github.com/google/fleetspeak@v0.1.15-0.20240426164851-4f31f62c1aea/fleetspeak/src/common/fscontext/fscontext.go (about)

     1  // Copyright 2024 Google LLC
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package fscontext contains helpers for migrating Fleetspeak to context.Context.
    16  //
    17  // Fleetspeak should better use context.Context for cancelation than chan struct{}s.
    18  package fscontext
    19  
    20  import (
    21  	"context"
    22  	"fmt"
    23  	"sync"
    24  )
    25  
    26  // ErrStopRequested is the cancelation cause to be used when callers
    27  // intentionally cancel a context from the outside.
    28  var ErrStopRequested = fmt.Errorf("%w: stop requested", context.Canceled)
    29  
    30  // WithDoneChan returns a new context and cancel function.
    31  //
    32  // The context is automatically canceled with the given cause as soon as done is
    33  // closed.
    34  //
    35  // This is not meant as a long-term solution, but as a migration path for code
    36  // which has a "done" channel but which needs to provide a context.
    37  //
    38  // Callers must always call cancel after the context is done.
    39  //
    40  // Example usage:
    41  //
    42  //	 errDoneClosed := fmt.Errorf("my magic done channel was closed: %w", fscontext.ErrStopRequested)
    43  //		ctx, cancel := fscontext.WithDoneChan(ctx, errDoneClosed, xyz.done)
    44  //		defer cancel(nil)
    45  func WithDoneChan(ctx context.Context, cause error, done <-chan struct{}) (newCtx context.Context, cancel context.CancelCauseFunc) {
    46  	myCtx, myCancel := context.WithCancelCause(ctx)
    47  	var wg sync.WaitGroup
    48  	wg.Add(1)
    49  	go func() {
    50  		defer wg.Done()
    51  		select {
    52  		case <-myCtx.Done():
    53  			// no need to cancel - it is already canceled
    54  		case <-done:
    55  			myCancel(cause)
    56  		}
    57  	}()
    58  	return myCtx, func(cause error) {
    59  		myCancel(cause)
    60  		wg.Wait()
    61  	}
    62  }