ddtrace/tracer: Add normalization of span duration (#973)

Duration could be negative, and that can cause the negative sketch to be
non empty, which is not desirable. This makes sure any span with negative
duration is given a duration of zero.

ddtrace/tracer/span.go

@@ -342,6 +342,9 @@ func (s *span) finish(finishTime int64) {
 	if s.Duration == 0 {
 		s.Duration = finishTime - s.Start
 	}
+	if s.Duration < 0 {
+		s.Duration = 0
+	}
 	s.finished = true
 
 	keep := true

ddtrace/tracer/span_test.go

@@ -163,6 +163,16 @@ func TestSpanFinishWithTime(t *testing.T) {
 	assert.Equal(duration, span.Duration)
 }
 
+func TestSpanFinishWithNegativeDuration(t *testing.T) {
+	assert := assert.New(t)
+	startTime := time.Now()
+	finishTime := startTime.Add(-10 * time.Second)
+	span := newBasicSpan("web.request")
+	span.Start = startTime.UnixNano()
+	span.Finish(FinishTime(finishTime))
+	assert.Equal(int64(0), span.Duration)
+}
+
 func TestSpanFinishWithError(t *testing.T) {
 	assert := assert.New(t)
 

profiler/profiler.go

@@ -15,10 +15,10 @@ import (
 	"sync"
 	"time"
 
-	pprofile "github.com/google/pprof/profile"
-
 	"gopkg.in/DataDog/dd-trace-go.v1/internal"
 	"gopkg.in/DataDog/dd-trace-go.v1/internal/log"
+
+	pprofile "github.com/google/pprof/profile"
 )
 
 // outChannelSize specifies the size of the profile output channel.