testing: fail Example tests that invoke runtime.Goexit

Previously, if an example test invoked runtime.Goexit, it would
pass yet hang until a timeout, while regular tests that invoke
runtime.Goexit do fail. This change removes that inconsistent
behavior and makes such example tests fail, and panic with an
indication of having invoked runtime.Goexit.

Fixes #41084

Change-Id: I0ffa152204f2b1580f4d5d6961ba1ce6b13fc022
Reviewed-on: https://go-review.googlesource.com/c/go/+/251857
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/test_example_goexit.txt

@@ -0,0 +1,25 @@
+# For issue golang.org/issue/41084
+[short] skip
+
+! go test -v examplegoexit
+stdout '(?s)--- PASS.*--- FAIL.*'
+stdout 'panic: test executed panic\(nil\) or runtime\.Goexit'
+
+-- examplegoexit/example_test.go --
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func ExamplePass() {
+	fmt.Println("pass")
+	// Output:
+	// pass
+}
+
+func ExampleGoexit() {
+	runtime.Goexit()
+	// Output:
+}

src/testing/example.go

@@ -62,9 +62,10 @@ func sortLines(output string) string {
 // If stdout doesn't match the expected output or if recovered is non-nil, it'll print the cause of failure to stdout.
 // If the test is chatty/verbose, it'll print a success message to stdout.
 // If recovered is non-nil, it'll panic with that value.
-func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Duration, recovered interface{}) (passed bool) {
+// If the test panicked with nil, or invoked runtime.Goexit, it'll be
+// made to fail and panic with errNilPanicOrGoexit
+func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Duration, finished bool, recovered interface{}) (passed bool) {
 	passed = true
-
 	dstr := fmtDuration(timeSpent)
 	var fail string
 	got := strings.TrimSpace(stdout)
@@ -78,16 +79,20 @@ func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Durati
 			fail = fmt.Sprintf("got:\n%s\nwant:\n%s\n", got, want)
 		}
 	}
-	if fail != "" || recovered != nil {
+	if fail != "" || !finished || recovered != nil {
 		fmt.Printf("--- FAIL: %s (%s)\n%s", eg.Name, dstr, fail)
 		passed = false
 	} else if *chatty {
 		fmt.Printf("--- PASS: %s (%s)\n", eg.Name, dstr)
 	}
+
 	if recovered != nil {
 		// Propagate the previously recovered result, by panicking.
 		panic(recovered)
 	}
+	if !finished && recovered == nil {
+		panic(errNilPanicOrGoexit)
+	}
 
 	return
 }

src/testing/run_example.go

@@ -43,6 +43,7 @@ func runExample(eg InternalExample) (ok bool) {
 		outC <- buf.String()
 	}()
 
+	finished := false
 	start := time.Now()
 
 	// Clean up in a deferred call so we can recover if the example panics.
@@ -55,10 +56,11 @@ func runExample(eg InternalExample) (ok bool) {
 		out := <-outC
 
 		err := recover()
-		ok = eg.processRunResult(out, timeSpent, err)
+		ok = eg.processRunResult(out, timeSpent, finished, err)
 	}()
 
 	// Run example.
 	eg.F()
+	finished = true
 	return
 }

src/testing/run_example_js.go

@@ -26,6 +26,7 @@ func runExample(eg InternalExample) (ok bool) {
 	stdout := os.Stdout
 	f := createTempFile(eg.Name)
 	os.Stdout = f
+	finished := false
 	start := time.Now()
 
 	// Clean up in a deferred call so we can recover if the example panics.
@@ -50,11 +51,12 @@ func runExample(eg InternalExample) (ok bool) {
 		}
 
 		err := recover()
-		ok = eg.processRunResult(out, timeSpent, err)
+		ok = eg.processRunResult(out, timeSpent, finished, err)
 	}()
 
 	// Run example.
 	eg.F()
+	finished = true
 	return
 }