archive/tar: add raw support for global PAX records

The PAX specification says the following:
<<<
'g' represents global extended header records for the following files in the archive.
The format of these extended header records shall be as described in pax Extended Header.
Each value shall affect all subsequent files that do not override that value
in their own extended header record and until another global extended header record
is reached that provides another value for the same field.
>>>

This CL adds support for parsing and composing global PAX records,
but intentionally does not provide support for automatically
persisting the global state across files.

Changes made:
* When Reader encounters a TypeXGlobalRecord header, it parses the
PAX records and returns them to the user ad-verbatim. Reader does not
store them in its state, ensuring it has no effect on future Next calls.
* When Writer receives a TypeXGlobalRecord header, it writes the
PAX records to the archive ad-verbatim. It does not store them in
its state, ensuring it has no effect on future WriteHeader calls.
* The restriction regarding empty record values is lifted since this
value is used to represent deletion in global headers.

Why provide raw support only:
* Some archives in the wild have a global header section (often empty)
and it is the user's responsibility to manually read and discard it's body.
The logic added here allows users to more easily skip over these sections.
* For users that do care about global headers, having access to the raw
records allows them to implement the functionality of global headers themselves
and manually persist the global state across files.
* We can still upgrade to a full implementation in the future.

Why we don't provide full support:
* Even though the PAX specification describes their operation in detail,
both the GNU and BSD tar tools (which are the most common implementations)
do not have a consistent interpretation of many details.
* Global headers were a controversial feature in PAX, by admission of the
specification itself:
  <<<
  The concept of a global extended header (typeflag g) was controversial.

  The typeflag g global headers should not be used with interchange media that
  could suffer partial data loss in transporting the archive.
  >>>
* Having state persist from entry-to-entry complicates the implementation
for a feature that is not widely used and not well supported.

Change-Id: I1d904cacc2623ddcaa91525a5470b7dbe226c7e8
Reviewed-on: https://go-review.googlesource.com/59190Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>

src/archive/tar/common.go

@@ -13,6 +13,7 @@ import (
 	"math"
 	"os"
 	"path"
+	"reflect"
 	"strconv"
 	"strings"
 	"time"
@@ -336,6 +337,9 @@ func (h *Header) allowedFormats() (format Format, paxHdrs map[string]string, err
 				paxHdrs[paxKey] = s
 			}
 		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == s {
+			paxHdrs[paxKey] = v
+		}
 	}
 	verifyNumeric := func(n int64, size int, name, paxKey string) {
 		if !fitsInBase256(size, n) {
@@ -352,6 +356,9 @@ func (h *Header) allowedFormats() (format Format, paxHdrs map[string]string, err
 				paxHdrs[paxKey] = strconv.FormatInt(n, 10)
 			}
 		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == strconv.FormatInt(n, 10) {
+			paxHdrs[paxKey] = v
+		}
 	}
 	verifyTime := func(ts time.Time, size int, name, paxKey string) {
 		if ts.IsZero() {
@@ -373,6 +380,9 @@ func (h *Header) allowedFormats() (format Format, paxHdrs map[string]string, err
 				paxHdrs[paxKey] = formatPAXTime(ts)
 			}
 		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == formatPAXTime(ts) {
+			paxHdrs[paxKey] = v
+		}
 	}
 
 	// Check basic fields.
@@ -396,6 +406,16 @@ func (h *Header) allowedFormats() (format Format, paxHdrs map[string]string, err
 
