testing: catch panicking example and report, just like tests

Fixes #4670.

R=rsc
CC=golang-dev
https://golang.org/cl/7148043

src/pkg/testing/example.go

@@ -24,8 +24,6 @@ func RunExamples(matchString func(pat, str string) (bool, error), examples []Int
 
 	var eg InternalExample
 
-	stdout := os.Stdout
-
 	for _, eg = range examples {
 		matched, err := matchString(*match, eg.Name)
 		if err != nil {
@@ -35,11 +33,21 @@ func RunExamples(matchString func(pat, str string) (bool, error), examples []Int
 		if !matched {
 			continue
 		}
+		if !runExample(eg) {
+			ok = false
+		}
+	}
+
+	return
+}
+
+func runExample(eg InternalExample) (ok bool) {
 	if *chatty {
 		fmt.Printf("=== RUN: %s\n", eg.Name)
 	}
 
-		// capture stdout
+	// Capture stdout.
+	stdout := os.Stdout
 	r, w, err := os.Pipe()
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
@@ -58,26 +66,33 @@ func RunExamples(matchString func(pat, str string) (bool, error), examples []Int
 		outC <- buf.String()
 	}()
 
-		// run example
-		t0 := time.Now()
-		eg.F()
-		dt := time.Now().Sub(t0)
+	start := time.Now()
 
-		// close pipe, restore stdout, get output
+	// Clean up in a deferred call so we can recover if the example panics.
+	defer func() {
+		d := time.Now().Sub(start)
+
+		// Close pipe, restore stdout, get output.
 		w.Close()
 		os.Stdout = stdout
 		out := <-outC
 
-		// report any errors
-		tstr := fmt.Sprintf("(%.2f seconds)", dt.Seconds())
-		if g, e := strings.TrimSpace(out), strings.TrimSpace(eg.Output); g != e {
-			fmt.Printf("--- FAIL: %s %s\ngot:\n%s\nwant:\n%s\n",
-				eg.Name, tstr, g, e)
-			ok = false
+		var fail string
+		err := recover()
+		if g, e := strings.TrimSpace(out), strings.TrimSpace(eg.Output); g != e && err == nil {
+			fail = fmt.Sprintf("got:\n%s\nwant:\n%s\n", g, e)
+		}
+		if fail != "" || err != nil {
+			fmt.Printf("--- FAIL: %s (%v)\n%s", eg.Name, d, fail)
 		} else if *chatty {
-			fmt.Printf("--- PASS: %s %s\n", eg.Name, tstr)
+			fmt.Printf("--- PASS: %s (%v)\n", eg.Name, d)
 		}
+		if err != nil {
+			panic(err)
 		}
+	}()
 
+	// Run example.
+	eg.F()
 	return
 }