runtime: prevent time.Timer.Reset(0) from deadlocking testing/synctest tests

In Go 1.23+, timer channels behave synchronously. When we have a timer
channel (i.e. !async && t.isChan) we would lock the
runtime.timer.sendLock mutex at the beginning of
runtime.timer.modify()'s execution.

Calling time.Timer.Reset(0) within a testing/synctest test,
unfortunately, causes it to hang indefinitely. This is because the
runtime.timer.sendLock mutex ends up being locked twice before it could
be unlocked:

- When calling time.Timer.Reset(), runtime.timer.modify() would lock the
  mutex per usual.
- Due to the 0 argument, runtime.timer.modify() would also try to
  execute the bubbled timer immediately rather than adding them to a
  heap. However, in doing so, it uses runtime.timer.unlockAndRun(),
  which also locks the same mutex.

This CL solves this issue by making sure that a locked
runtime.timer.sendLock mutex is unlocked first, whenever we try to
execute bubbled timer immediately in the stack.

Fixes #76052

Change-Id: I66429b9bf6971400de95dcf2d5dc9670c3135492
Reviewed-on: https://go-review.googlesource.com/c/go/+/716883
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions