cmd/trace: make binary argument optional

1.7 traces embed symbol info and we now generate symbolized pprof profiles,
so we don't need the binary. Make binary argument optional as 1.5 traces
still need it.

Change-Id: I65eb13e3d20ec765acf85c42d42a8d7aae09854c
Reviewed-on: https://go-review.googlesource.com/22410Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/go/test.go

@@ -233,7 +233,6 @@ const testFlag2 = `
 
 	-trace trace.out
 	    Write an execution trace to the specified file before exiting.
-	    Writes test binary as -c would.
 
 	-v
 	    Verbose output: log all tests as they are run. Also print all

src/cmd/go/testflag.go

@@ -149,9 +149,11 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 				testBench = true
 			case "timeout":
 				testTimeout = value
-			case "blockprofile", "cpuprofile", "memprofile", "trace":
+			case "blockprofile", "cpuprofile", "memprofile":
 				testProfile = true
 				testNeedBinary = true
+			case "trace":
+				testProfile = true
 			case "coverpkg":
 				testCover = true
 				if value == "" {

src/cmd/trace/main.go

@@ -14,7 +14,7 @@ Example usage:
 Generate a trace file with 'go test':
 	go test -trace trace.out pkg
 View the trace in a web browser:
-	go tool trace pkg.test trace.out
+	go tool trace trace.out
 */
 package main
 
@@ -37,7 +37,9 @@ Given a trace file produced by 'go test':
 	go test -trace=trace.out pkg
 
 Open a web browser displaying trace:
-	go tool trace [flags] pkg.test trace.out
+	go tool trace [flags] [pkg.test] trace.out
+[pkg.test] argument is required for traces produced by Go 1.6 and below.
+Go 1.7 does not require the binary argument.
 
 Flags:
 	-http=addr: HTTP service address (e.g., ':6060')
@@ -58,12 +60,17 @@ func main() {
 	}
 	flag.Parse()
 
-	// Usage information when no arguments.
-	if flag.NArg() != 2 {
+	// Go 1.7 traces embed symbol info and does not require the binary.
+	// But we optionally accept binary as first arg for Go 1.5 traces.
+	switch flag.NArg() {
+	case 1:
+		traceFile = flag.Arg(0)
+	case 2:
+		programBinary = flag.Arg(0)
+		traceFile = flag.Arg(1)
+	default:
 		flag.Usage()
 	}
-	programBinary = flag.Arg(0)
-	traceFile = flag.Arg(1)
 
 	ln, err := net.Listen("tcp", *httpFlag)
 	if err != nil {
@@ -91,7 +98,7 @@ var loader struct {
 
 func parseEvents() ([]*trace.Event, error) {
 	loader.once.Do(func() {
-		tracef, err := os.Open(flag.Arg(1))
+		tracef, err := os.Open(traceFile)
 		if err != nil {
 			loader.err = fmt.Errorf("failed to open trace file: %v", err)
 			return