 	// Check for header-only types.
 	var whyOnlyPAX, whyOnlyGNU string
+	switch h.Typeflag {
+	case TypeXHeader, TypeGNULongName, TypeGNULongLink:
+		return FormatUnknown, nil, headerError{"cannot manually encode TypeXHeader, TypeGNULongName, or TypeGNULongLink headers"}
+	case TypeXGlobalHeader:
+		if !reflect.DeepEqual(h, &Header{Typeflag: h.Typeflag, Xattrs: h.Xattrs, PAXRecords: h.PAXRecords, Format: h.Format}) {
+			return FormatUnknown, nil, headerError{"only PAXRecords may be set for TypeXGlobalHeader"}
+		}
+		whyOnlyPAX = "only PAX supports TypeXGlobalHeader"
+		format.mayOnlyBe(FormatPAX)
+	}
 	if !isHeaderOnlyType(h.Typeflag) && h.Size < 0 {
 		return FormatUnknown, nil, headerError{"negative size on header-only type"}
 	}
@@ -410,19 +430,20 @@ func (h *Header) allowedFormats() (format Format, paxHdrs map[string]string, err
 	}
 	if len(h.PAXRecords) > 0 {
 		for k, v := range h.PAXRecords {
-			_, exists := paxHdrs[k]
-			ignore := exists || basicKeys[k] || strings.HasPrefix(k, paxGNUSparse)
-			if !ignore {
-				paxHdrs[k] = v
+			switch _, exists := paxHdrs[k]; {
+			case exists:
+				continue // Do not overwrite existing records
+			case h.Typeflag == TypeXGlobalHeader:
+				paxHdrs[k] = v // Copy all records
+			case !basicKeys[k] && !strings.HasPrefix(k, paxGNUSparse):
+				paxHdrs[k] = v // Ignore local records that may conflict
 			}
 		}
 		whyOnlyPAX = "only PAX supports PAXRecords"
 		format.mayOnlyBe(FormatPAX)
 	}
 	for k, v := range paxHdrs {
-		// Forbid empty values (which represent deletion) since usage of
-		// them are non-sensible without global PAX record support.
-		if !validPAXRecord(k, v) || v == "" {
+		if !validPAXRecord(k, v) {
 			return FormatUnknown, nil, headerError{fmt.Sprintf("invalid PAX record: %q", k+" = "+v)}
 		}
 	}

src/archive/tar/reader.go

@@ -85,12 +85,21 @@ loop:
 
 		// Check for PAX/GNU special headers and files.
 		switch hdr.Typeflag {
-		case TypeXHeader:
+		case TypeXHeader, TypeXGlobalHeader:
 			format.mayOnlyBe(FormatPAX)
 			paxHdrs, err = parsePAX(tr)
 			if err != nil {
 				return nil, err
 			}
+			if hdr.Typeflag == TypeXGlobalHeader {
+				mergePAX(hdr, paxHdrs)
+				return &Header{
+					Typeflag:   hdr.Typeflag,
+					Xattrs:     hdr.Xattrs,
+					PAXRecords: hdr.PAXRecords,
+					Format:     format,
+				}, nil
+			}
 			continue loop // This is a meta header affecting the next header
 		case TypeGNULongName, TypeGNULongLink:
 			format.mayOnlyBe(FormatGNU)
@@ -230,14 +239,13 @@ func (tr *Reader) readGNUSparsePAXHeaders(hdr *Header) (sparseDatas, error) {
 	}
 }
 
-// mergePAX merges well known headers according to PAX standard.
-// In general headers with the same name as those found
-// in the header struct overwrite those found in the header
-// struct with higher precision or longer values. Esp. useful
-// for name and linkname fields.
-func mergePAX(hdr *Header, headers map[string]string) (err error) {
+// mergePAX merges paxHdrs into hdr for all relevant fields of Header.
+func mergePAX(hdr *Header, paxHdrs map[string]string) (err error) {
+	for k, v := range paxHdrs {
+		if v == "" {
+			continue // Keep the original USTAR value
+		}
 		var id64 int64
-	for k, v := range headers {
 		switch k {
 		case paxPath:
 			hdr.Name = v
@@ -273,7 +281,7 @@ func mergePAX(hdr *Header, headers map[string]string) (err error) {
 			return ErrHeader
 		}
 	}
-	hdr.PAXRecords = headers
+	hdr.PAXRecords = paxHdrs
 	return nil
 }
 
@@ -309,13 +317,7 @@ func parsePAX(r io.Reader) (map[string]string, error) {
 			}
 			sparseMap = append(sparseMap, value)
 		default:
-			// According to PAX specification, a value is stored only if it is
-			// non-empty. Otherwise, the key is deleted.
-			if len(value) > 0 {
 			paxHdrs[key] = value
-			} else {
-				delete(paxHdrs, key)
-			}
 		}
 	}
 	if len(sparseMap) > 0 {

