| // Copyright 2019 The LUCI 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 errctx provides implementations of context.Context that allow for |
| // cancellation or deadline expiration with custom error messages. |
| package errctx |
| |
| import ( |
| "context" |
| "sync" |
| "time" |
| ) |
| |
| // cancelContext is an implementation of context.Context, in which the context |
| // can be cancelled with a custom error. |
| type cancelContext struct { |
| parent context.Context |
| |
| cancelOnce sync.Once |
| errMutex sync.RWMutex |
| done chan struct{} |
| err error |
| } |
| |
| // Deadline implements context.Context. |
| func (c *cancelContext) Deadline() (time.Time, bool) { |
| return c.parent.Deadline() |
| } |
| |
| // Done implements context.Context. |
| func (c *cancelContext) Done() <-chan struct{} { |
| return c.done |
| } |
| |
| // Err implements context.Context. |
| func (c *cancelContext) Err() error { |
| c.errMutex.RLock() |
| e := c.err |
| c.errMutex.RUnlock() |
| return e |
| } |
| |
| // Value implements context.Context. |
| func (c *cancelContext) Value(key interface{}) interface{} { |
| return c.parent.Value(key) |
| } |
| |
| // cancel cancels the context with the given error. |
| // |
| // If no error is provided, context.Canceled is inferred. |
| func (c *cancelContext) cancel(err error) { |
| c.cancelOnce.Do(func() { |
| c.errMutex.Lock() |
| if err == nil { |
| c.err = context.Canceled |
| } else { |
| c.err = err |
| } |
| close(c.done) |
| c.errMutex.Unlock() |
| }) |
| } |
| |
| // propagate launches a goroutine that propagates context cancellation from |
| // parent context. |
| func (c *cancelContext) propagate() { |
| go func() { |
| select { |
| case <-c.parent.Done(): |
| c.cancel(c.parent.Err()) |
| case <-c.Done(): |
| } |
| }() |
| } |
| |
| func newCancelContext(parent context.Context) *cancelContext { |
| c := &cancelContext{ |
| parent: parent, |
| done: make(chan struct{}), |
| } |
| c.propagate() |
| return c |
| } |
| |
| // WithCancel returns a child context with a cancellation function that accepts |
| // custom errors. |
| func WithCancel(parent context.Context) (context.Context, func(error)) { |
| c := newCancelContext(parent) |
| return c, c.cancel |
| } |