semasleep on Darwin was refactored in https://golang.org/cl/118736 to
use the pthread_cond_timedwait function from libc. The new code
incorrectly assumed that pthread_cond_timedwait took a timeout relative
to the current time, when it in fact it takes a timeout specified in
absolute time. semasleep thus specified a timeout well in the past,
causing it to immediately exceed the timeout and spin hot. This was the
source of a large performance hit to CockroachDB (#26019).

Adjust semasleep to instead call pthread_cond_timedwait_relative_np,
which properly interprets its timeout parameter as relative to the
current time.

pthread_cond_timedwait_relative_np is non-portable, but using
pthread_cond_timedwait correctly would require two calls to
gettimeofday: one in the runtime package to convert the relative timeout
to absolute time, then another in the pthread library to convert back to
a relative offset [0], as the Darwin kernel expects a relative offset.

[0]: https://opensource.apple.com/source/libpthread/libpthread-301.30.1/src/pthread_cond.c.auto.html

Fix #26019.

Change-Id: I1a8c2429f79513b43d2b256365cd9166d235af8b
Reviewed-on: https://go-review.googlesource.com/120635Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>