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: 83f64d7
GitHub-Pull-Request: #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>

doc/go1.15.html

@@ -119,6 +119,21 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 TODO
 </p>
 
+<dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
+  <dd>
+    <p><!-- CL 221427 -->
+      When the flag package sees <code>-h</code> or <code>-help</code>, and
+      those flags are not defined, the flag package prints a usage message.
+      If the <a href=/pkg/flag/#FlagSet><code>FlagSet</code></a> was created with
+      <a href=/pkg/flag/#ExitOnError><code>ExitOnError</code></a>,
+      <a href=/pkg/flag/#FlagSet.Parse><code>FlagSet.Parse</code></a> would then
+      exit with a status of 2. In this release, the exit status for <code>-h</code>
+      or <code>-help</code> has been changed to 0. In particular, this applies to
+      the default handling of command line flags.
+    </p>
+  </dd>
+</dl>
+
 <dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p><!-- CL 221779 -->

src/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)

src/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)
+		}
+	}
+}