runtime: make tidExists more robust

The LockThreadExit tests in the runtime have been observed to fail after
reading /proc/self/task/<tid>/stat and blindly assuming its contents
followed a specific format. The parsing code is also wrong, because
splitting by spaces doesn't work when the comm name contains a space.
It also ignores errors without reporting them, which isn't great.

This change rewrites tidExists to be more robust by using
/proc/self/task/<tid>/status instead. It also modifies tidExists'
signature to report an error to its caller. Its caller then prints that
error.

Ignoring a non-not-exist error with opening this file is the likely but
unconfirmed cause of #65736 (ESRCH). This change also checks for that
error explicitly as an optimistic fix.

Fixes #65736.

Change-Id: Iea560b457d514426da2781b7eb7b8616a91ec23b
Reviewed-on: https://go-review.googlesource.com/c/go/+/567938
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

0 files changed, 0 insertions, 0 deletions