semaphore: unblock waiters when the front waiter cancels

When `Release`, if the remaining tokens are not enough for the front waiter, no waiters will be notified.

So if the canceled waiter is the front one, it should try to notify following waiters if there are remaining tokens.

I found this bug when implementing a cancelable rwmutex based on semaphore:

https://github.com/zhiqiangxu/util/blob/master/mutex/crwmutex.go

This bug can be verified by this test:

https://github.com/zhiqiangxu/util/blob/master/mutex/crwmutex_test.go#L43

Change-Id: Id8564976bd375a82c4fbc6cb08b0bb83118a346c
GitHub-Last-Rev: 29b6ff26bf779d23239cfe64a395378a9e41d1fc
GitHub-Pull-Request: golang/sync#10
Reviewed-on: https://go-review.googlesource.com/c/sync/+/223418
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
2 files changed
tree: 0b1ec05e469fc2098a516643847d609b15aaeb9a
  1. AUTHORS
  2. CONTRIBUTING.md
  3. CONTRIBUTORS
  4. LICENSE
  5. PATENTS
  6. README.md
  7. codereview.cfg
  8. errgroup/
  9. go.mod
  10. semaphore/
  11. singleflight/
  12. syncmap/
README.md

Go Sync

This repository provides Go concurrency primitives in addition to the ones provided by the language and “sync” and “sync/atomic” packages.

Download/Install

The easiest way to install is to run go get -u golang.org/x/sync. You can also manually git clone the repository to $GOPATH/src/golang.org/x/sync.

Report Issues / Send Patches

This repository uses Gerrit for code changes. To learn how to submit changes to this repository, see https://golang.org/doc/contribute.html.

The main issue tracker for the sync repository is located at https://github.com/golang/go/issues. Prefix your issue with “x/sync:” in the subject line, so it is easy to find.