archive/tar: skip NUL-filled unused octal fields

Fixes #5290.

R=golang-dev, dave, bradfitz, r
CC=golang-dev
https://golang.org/cl/8763044

src/pkg/archive/tar/reader.go

@@ -243,13 +243,15 @@ func (tr *Reader) octal(b []byte) int64 {
 		return x
 	}
 
-	// Removing leading spaces.
-	for len(b) > 0 && b[0] == ' ' {
-		b = b[1:]
-	}
-	// Removing trailing NULs and spaces.
-	for len(b) > 0 && (b[len(b)-1] == ' ' || b[len(b)-1] == '\x00') {
-		b = b[0 : len(b)-1]
+	// Because unused fields are filled with NULs, we need
+	// to skip leading NULs. Fields may also be padded with
+	// spaces or NULs.
+	// So we remove leading and trailing NULs and spaces to
+	// be sure.
+	b = bytes.Trim(b, " \x00")
+
+	if len(b) == 0 {
+		return 0
 	}
 	x, err := strconv.ParseUint(cString(b), 8, 64)
 	if err != nil {

src/pkg/archive/tar/reader_test.go

@@ -142,6 +142,25 @@ var untarTests = []*untarTest{
 			},
 		},
 	},
+	{
+		file: "testdata/nil-uid.tar", // golang.org/issue/5290
+		headers: []*Header{
+			{
+				Name:     "P1050238.JPG.log",
+				Mode:     0664,
+				Uid:      0,
+				Gid:      0,
+				Size:     14,
+				ModTime:  time.Unix(1365454838, 0),
+				Typeflag: TypeReg,
+				Linkname: "",
+				Uname:    "eyefi",
+				Gname:    "eyefi",
+				Devmajor: 0,
+				Devminor: 0,
+			},
+		},
+	},
 }
 
 func TestReader(t *testing.T) {