flag: exit 0 when -h or -help invoked but undefined

flag treats -h or -help as a special case to print a nice help
message, but exit with a status code of 2. This update makes
that status code 0.

Fixes #37533

Change-Id: I7e0bd29944ce46607fb7cfc6740734f7444a151a
GitHub-Last-Rev: 83f64d757bc3a9957c49caa5de74d05a96724771
GitHub-Pull-Request: golang/go#37530
Reviewed-on: https://go-review.googlesource.com/c/go/+/221427
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

flag/flag.go

@@ -308,7 +308,7 @@ type ErrorHandling int
 // These constants cause FlagSet.Parse to behave as described if the parse fails.
 const (
 	ContinueOnError ErrorHandling = iota // Return a descriptive error.
-	ExitOnError                          // Call os.Exit(2).
+	ExitOnError                          // Call os.Exit(2) or for -h/-help Exit(0).
 	PanicOnError                         // Call panic with a descriptive error.
 )
 
@@ -979,6 +979,9 @@ func (f *FlagSet) Parse(arguments []string) error {
 		case ContinueOnError:
 			return err
 		case ExitOnError:
+			if err == ErrHelp {
+				os.Exit(0)
+			}
 			os.Exit(2)
 		case PanicOnError:
 			panic(err)

flag/flag_test.go

@@ -8,9 +8,11 @@ import (
 	"bytes"
 	. "flag"
 	"fmt"
+	"internal/testenv"
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"sort"
 	"strconv"
 	"strings"
@@ -544,3 +546,62 @@ func TestRangeError(t *testing.T) {
 		}
 	}
 }
+
+func TestExitCode(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	magic := 123
+	if os.Getenv("GO_CHILD_FLAG") != "" {
+		fs := NewFlagSet("test", ExitOnError)
+		if os.Getenv("GO_CHILD_FLAG_HANDLE") != "" {
+			var b bool
+			fs.BoolVar(&b, os.Getenv("GO_CHILD_FLAG_HANDLE"), false, "")
+		}
+		fs.Parse([]string{os.Getenv("GO_CHILD_FLAG")})
+		os.Exit(magic)
+	}
+
+	tests := []struct {
+		flag       string
+		flagHandle string
+		expectExit int
+	}{
+		{
+			flag:       "-h",
+			expectExit: 0,
+		},
+		{
+			flag:       "-help",
+			expectExit: 0,
+		},
+		{
+			flag:       "-undefined",
+			expectExit: 2,
+		},
+		{
+			flag:       "-h",
+			flagHandle: "h",
+			expectExit: magic,
+		},
+		{
+			flag:       "-help",
+			flagHandle: "help",
+			expectExit: magic,
+		},
+	}
+
+	for _, test := range tests {
+		cmd := exec.Command(os.Args[0], "-test.run=TestExitCode")
+		cmd.Env = append(
+			os.Environ(),
+			"GO_CHILD_FLAG="+test.flag,
+			"GO_CHILD_FLAG_HANDLE="+test.flagHandle,
+		)
+		cmd.Run()
+		got := cmd.ProcessState.ExitCode()
+		if got != test.expectExit {
+			t.Errorf("unexpected exit code for test case %+v \n: got %d, expect %d",
+				test, got, test.expectExit)
+		}
+	}
+}