runtime/trace: test detection of broken timestamps

On some processors cputicks (used to generate trace timestamps)
produce non-monotonic timestamps. It is important that the parser
distinguishes logically inconsistent traces (e.g. missing, excessive
or misordered events) from broken timestamps. The former is a bug
in tracer, the latter is a machine issue.

Test that (1) parser does not return a logical error in case of
broken timestamps and (2) broken timestamps are eventually detected
and reported.

Change-Id: Ib4b1eb43ce128b268e754400ed8b5e8def04bd78
Reviewed-on: https://go-review.googlesource.com/21608
Reviewed-by: Austin Clements <austin@google.com>

src/internal/trace/parser.go

@@ -9,10 +9,12 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"math/rand"
 	"os"
 	"os/exec"
 	"strconv"
 	"strings"
+	_ "unsafe"
 )
 
 // Event describes one event in the trace.
@@ -371,6 +373,16 @@ func parseEvents(ver int, rawEvents []rawEvent, strings map[uint64]string) (even
 		err = fmt.Errorf("no EvFrequency event")
 		return
 	}
+	if BreakTimestampsForTesting {
+		var batchArr [][]*Event
+		for _, batch := range batches {
+			batchArr = append(batchArr, batch)
+		}
+		for i := 0; i < 5; i++ {
+			batch := batchArr[rand.Intn(len(batchArr))]
+			batch[rand.Intn(len(batch))].Ts += int64(rand.Intn(2000) - 1000)
+		}
+	}
 	if ver < 1007 {
 		events, err = order1005(batches)
 	} else {
@@ -813,6 +825,9 @@ func argNum(raw rawEvent, ver int) int {
 	return narg
 }
 
+// BreakTimestampsForTesting causes the parser to randomly alter timestamps (for testing of broken cputicks).
+var BreakTimestampsForTesting bool
+
 // Event types in the trace.
 // Verbatim copy from src/runtime/trace.go.
 const (