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

[davidteather]

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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Patch Set 1:

(1 comment)

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Patch Set 2:

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Patch Set 2: Commit-Queue+1

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