archive/tar: Fix bug preventing untar

Do not use ustar format if we need the GNU one.
Change \000 to \x00 for consistency
Check for "ustar\x00" instead of "ustar\x00\x00" for conistency with tar
and compatiblity with archive generated with older code (which was ustar\x00\x20\x00)
Add test for long name + big file.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/99050043

src/pkg/archive/tar/reader.go

@@ -468,14 +468,14 @@ func (tr *Reader) readHeader() *Header {
 	// so its magic bytes, like the rest of the block, are NULs.
 	magic := string(s.next(8)) // contains version field as well.
 	var format string
-	switch magic {
-	case "ustar\x0000": // POSIX tar (1003.1-1988)
+	switch {
+	case magic[:6] == "ustar\x00": // POSIX tar (1003.1-1988)
 		if string(header[508:512]) == "tar\x00" {
 			format = "star"
 		} else {
 			format = "posix"
 		}
-	case "ustar  \x00": // old GNU tar
+	case magic == "ustar  \x00": // old GNU tar
 		format = "gnu"
 	}
 

src/pkg/archive/tar/writer.go

@@ -218,8 +218,8 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 				tw.cString(prefixHeaderBytes, prefix, false, paxNone, nil)
 
 				// Use the ustar magic if we used ustar long names.
-				if len(prefix) > 0 {
-					copy(header[257:265], []byte("ustar\000"))
+				if len(prefix) > 0 && !tw.usedBinary {
+					copy(header[257:265], []byte("ustar\x00"))
 				}
 			}
 		}

src/pkg/archive/tar/writer_test.go

@@ -103,6 +103,29 @@ var writerTests = []*writerTest{
 			},
 		},
 	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > (longname/)*15 /16gig.txt
+	//   tar -b 1 -c -f- (longname/)*15 /16gig.txt | dd bs=512 count=8 > writer-big-long.tar
+	{
+		file: "testdata/writer-big-long.tar",
+		entries: []*writerTestEntry{
+			{
+				header: &Header{
+					Name:     strings.Repeat("longname/", 15) + "16gig.txt",
+					Mode:     0644,
+					Uid:      1000,
+					Gid:      1000,
+					Size:     16 << 30,
+					ModTime:  time.Unix(1399583047, 0),
+					Typeflag: '0',
+					Uname:    "guillaume",
+					Gname:    "guillaume",
+				},
+				// fake contents
+				contents: strings.Repeat("\x00", 4<<10),
+			},
+		},
+	},
 	// This file was produced using gnu tar 1.17
 	// gnutar  -b 4 --format=ustar (longname/)*15 + file.txt
 	{