fmt: integer formatting should not permanently change padding

Changes the integer function to restore the original f.zero value
and therefore padding type before returning.

Change-Id: I456449259a3d39bd6d62e110553120c31ec63f23
Reviewed-on: https://go-review.googlesource.com/20512Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/fmt/fmt_test.go

@@ -914,6 +914,9 @@ var fmtTests = []struct {
 	{"%06v", []interface{}{+10.0 + 10i, 10}, "[(000010+00010i) 000010]"},
 	{"%06v", []interface{}{-10.0 + 10i, 10}, "[(-00010+00010i) 000010]"},
 
+	// integer formatting should not alter padding for other elements.
+	{"%03.6v", []interface{}{1, 2.0, "x"}, "[000001 002 00x]"},
+
 	// Complex fmt used to leave the plus flag set for future entries in the array
 	// causing +2+0i and +3+0i instead of 2+0i and 3+0i.
 	{"%v", []complex64{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},

src/fmt/format.go

@@ -221,6 +221,7 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 
 	// two ways to ask for extra leading zero digits: %.3d or %03d.
 	// apparently the first cancels the second.
+	oldZero := f.zero // f.zero is used in f.pad but modified below; restored at end of function.
 	prec := 0
 	if f.precPresent {
 		prec = f.prec
@@ -305,6 +306,7 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	}
 
 	f.pad(buf[i:])
+	f.zero = oldZero
 }
 
 // truncate truncates the string to the specified precision, if present.