testing: add Get method for -test.v option

There is existing code that calls flag.Lookup("test.v") and inspects
the value. That stopped working as of CL 443596. Make code like that
continue to work at least for the case where we aren't using
-test.v=test2json.

Change-Id: Idb30b149b48ee3987a201e349cf4d9bfe9ddee56
Reviewed-on: https://go-review.googlesource.com/c/go/+/447796
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: ianlancetaylor

src/testing/flag_test.go

@@ -0,0 +1,86 @@
+// Copyright 2022 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"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+var testFlagArg = flag.String("test_flag_arg", "", "TestFlag: passing -v option")
+
+const flagTestEnv = "GO_WANT_FLAG_HELPER_PROCESS"
+
+func TestFlag(t *testing.T) {
+	if os.Getenv(flagTestEnv) == "1" {
+		testFlagHelper(t)
+		return
+	}
+
+	testenv.MustHaveExec(t)
+
+	for _, flag := range []string{"", "-test.v", "-test.v=test2json"} {
+		flag := flag
+		t.Run(flag, func(t *testing.T) {
+			t.Parallel()
+			exe, err := os.Executable()
+			if err != nil {
+				exe = os.Args[0]
+			}
+			cmd := exec.Command(exe, "-test.run=TestFlag", "-test_flag_arg="+flag)
+			if flag != "" {
+				cmd.Args = append(cmd.Args, flag)
+			}
+			cmd.Env = append(cmd.Environ(), flagTestEnv+"=1")
+			b, err := cmd.CombinedOutput()
+			if len(b) > 0 {
+				t.Logf("%s", b)
+			}
+			if err != nil {
+				t.Error(err)
+			}
+		})
+	}
+}
+
+// testFlagHelper is called by the TestFlagHelper subprocess.
+func testFlagHelper(t *testing.T) {
+	f := flag.Lookup("test.v")
+	if f == nil {
+		t.Fatal(`flag.Lookup("test.v") failed`)
+	}
+
+	bf, ok := f.Value.(interface{ IsBoolFlag() bool })
+	if !ok {
+		t.Errorf("test.v flag (type %T) does not have IsBoolFlag method", f)
+	} else if !bf.IsBoolFlag() {
+		t.Error("test.v IsBoolFlag() returned false")
+	}
+
+	gf, ok := f.Value.(flag.Getter)
+	if !ok {
+		t.Fatalf("test.v flag (type %T) does not have Get method", f)
+	}
+	v := gf.Get()
+
+	var want any
+	switch *testFlagArg {
+	case "":
+		want = false
+	case "-test.v":
+		want = true
+	case "-test.v=test2json":
+		want = "test2json"
+	default:
+		t.Fatalf("unexpected test_flag_arg %q", *testFlagArg)
+	}
+
+	if v != want {
+		t.Errorf("test.v is %v want %v", v, want)
+	}
+}

src/testing/testing.go

@@ -513,6 +513,13 @@ func (f *chattyFlag) String() string {
 	return "false"
 }
 
+func (f *chattyFlag) Get() any {
+	if f.json {
+		return "test2json"
+	}
+	return f.on
+}
+
 const marker = byte(0x16) // ^V for framing
 
 func (f *chattyFlag) prefix() string {