stats/view: implement Flush

Flush is a global function that immediately reports
all collected data by the worker, regardless of the
reporting duration or buffering.

Added tests but also ensured that calling (*worker).stop()
then Flush doesn't block and returns ASAP if the state
is "quit". The required protected w.quit from double close
but also using a select to detect cases of forever waiting
channel `w.quit` which was closed.

Fixes census-instrumentation#862

stats/view/view_measure_test.go

@@ -8,6 +8,8 @@ import (
 )
 
 func TestMeasureFloat64AndInt64(t *testing.T) {
+	restart()
+
 	// Recording through both a Float64Measure and Int64Measure with the
 	// same name should work.
 

stats/view/worker.go

@@ -17,6 +17,7 @@ package view
 
 import (
 	"fmt"
+	"sync"
 	"time"
 
 	"go.opencensus.io/stats"
@@ -43,6 +44,8 @@ type worker struct {
 	timer      *time.Ticker
 	c          chan command
 	quit, done chan bool
+	flushCh    chan bool
+	quitOnce   sync.Once
 }
 
 var defaultWorker *worker
@@ -139,6 +142,21 @@ func newWorker() *worker {
 		c:          make(chan command, 1024),
 		quit:       make(chan bool),
 		done:       make(chan bool),
+		flushCh:    make(chan bool),
+	}
+}
+
+// Flush reports all collected points regardless
+// of the time reporting period or buffering.
+func Flush() {
+	select {
+	case <-defaultWorker.quit:
+		// If this channel is closed i.e. we quit, do nothing.
+		return
+	default: // Otherwise we can proceed with flushing.
+		req := &flushReq{c: make(chan bool)}
+		defaultWorker.c <- req
+		<-req.c // don't return until the flush is complete.
 	}
 }
 
@@ -159,8 +177,13 @@ func (w *worker) start() {
 }
 
 func (w *worker) stop() {
-	w.quit <- true
-	<-w.done
+	w.quitOnce.Do(func() {
+		// Close w.quit so that any operations that need
+		// to check if we've stopped/quit will immediately
+		// select on w.quit.
+		close(w.quit)
+		<-w.done
+	})
 }
 
 func (w *worker) getMeasureRef(name string) *measureRef {

stats/view/worker_commands.go

@@ -81,6 +81,17 @@ func (cmd *registerViewReq) handleCommand(w *worker) {
 	}
 }
 
+// flushReq is the command to flush all recorded
+// data regardless of time period and buffering.
+type flushReq struct {
+	c chan bool
+}
+
+func (fr *flushReq) handleCommand(w *worker) {
+	w.reportUsage(time.Now())
+	fr.c <- true
+}
+
 // unregisterFromViewReq is the command to unregister to a view. Has no
 // impact on the data collection for client that are pulling data from the
 // library.

stats/view/worker_test.go

@@ -397,6 +397,68 @@ func TestUnregisterReportsUsage(t *testing.T) {
 	}
 }
 
+func TestFlush(t *testing.T) {
+	restart()
+	ctx := context.Background()
+
+	SetReportingPeriod(time.Hour)
+
+	m1 := stats.Int64("measure", "desc", "unit")
+	view1 := &View{Name: "count", Measure: m1, Aggregation: Count()}
+	m2 := stats.Int64("measure2", "desc", "unit")
+	view2 := &View{Name: "count2", Measure: m2, Aggregation: Count()}
+
+	if err := Register(view1, view2); err != nil {
+		t.Fatalf("cannot register: %v", err)
+	}
+
+	e := &countExporter{}
+	RegisterExporter(e)
+
+	// Irrespective of the reporting period, with Flush
+	// all the recorded points should be reported. Hence we'll
+	// set an arbitrarily large period of 1 hr.
+	SetReportingPeriod(time.Hour)
+
+	stats.Record(ctx, m1.M(1))
+	stats.Record(ctx, m2.M(3))
+	stats.Record(ctx, m2.M(1))
+
+	<-time.After(40 * time.Millisecond)
+	Flush()
+	<-time.After(40 * time.Millisecond)
+
+	e.Lock()
+	got := e.totalCount
+	e.Unlock()
+	want := int64(3) // Number of wanted data points
+	if got != want {
+		t.Errorf("Count data\nGot:  %d\nWant: %v", got, want)
+	}
+}
+
+func TestFlush_afterStopDoesnotBlock(t *testing.T) {
+	restart()
+
+	doneCh := make(chan bool)
+	go func() {
+		defer close(doneCh)
+
+		for i := 0; i < 10; i++ {
+			Flush()
+			defaultWorker.stop()
+			Flush()
+		}
+	}()
+
+	select {
+	case <-time.After(300 * time.Microsecond): // Arbitrary duration that's considered "long"
+		t.Fatal("Flush + stop goroutine did not return on time")
+	case <-doneCh:
+		// returned ASAP so okay
+	}
+}
+
 type countExporter struct {
 	sync.Mutex
 	count      int64