fmt: fix up zero padding

If the padding is huge, we crashed by blowing the buffer. That's easy: make sure
we have a big enough buffer by allocating in problematic cases.
Zero padding floats was just wrong in general: the space would appear in the
middle.

Fixes #6044.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12498043

src/pkg/fmt/fmt_test.go

@@ -493,6 +493,17 @@ var fmtTests = []struct {
 	// Used to crash because nByte didn't allow for a sign.
 	{"%b", int64(-1 << 63), "-1000000000000000000000000000000000000000000000000000000000000000"},
 
+	// Used to panic.
+	{"%0100d", 1, "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001"},
+	{"%0100d", -1, "-000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001"},
+	{"%0.100f", 1.0, "1.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"},
+	{"%0.100f", -1.0, "-1.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"},
+
+	// Zero padding floats used to put the minus sign in the middle.
+	{"%020f", -1.0, "-000000000001.000000"},
+	{"%20f", -1.0, "           -1.000000"},
+	{"%0100f", -1.0, "-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.000000"},
+
 	// 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/pkg/fmt/format.go

@@ -160,6 +160,11 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	}
 
 	var buf []byte = f.intbuf[0:]
+	if f.widPresent && f.wid > nByte {
+		// We're going to need a bigger boat.
+		buf = make([]byte, f.wid)
+	}
+
 	negative := signedness == signed && a < 0
 	if negative {
 		a = -a
@@ -182,7 +187,7 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	// a is made into unsigned ua.  we could make things
 	// marginally faster by splitting the 32-bit case out into a separate
 	// block but it's not worth the duplication, so ua has 64 bits.
-	i := len(f.intbuf)
+	i := len(buf)
 	ua := uint64(a)
 	for ua >= base {
 		i--
@@ -191,7 +196,7 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	}
 	i--
 	buf[i] = digits[ua]
-	for i > 0 && prec > nByte-i {
+	for i > 0 && prec > len(buf)-i {
 		i--
 		buf[i] = '0'
 	}
@@ -354,6 +359,14 @@ func (f *fmt) formatFloat(v float64, verb byte, prec, n int) {
 	// The formatted number starts at slice[1].
 	switch slice[1] {
 	case '-', '+':
+		// If we're zero padding, want the sign before the leading zeros.
+		// Achieve this by writing the sign out and padding the postive number.
+		if f.zero && f.widPresent && f.wid > len(slice) {
+			f.buf.WriteByte(slice[1])
+			f.wid--
+			f.pad(slice[2:])
+			return
+		}
 		// We're set; drop the leading space.
 		slice = slice[1:]
 	default: