Use idiomatic one-to-many one-time signal pattern.

The idiomatic pattern for signalling a one-time message to multiple
consumers from a single producer is as follows:

```
  c := make(chan struct{})
  w := new(sync.WaitGroup)  // Boilerplate to ensure synchronization.

  for i := 0; i < 1000; i++ {
    w.Add(1)
    go func() {
      defer w.Done()

      for {
        select {
        case _, ok := <- c:
          if !ok {
            return
          }
        default:
          // Do something here.
        }
      }
    }()
  }

  close(c)  // Signal the one-to-many single-use message.
  w.Wait()

```

Change-Id: I755f73ba4c70a923afd342a4dea63365bdf2144b

main.go

@@ -75,7 +75,7 @@ type prometheus struct {
 	deletionTimer   *time.Ticker
 
 	curationSema             chan struct{}
-	stopBackgroundOperations chan bool
+	stopBackgroundOperations chan struct{}
 
 	unwrittenSamples chan *extraction.Result
 
@@ -167,8 +167,10 @@ func (p *prometheus) delete(olderThan time.Duration, batchSize int) error {
 }
 
 func (p *prometheus) close() {
+	// Disallow further curation work.
 	close(p.curationSema)
 
+	// Stop curation timers.
 	if p.compactionTimer != nil {
 		p.compactionTimer.Stop()
 	}
@@ -177,9 +179,7 @@ func (p *prometheus) close() {
 	}
 
 	// Stop any currently active curation (deletion or compaction).
-	if len(p.stopBackgroundOperations) == 0 {
-		p.stopBackgroundOperations <- true
-	}
+	close(p.stopBackgroundOperations)
 
 	p.ruleManager.Stop()
 	p.targetManager.Stop()
@@ -192,7 +192,6 @@ func (p *prometheus) close() {
 	}
 
 	close(p.notifications)
-	close(p.stopBackgroundOperations)
 }
 
 func main() {
@@ -306,7 +305,7 @@ func main() {
 
 		unwrittenSamples: unwrittenSamples,
 
-		stopBackgroundOperations: make(chan bool, 1),
+		stopBackgroundOperations: make(chan struct{}),
 
 		ruleManager:     ruleManager,
 		targetManager:   targetManager,

storage/metric/compaction_regression_test.go

@@ -140,7 +140,7 @@ func (s compactionTestScenario) test(t *testing.T) {
 	defer processor.Close()
 
 	curator := NewCurator(&CuratorOptions{
-		Stop:      make(chan bool),
+		Stop:      make(chan struct{}),
 		ViewQueue: ts.ViewQueue,
 	})
 	defer curator.Close()

storage/metric/curator.go

@@ -52,10 +52,7 @@ type CurationState struct {
 
 // CuratorOptions bundles the parameters needed to create a Curator.
 type CuratorOptions struct {
-	// Stop functions as a channel that when empty allows the curator to operate.
-	// The moment a value is ingested inside of it, the curator goes into drain
-	// mode.
-	Stop chan bool
+	Stop chan struct{}
 
 	ViewQueue chan viewJob
 }
@@ -64,7 +61,7 @@ type CuratorOptions struct {
 // stored samples on-disk.  This is useful to compact sparse sample values into
 // single sample entities to reduce keyspace load on the datastore.
 type Curator struct {
-	stop chan bool
+	stop chan struct{}
 
 	viewQueue chan viewJob
 
@@ -112,7 +109,7 @@ type watermarkScanner struct {
 	stopAt clientmodel.Timestamp
 
 	// stop functions as the global stop channel for all future operations.
-	stop chan bool
+	stop chan struct{}
 	// status is the outbound channel for notifying the status page of its state.
 	status CurationStateUpdater
 
@@ -216,14 +213,6 @@ func (c *Curator) Run(ignoreYoungerThan time.Duration, instant clientmodel.Times
 	return
 }
 
-// Drain instructs the curator to stop at the next convenient moment as to not
-// introduce data inconsistencies.
-func (c *Curator) Drain() {
-	if len(c.stop) == 0 {
-		c.stop <- true
-	}
-}
-
 // Close needs to be called to cleanly dispose of a curator.
 func (c *Curator) Close() {
 	c.dtoSampleKeys.Close()
@@ -259,7 +248,15 @@ func (w *watermarkScanner) DecodeValue(in interface{}) (interface{}, error) {
 }
 
 func (w *watermarkScanner) shouldStop() bool {
-	return len(w.stop) != 0
+	select {
+	case _, ok := <-w.stop:
+		if ok {
+			panic("channel should be closed only")
+		}
+		return true
+	default:
+		return false
+	}
 }
 
 func (w *watermarkScanner) Filter(key, value interface{}) (r storage.FilterResult) {

storage/metric/processor_test.go

@@ -875,7 +875,7 @@ func TestCuratorCompactionProcessor(t *testing.T) {
 
 		updates := &noopUpdater{}
 
-		stop := make(chan bool)
+		stop := make(chan struct{})
 		defer close(stop)
 
 		c := NewCurator(&CuratorOptions{
@@ -1401,7 +1401,7 @@ func TestCuratorDeletionProcessor(t *testing.T) {
 
 		updates := &noopUpdater{}
 
-		stop := make(chan bool)
+		stop := make(chan struct{})
 		defer close(stop)
 
 		c := NewCurator(&CuratorOptions{