runtime: skip /proc reads when containermaxprocs=0

[zheliu2]

When GODEBUG=containermaxprocs=0, skip reading /proc/self/cgroup and
/proc/self/mountinfo entirely in defaultGOMAXPROCSInit. Previously
these files were read unconditionally at startup, then the result was
discarded when the GODEBUG check failed. This caused permission denied
log noise in confined environments like Snap packages.

The fix moves the debug.containermaxprocs check before the
cgroup.OpenCPU call so no file I/O occurs when the feature is disabled.

Fixes #77911

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Jorropo:

Patch Set 1: Commit-Queue+1

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-03-13T01:05:32Z","revision":"3d57cadbea06e3f47b425573f5f74018f07abebc"}

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Patch Set 1: -Commit-Queue

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Message from Go LUCI:

Patch Set 1:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-amd64-sizespecializedmalloc has failed with summary (view all results):

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• go on master (change 754941)

To reproduce, try gomote repro 8687486837906255265.

Additional links for debugging:

• go tool dist test -json output [shard 1]

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result-1

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Message from Michael Knyszek:

Patch Set 2:

(1 comment)

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Message from zhe liu:

Patch Set 2:

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