flag: align multi-line usage strings

Previously, a multi-line flag usage string would not be indented with the
rest of the usage strings. This made long usage strings difficult to read.

For example, the usage for flag.String("A", "", "1\n2\n3") would be printed
as:

  -A	1
2
3

But will now be printed as:

  -A	1
    	2
    	3

Also fixes a slight error in the FlagSet.PrintDefaults documentation.

Fixes #20799

Change-Id: I4379c6b7590fdb93a2809a01046a0f6ae32c3e5d
Reviewed-on: https://go-review.googlesource.com/66711
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/flag/flag.go

@@ -71,6 +71,7 @@ import (
 	"reflect"
 	"sort"
 	"strconv"
+	"strings"
 	"time"
 )
 
@@ -448,9 +449,9 @@ func UnquoteUsage(flag *Flag) (name string, usage string) {
 	return
 }
 
-// PrintDefaults prints to standard error the default values of all
-// defined command-line flags in the set. See the documentation for
-// the global function PrintDefaults for more information.
+// PrintDefaults prints, to standard error unless configured otherwise, the
+// default values of all defined command-line flags in the set. See the
+// documentation for the global function PrintDefaults for more information.
 func (f *FlagSet) PrintDefaults() {
 	f.VisitAll(func(flag *Flag) {
 		s := fmt.Sprintf("  -%s", flag.Name) // Two spaces before -; see next two comments.
@@ -467,7 +468,8 @@ func (f *FlagSet) PrintDefaults() {
 			// for both 4- and 8-space tab stops.
 			s += "\n    \t"
 		}
-		s += usage
+		s += strings.Replace(usage, "\n", "\n    \t", -1)
+
 		if !isZeroValue(flag, flag.DefValue) {
 			if _, ok := flag.Value.(*stringValue); ok {
 				// put quotes on the value

src/flag/flag_test.go

@@ -389,8 +389,14 @@ const defaultOutput = `  -A	for bootstrapping, allow 'any' type
     	a non-zero number (default 2.7)
   -G float
     	a float that defaults to zero
+  -M string
+    	a multiline
+    	help
+    	string
   -N int
     	a non-zero int (default 27)
+  -O	a flag
+    	multiline help string (default true)
   -Z int
     	an int that defaults to zero
   -maxT timeout
@@ -407,7 +413,9 @@ func TestPrintDefaults(t *testing.T) {
 	fs.String("D", "", "set relative `path` for local imports")
 	fs.Float64("F", 2.7, "a non-zero `number`")
 	fs.Float64("G", 0, "a float that defaults to zero")
+	fs.String("M", "", "a multiline\nhelp\nstring")
 	fs.Int("N", 27, "a non-zero int")
+	fs.Bool("O", true, "a flag\nmultiline help string")
 	fs.Int("Z", 0, "an int that defaults to zero")
 	fs.Duration("maxT", 0, "set `timeout` for dial")
 	fs.PrintDefaults()