testing: add tracing support

testing part of tracing functionality:
https://docs.google.com/document/u/1/d/1FP5apqzBgr7ahCCgFO-yoVhk4YZrNIDNf9RybngBc14/pub
Full change:
https://codereview.appspot.com/146920043

Change-Id: Ia3c2c4417106937d5775b0e7064db92c1fc36679
Reviewed-on: https://go-review.googlesource.com/1461Reviewed-by: Russ Cox <rsc@golang.org>

src/testing/testing.go

@@ -175,6 +175,7 @@ var (
 	cpuProfile       = flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
 	blockProfile     = flag.String("test.blockprofile", "", "write a goroutine blocking profile to the named file after execution")
 	blockProfileRate = flag.Int("test.blockprofilerate", 1, "if >= 0, calls runtime.SetBlockProfileRate()")
+	trace            = flag.String("test.trace", "", "write an execution trace to the named file after execution")
 	timeout          = flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
 	cpuListStr       = flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
 	parallel         = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
@@ -600,6 +601,19 @@ func before() {
 		}
 		// Could save f so after can call f.Close; not worth the effort.
 	}
+	if *trace != "" {
+		f, err := os.Create(toOutputDir(*trace))
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err := pprof.StartTrace(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't start tracing: %s", err)
+			f.Close()
+			return
+		}
+		// Could save f so after can call f.Close; not worth the effort.
+	}
 	if *blockProfile != "" && *blockProfileRate >= 0 {
 		runtime.SetBlockProfileRate(*blockProfileRate)
 	}
@@ -614,6 +628,9 @@ func after() {
 	if *cpuProfile != "" {
 		pprof.StopCPUProfile() // flushes profile to disk
 	}
+	if *trace != "" {
+		pprof.StopTrace() // flushes trace to disk
+	}
 	if *memProfile != "" {
 		f, err := os.Create(toOutputDir(*memProfile))
 		if err != nil {