archive/tar: accept binary format when reading numeric header fields.

Fixes #4358.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6840043

src/pkg/archive/tar/reader.go

@@ -72,6 +72,18 @@ func cString(b []byte) string {
 }
 
 func (tr *Reader) octal(b []byte) int64 {
+	// Check for binary format first.
+	if len(b) > 0 && b[0]&0x80 != 0 {
+		var x int64
+		for i, c := range b {
+			if i == 0 {
+				c &= 0x7f // ignore signal bit in first byte
+			}
+			x = x<<8 | int64(c)
+		}
+		return x
+	}
+
 	// Removing leading spaces.
 	for len(b) > 0 && b[0] == ' ' {
 		b = b[1:]

src/pkg/archive/tar/tar_test.go

@@ -65,6 +65,7 @@ func TestRoundTrip(t *testing.T) {
 	tw := NewWriter(&b)
 	hdr := &Header{
 		Name:    "file.txt",
+		Uid:     1 << 21, // too big for 8 octal digits
 		Size:    int64(len(data)),
 		ModTime: time.Now(),
 	}