os: follow-up pidfd fixes [freeze exception]

[prattmic]

This is a freeze exception request for feedback for https://go.dev/cl/588675. This CL is fixing #67634, #67640, and #67641, which are issues with the new pidfd support added in https://go.dev/cl/570036 and https://go.dev/cl/570681.

Since these are regressions, I don't think a freeze exception is strictly necessary, however the fix is a fairly invasive overhaul of the internals of os.Process, which I'd say has a medium risk of introducing new problems. The alternative to submitting this fix now would be to revert the pidfd changes and try again when the dev cycles opens again.

cc @golang/release @kolyshkin

[kolyshkin]

I'm still wrapping my head around CL 588675 but overall it makes sense. At the very least #67634 is definitely a bad regression that could hit users hard.

[prattmic]

At the very least #67634 is definitely a bad regression that could hit users hard.

Yes, this is the one issue where I am aware of actual failures (some tests inside Google fail). The others are just problems I noticed when fixing the first.

[dmitshur]

We reviewed this in a release meeting, and agreed to approve its "freeze exception" bit. Thanks for letting us know.

[prattmic]

https://go.dev/cl/588675 is merged.