8% -98.06% (p=0.000 n=25)
ErrCanceledParallelDistinct-10 36.8500n ± 2% 0.6554n ± 9% -98.22% (p=0.000 n=25)
geomean 6.496n 1.029n -84.15%

B/op and allocs/op are unchanged. ErrOK and ErrOKParallel return before
the changed branch.

Fixes #80914

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gopherbot commented Aug 17, 2026

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This PR (HEAD: f2884a2) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/815961.

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@gopherbot

gopherbot commented Aug 17, 2026

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
After addressing review feedback, remember to publish your drafts!

@davidteather
context: skip the Done receive in Err when the channel is closedchan
86eaaf2 
Err receives from the Done channel so that a non-nil error implies the
channel is closed. A blocking receive takes hchan.lock even when the
channel is already closed, and Done is lazy. Until something calls it,
cancel stores the package-level closedchan. Every canceled context in
the program shares one lock.

BenchmarkErrCanceledParallelDistinct gives each goroutine its own
context and is just as slow as the shared-context case, which is what
identifies the shared global rather than any per-context state.

closedchan is closed at init and cannot reopen, so observing it needs
no synchronization. Skip the receive in that case. Done still supplies
the channel, so a cancel in flight blocks on c.mu as before.

goos: darwin
goarch: arm64
pkg: context
cpu: Apple M2 Pro
                               │      old      │                 new                  │
                               │    sec/op     │    sec/op     vs base                │
ErrOK-10                           2.017n ± 2%    1.979n ± 2%        ~ (p=0.427 n=25)
ErrCanceled-10                    14.000n ± 2%    3.973n ± 2%  -71.62% (p=0.000 n=25)
ErrOKParallel-10                  0.3072n ± 7%   0.3191n ± 3%        ~ (p=0.184 n=25)
ErrCanceledParallel-10           36.1900n ± 1%   0.7030n ± 8%  -98.06% (p=0.000 n=25)
ErrCanceledParallelDistinct-10   36.8500n ± 2%   0.6554n ± 9%  -98.22% (p=0.000 n=25)
geomean                            6.496n         1.029n       -84.15%

B/op and allocs/op are unchanged. ErrOK and ErrOKParallel return before
the changed branch.

Fixes golang#80914
@davidteather
davidteather force-pushed the dteather/context-closedchan branch from f2884a2 to 86eaaf2 Compare August 18, 2026 00:02
@gopherbot

gopherbot commented Aug 18, 2026

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This PR (HEAD: 86eaaf2) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/815961.

Important tips:

  • Don't comment on this PR. All discussion takes place in Gerrit.
  • You need a Gmail or other Google account to log in to Gerrit.
  • To change your code in response to feedback:
    • Push a new commit to the branch used by your GitHub PR.
    • A new "patch set" will then appear in Gerrit.
    • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
    • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
    • Multiple commits in the PR will be squashed by GerritBot.
  • The title and description of the GitHub PR are used to construct the final commit message.
    • Edit these as needed via the GitHub web interface (not via Gerrit or git).
    • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
  • See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

@gopherbot

gopherbot commented Aug 18, 2026

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Message from David Teather:

Patch Set 2:

(1 comment)

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
After addressing review feedback, remember to publish your drafts!

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gopherbot commented Aug 18, 2026

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Message from Ian Lance Taylor:

Patch Set 2: Commit-Queue+1

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
After addressing review feedback, remember to publish your drafts!

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gopherbot commented Aug 18, 2026

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-08-18T00:18:48Z","revision":"3ba348a463a306afc069c4b2d024136c6f4695da"}

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
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gopherbot commented Aug 18, 2026

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Message from Ian Lance Taylor:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_5206>)

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
After addressing review feedback, remember to publish your drafts!

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

This CL has passed the run

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
After addressing review feedback, remember to publish your drafts!

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2: LUCI-TryBot-Result+1

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
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Message from Ian Lance Taylor:

Patch Set 2: Auto-Submit+1 Code-Review+2

(1 comment)

Please don’t reply on this GitHub thread. Visit golang.org/cl/815961.
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context: Err on a canceled context contends on a process global channel lock

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