Return []error from Stop() channel, allow recursive stop groups

pkg/stop/stop.go

@@ -5,17 +5,45 @@ import (
 	"sync"
 )
 
+// Channel is used to return zero or more errors asynchronously. Call Done()
+// once to pass errors to the Channel.
+type Channel chan []error
+
+// Result is a receive-only version of Channel. Call Wait() once to receive any
+// returned errors.
+type Result <-chan []error
+
+// Done adds zero or more errors to the Channel and closes it, indicating the
+// caller has finished stopping. It should be called exactly once.
+func (ch Channel) Done(errs ...error) {
+	if len(errs) > 0 && errs[0] != nil {
+		ch <- errs
+	}
+	close(ch)
+}
+
+// Result converts a Channel to a Result.
+func (ch Channel) Result() <-chan []error {
+	return ch
+}
+
+// Wait blocks until Done() is called on the underlying Channel and returns any
+// errors. It should be called exactly once.
+func (r Result) Wait() []error {
+	return <-r
+}
+
 // AlreadyStopped is a closed error channel to be used by Funcs when
 // an element was already stopped.
-var AlreadyStopped <-chan error
+var AlreadyStopped Result
 
 // AlreadyStoppedFunc is a Func that returns AlreadyStopped.
-var AlreadyStoppedFunc = func() <-chan error { return AlreadyStopped }
+var AlreadyStoppedFunc = func() Result { return AlreadyStopped }
 
 func init() {
-	closeMe := make(chan error)
+	closeMe := make(Channel)
 	close(closeMe)
-	AlreadyStopped = closeMe
+	AlreadyStopped = closeMe.Result()
 }
 
 // Stopper is an interface that allows a clean shutdown.
@@ -27,11 +55,11 @@ type Stopper interface {
 	// Closing the channel signals a clean shutdown.
 	// Stop() should return immediately and perform the actual shutdown in a
 	// separate goroutine.
-	Stop() <-chan error
+	Stop() Result
 }
 
 // Func is a function that can be used to provide a clean shutdown.
-type Func func() <-chan error
+type Func func() Result
 
 // Group is a collection of Stoppers that can be stopped all at once.
 type Group struct {
@@ -67,14 +95,13 @@ func (cg *Group) AddFunc(toAddFunc Func) {
 // Stopping will be done in a concurrent fashion.
 // The slice of errors returned contains all errors returned by stopping the
 // members.
-func (cg *Group) Stop() []error {
+func (cg *Group) Stop() Result {
 	cg.Lock()
 	defer cg.Unlock()
 
-	var errors []error
-	whenDone := make(chan struct{})
+	whenDone := make(Channel)
 
-	waitChannels := make([]<-chan error, 0, len(cg.stoppables))
+	waitChannels := make([]Result, 0, len(cg.stoppables))
 	for _, toStop := range cg.stoppables {
 		waitFor := toStop()
 		if waitFor == nil {
@@ -84,15 +111,15 @@ func (cg *Group) Stop() []error {
 	}
 
 	go func() {
+		var errors []error
 		for _, waitForMe := range waitChannels {
-			err := <-waitForMe
-			if err != nil {
-				errors = append(errors, err)
+			childErrors := waitForMe.Wait()
+			if len(childErrors) > 0 {
+				errors = append(errors, childErrors...)
 			}
 		}
-		close(whenDone)
+		whenDone.Done(errors...)
 	}()
 
-	<-whenDone
-	return errors
+	return whenDone.Result()
 }