testing: provide additional information when test funcs panic

Flush the output log up to the root when a test panics. Prior to
this change, only the current test's output log was flushed to its
parent, resulting in no output when a subtest panics.

For the following test function:

	func Test(t *testing.T) {
		for i, test := range []int{1, 0, 2} {
			t.Run(fmt.Sprintf("%v/%v", i, test), func(t *testing.T) {
				_ = 1 / test
			})
		}
	}

Output before this change:

	panic: runtime error: integer divide by zero [recovered]
		panic: runtime error: integer divide by zero
	(stack trace follows)

Output after this change:

	--- FAIL: Test (0.00s)
	    --- FAIL: Test/1/0 (0.00s)
	panic: runtime error: integer divide by zero [recovered]
	(stack trace follows)

Fixes #32121

Change-Id: Ifee07ccc005f0493a902190a8be734943123b6b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/179599
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 83 insertions, 0 deletions

diff --git a/src/testing/panic_test.go b/src/testing/panic_test.go
new file mode 100644
index 0000000000..3491510b81
--- /dev/null
+++ b/src/testing/panic_test.go
@@ -0,0 +1,83 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing_test
+
+import (
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+var testPanicTest = flag.String("test_panic_test", "", "TestPanic: indicates which test should panic")
+
+func TestPanic(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	testCases := []struct {
+		desc  string
+		flags []string
+		want  string
+	}{{
+		desc:  "root test panics",
+		flags: []string{"-test_panic_test=TestPanicHelper"},
+		want: `
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+`,
+	}, {
+		desc:  "subtest panics",
+		flags: []string{"-test_panic_test=TestPanicHelper/1"},
+		want: `
+--- FAIL: TestPanicHelper (N.NNs)
+    panic_test.go:NNN: TestPanicHelper
+    --- FAIL: TestPanicHelper/1 (N.NNs)
+        panic_test.go:NNN: TestPanicHelper/1
+`,
+	}}
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			cmd := exec.Command(os.Args[0], "-test.run=TestPanicHelper")
+			cmd.Args = append(cmd.Args, tc.flags...)
+			cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+			b, _ := cmd.CombinedOutput()
+			got := string(b)
+			want := strings.TrimSpace(tc.want)
+			re := makeRegexp(want)
+			if ok, err := regexp.MatchString(re, got); !ok || err != nil {
+				t.Errorf("output:\ngot:\n%s\nwant:\n%s", got, want)
+			}
+		})
+	}
+}
+
+func makeRegexp(s string) string {
+	s = regexp.QuoteMeta(s)
+	s = strings.ReplaceAll(s, ":NNN:", `:\d+:`)
+	s = strings.ReplaceAll(s, "N\\.NNs", `\d*\.\d*s`)
+	return s
+}
+
+func TestPanicHelper(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+	t.Log(t.Name())
+	if t.Name() == *testPanicTest {
+		panic("panic")
+	}
+	for i := 0; i < 3; i++ {
+		t.Run(fmt.Sprintf("%v", i), func(t *testing.T) {
+			t.Log(t.Name())
+			if t.Name() == *testPanicTest {
+				panic("panic")
+			}
+		})
+	}
+}