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fix: avoid panic from releasing unacquired semaphore (#363)
When using this pattern:
```go
eg, egCtx := errgroup.WithContext(ctx)
for /* ... */ {
limiter.Acquire(ctx, 1)
eg.Go(func() error {
defer limiter.Release(1)
// ...
return nil
})
}
if err := eg.Wait(); err != nil {
return err
}
```
If `ctx` is cancelled then `Acquire` may return an error, which will
then cause `Release` to panic after it's called more times than the
semaphore was successfully acquired.
This change uses the
[SetLimit](https://pkg.go.dev/golang.org/x/sync@v0.1.0/errgroup#Group.SetLimit)
functionality which has been added to `errgroup.Group` and provides a
small wrapper around it in the internal/syncutil package to improve the
ergonomics of its usage.
Signed-off-by: Andy Bursavich <abursavich@gmail.com>- Loading branch information
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,67 @@ | ||
| /* | ||
| Copyright The ORAS Authors. | ||
| Licensed under the Apache License, Version 2.0 (the "License"); | ||
| you may not use this file except in compliance with the License. | ||
| You may obtain a copy of the License at | ||
| http://www.apache.org/licenses/LICENSE-2.0 | ||
| Unless required by applicable law or agreed to in writing, software | ||
| distributed under the License is distributed on an "AS IS" BASIS, | ||
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| See the License for the specific language governing permissions and | ||
| limitations under the License. | ||
| */ | ||
|
|
||
| package syncutil | ||
|
|
||
| import ( | ||
| "context" | ||
|
|
||
| "golang.org/x/sync/errgroup" | ||
| ) | ||
|
|
||
| // A LimitedGroup is a collection of goroutines working on subtasks that are part of | ||
| // the same overall task. | ||
| type LimitedGroup struct { | ||
| grp *errgroup.Group | ||
| ctx context.Context | ||
| } | ||
|
|
||
| // LimitGroup returns a new LimitedGroup and an associated Context derived from ctx. | ||
| // | ||
| // The number of active goroutines in this group is limited to the given limit. | ||
| // A negative value indicates no limit. | ||
| // | ||
| // The derived Context is canceled the first time a function passed to Go | ||
| // returns a non-nil error or the first time Wait returns, whichever occurs | ||
| // first. | ||
| func LimitGroup(ctx context.Context, limit int) (*LimitedGroup, context.Context) { | ||
| grp, ctx := errgroup.WithContext(ctx) | ||
| grp.SetLimit(limit) | ||
| return &LimitedGroup{grp: grp, ctx: ctx}, ctx | ||
| } | ||
|
|
||
| // Go calls the given function in a new goroutine. | ||
| // It blocks until the new goroutine can be added without the number of | ||
| // active goroutines in the group exceeding the configured limit. | ||
| // | ||
| // The first call to return a non-nil error cancels the group's context. | ||
| // After which, any subsequent calls to Go will not execute their given function. | ||
| // The error will be returned by Wait. | ||
| func (g *LimitedGroup) Go(f func() error) { | ||
| g.grp.Go(func() error { | ||
| select { | ||
| case <-g.ctx.Done(): | ||
| return g.ctx.Err() | ||
| default: | ||
| return f() | ||
| } | ||
| }) | ||
| } | ||
|
|
||
| // Wait blocks until all function calls from the Go method have returned, then | ||
| // returns the first non-nil error (if any) from them. | ||
| func (g *LimitedGroup) Wait() error { | ||
| return g.grp.Wait() | ||
| } |