flag: add (*FlagSet).Name, (*FlagSet).ErrorHandling, export (*FlagSet).Output

Allows code that operates on a FlagSet to know the name and error
handling behavior of the FlagSet without having to call FlagSet.Init.

Fixes #17628
Fixes #21888

Change-Id: Ib0fe4c8885f9ccdacf5a7fb761d5ecb23f3bb055
Reviewed-on: https://go-review.googlesource.com/70391
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/flag/flag.go

@@ -308,13 +308,25 @@ func sortFlags(flags map[string]*Flag) []*Flag {
 	return result
 }
 
-func (f *FlagSet) out() io.Writer {
+// Output returns the destination for usage and error messages. os.Stderr is returned if
+// output was not set or was set to nil.
+func (f *FlagSet) Output() io.Writer {
 	if f.output == nil {
 		return os.Stderr
 	}
 	return f.output
 }
 
+// Name returns the name of the flag set.
+func (f *FlagSet) Name() string {
+	return f.name
+}
+
+// ErrorHandling returns the error handling behavior of the flag set.
+func (f *FlagSet) ErrorHandling() ErrorHandling {
+	return f.errorHandling
+}
+
 // SetOutput sets the destination for usage and error messages.
 // If output is nil, os.Stderr is used.
 func (f *FlagSet) SetOutput(output io.Writer) {
@@ -474,7 +486,7 @@ func (f *FlagSet) PrintDefaults() {
 				s += fmt.Sprintf(" (default %v)", flag.DefValue)
 			}
 		}
-		fmt.Fprint(f.out(), s, "\n")
+		fmt.Fprint(f.Output(), s, "\n")
 	})
 }
 
@@ -504,9 +516,9 @@ func PrintDefaults() {
 // defaultUsage is the default function to print a usage message.
 func (f *FlagSet) defaultUsage() {
 	if f.name == "" {
-		fmt.Fprintf(f.out(), "Usage:\n")
+		fmt.Fprintf(f.Output(), "Usage:\n")
 	} else {
-		fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
+		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
 	}
 	f.PrintDefaults()
 }
@@ -525,7 +537,7 @@ func (f *FlagSet) defaultUsage() {
 // happens anyway as the command line's error handling strategy is set to
 // ExitOnError.
 var Usage = func() {
-	fmt.Fprintf(CommandLine.out(), "Usage of %s:\n", os.Args[0])
+	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
 	PrintDefaults()
 }
 
@@ -793,7 +805,7 @@ func (f *FlagSet) Var(value Value, name string, usage string) {
 		} else {
 			msg = fmt.Sprintf("%s flag redefined: %s", f.name, name)
 		}
-		fmt.Fprintln(f.out(), msg)
+		fmt.Fprintln(f.Output(), msg)
 		panic(msg) // Happens only if flags are declared with identical names
 	}
 	if f.formal == nil {
@@ -816,7 +828,7 @@ func Var(value Value, name string, usage string) {
 // returns the error.
 func (f *FlagSet) failf(format string, a ...interface{}) error {
 	err := fmt.Errorf(format, a...)
-	fmt.Fprintln(f.out(), err)
+	fmt.Fprintln(f.Output(), err)
 	f.usage()
 	return err
 }

src/flag/flag_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	. "flag"
 	"fmt"
+	"io"
 	"os"
 	"sort"
 	"strconv"
@@ -454,3 +455,36 @@ func TestUsageOutput(t *testing.T) {
 		t.Errorf("output = %q; want %q", got, want)
 	}
 }
+
+func TestGetters(t *testing.T) {
+	expectedName := "flag set"
+	expectedErrorHandling := ContinueOnError
+	expectedOutput := io.Writer(os.Stderr)
+	fs := NewFlagSet(expectedName, expectedErrorHandling)
+
+	if fs.Name() != expectedName {
+		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
+	}
+	if fs.ErrorHandling() != expectedErrorHandling {
+		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
+	}
+	if fs.Output() != expectedOutput {
+		t.Errorf("unexpected output: got %#v, expected %#v", fs.Output(), expectedOutput)
+	}
+
+	expectedName = "gopher"
+	expectedErrorHandling = ExitOnError
+	expectedOutput = os.Stdout
+	fs.Init(expectedName, expectedErrorHandling)
+	fs.SetOutput(expectedOutput)
+
+	if fs.Name() != expectedName {
+		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
+	}
+	if fs.ErrorHandling() != expectedErrorHandling {
+		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
+	}
+	if fs.Output() != expectedOutput {
+		t.Errorf("unexpected output: got %v, expected %v", fs.Output(), expectedOutput)
+	}
+}