flag: make tests silent

A few of the tests were printing garbage to stderr,
since FlagSet's default Output is os.Stderr:

	$ go test
	flag provided but not defined: -x
	invalid value "1" for flag -v: test error
	Usage of test:
	flag needs an argument: -b
	Usage of test:
	  -b	usage
	PASS
	ok  	flag	0.008s

Add the remaining SetOutput(io.Discard) method calls.

Note that TestUserDefinedFunc was a tricky one.
Even with the added SetOutput calls,
the last part of the test would still print usage text to stderr.
It took me a while to figure out the problem was copying FlagSet.
I've filed go.dev/issue/51507 to record this particular sharp edge,
and the test code now avoids making FlagSet copies to avoid the bug.

Change-Id: I323f24091b98386312aa72df3eb890af6625628d
Reviewed-on: https://go-review.googlesource.com/c/go/+/390234
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 11 insertions, 4 deletions

diff --git a/src/flag/export_test.go b/src/flag/export_test.go
index 838cfaf6a4..9ef93ed6c5 100644
--- a/src/flag/export_test.go
+++ b/src/flag/export_test.go
@@ -4,7 +4,10 @@
 
 package flag
 
-import "os"
+import (
+	"io"
+	"os"
+)
 
 // Additional routines compiled into the package only during testing.
 
@@ -15,6 +18,7 @@ var DefaultUsage = Usage
 // exit the program.
 func ResetForTesting(usage func()) {
 	CommandLine = NewFlagSet(os.Args[0], ContinueOnError)
+	CommandLine.SetOutput(io.Discard)
 	CommandLine.Usage = commandLineUsage
 	Usage = usage
 }
diff --git a/src/flag/flag_test.go b/src/flag/flag_test.go
index 5835fcf22c..d5c443d3c6 100644
--- a/src/flag/flag_test.go
+++ b/src/flag/flag_test.go
@@ -246,6 +246,7 @@ func (f *flagVar) Set(value string) error {
 func TestUserDefined(t *testing.T) {
 	var flags FlagSet
 	flags.Init("test", ContinueOnError)
+	flags.SetOutput(io.Discard)
 	var v flagVar
 	flags.Var(&v, "v", "usage")
 	if err := flags.Parse([]string{"-v", "1", "-v", "2", "-v=3"}); err != nil {
@@ -261,8 +262,8 @@ func TestUserDefined(t *testing.T) {
 }
 
 func TestUserDefinedFunc(t *testing.T) {
-	var flags FlagSet
-	flags.Init("test", ContinueOnError)
+	flags := NewFlagSet("test", ContinueOnError)
+	flags.SetOutput(io.Discard)
 	var ss []string
 	flags.Func("v", "usage", func(s string) error {
 		ss = append(ss, s)
@@ -286,7 +287,8 @@ func TestUserDefinedFunc(t *testing.T) {
 		t.Errorf("usage string not included: %q", usage)
 	}
 	// test Func error
-	flags = *NewFlagSet("test", ContinueOnError)
+	flags = NewFlagSet("test", ContinueOnError)
+	flags.SetOutput(io.Discard)
 	flags.Func("v", "usage", func(s string) error {
 		return fmt.Errorf("test error")
 	})
@@ -335,6 +337,7 @@ func (b *boolFlagVar) IsBoolFlag() bool {
 func TestUserDefinedBool(t *testing.T) {
 	var flags FlagSet
 	flags.Init("test", ContinueOnError)
+	flags.SetOutput(io.Discard)
 	var b boolFlagVar
 	var err error
 	flags.Var(&b, "b", "usage")