src/archive/tar/reader_test.go

@@ -348,6 +348,39 @@ func TestReader(t *testing.T) {
 			},
 			Format: FormatPAX,
 		}},
+	}, {
+		file: "testdata/pax-global-records.tar",
+		headers: []*Header{{
+			Typeflag:   TypeXGlobalHeader,
+			PAXRecords: map[string]string{"path": "global1", "mtime": "1500000000.0"},
+			Format:     FormatPAX,
+		}, {
+			Typeflag: TypeReg,
+			Name:     "file1",
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatUSTAR,
+		}, {
+			Typeflag:   TypeReg,
+			Name:       "file2",
+			PAXRecords: map[string]string{"path": "file2"},
+			ModTime:    time.Unix(0, 0),
+			Format:     FormatPAX,
+		}, {
+			Typeflag:   TypeXGlobalHeader,
+			PAXRecords: map[string]string{"path": ""},
+			Format:     FormatPAX,
+		}, {
+			Typeflag: TypeReg,
+			Name:     "file3",
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatUSTAR,
+		}, {
+			Typeflag:   TypeReg,
+			Name:       "file4",
+			ModTime:    time.Unix(1400000000, 0),
+			PAXRecords: map[string]string{"mtime": "1400000000"},
+			Format:     FormatPAX,
+		}},
 	}, {
 		file: "testdata/nil-uid.tar", // golang.org/issue/5290
 		headers: []*Header{{
@@ -965,12 +998,18 @@ func TestMergePAX(t *testing.T) {
 			Name:    "a/b/c",
 			Uid:     1000,
 			ModTime: time.Unix(1350244992, 23960108),
+			PAXRecords: map[string]string{
+				"path":  "a/b/c",
+				"uid":   "1000",
+				"mtime": "1350244992.023960108",
+			},
 		},
 		ok: true,
 	}, {
 		in: map[string]string{
 			"gid": "gtgergergersagersgers",
 		},
+		ok: false,
 	}, {
 		in: map[string]string{
 			"missing":          "missing",
@@ -978,6 +1017,10 @@ func TestMergePAX(t *testing.T) {
 		},
 		want: &Header{
 			Xattrs: map[string]string{"key": "value"},
+			PAXRecords: map[string]string{
+				"missing":          "missing",
+				"SCHILY.xattr.key": "value",
+			},
 		},
 		ok: true,
 	}}
@@ -985,8 +1028,6 @@ func TestMergePAX(t *testing.T) {
 	for i, v := range vectors {
 		got := new(Header)
 		err := mergePAX(got, v.in)
-		// TODO(dsnet): Test more combinations with global record support.
-		got.PAXRecords = nil
 		if v.ok && !reflect.DeepEqual(*got, *v.want) {
 			t.Errorf("test %d, mergePAX(...):\ngot  %+v\nwant %+v", i, *got, *v.want)
 		}
@@ -1012,7 +1053,7 @@ func TestParsePAX(t *testing.T) {
 		{"13 key1=haha\n13 key2=nana\n13 key3=kaka\n",
 			map[string]string{"key1": "haha", "key2": "nana", "key3": "kaka"}, true},
 		{"13 key1=val1\n13 key2=val2\n8 key1=\n",
-			map[string]string{"key2": "val2"}, true},
+			map[string]string{"key1": "", "key2": "val2"}, true},
 		{"22 GNU.sparse.size=10\n26 GNU.sparse.numblocks=2\n" +
 			"23 GNU.sparse.offset=1\n25 GNU.sparse.numbytes=2\n" +
 			"23 GNU.sparse.offset=3\n25 GNU.sparse.numbytes=4\n",
@@ -1029,10 +1070,10 @@ func TestParsePAX(t *testing.T) {
 		r := strings.NewReader(v.in)
 		got, err := parsePAX(r)
 		if !reflect.DeepEqual(got, v.want) && !(len(got) == 0 && len(v.want) == 0) {
-			t.Errorf("test %d, parsePAX(...):\ngot  %v\nwant %v", i, got, v.want)
+			t.Errorf("test %d, parsePAX():\ngot  %v\nwant %v", i, got, v.want)
 		}
 		if ok := err == nil; ok != v.ok {
-			t.Errorf("test %d, parsePAX(...): got %v, want %v", i, ok, v.ok)
+			t.Errorf("test %d, parsePAX(): got %v, want %v", i, ok, v.ok)
 		}
 	}
 }

src/archive/tar/tar_test.go

@@ -559,7 +559,8 @@ func TestHeaderAllowedFormats(t *testing.T) {
 		formats: FormatUnknown,
 	}, {
 		header:  &Header{Xattrs: map[string]string{"foo": ""}},
-		formats: FormatUnknown,
+		paxHdrs: map[string]string{paxSchilyXattr + "foo": ""},
+		formats: FormatPAX,
 	}, {
 		header:  &Header{ModTime: time.Unix(0, 0)},
 		formats: FormatUSTAR | FormatPAX | FormatGNU,

src/archive/tar/writer.go

@@ -141,7 +141,8 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 	}
 
 	// Write PAX records to the output.
-	if len(paxHdrs) > 0 {
+	isGlobal := hdr.Typeflag == TypeXGlobalHeader
+	if len(paxHdrs) > 0 || isGlobal {
 		// Sort keys for deterministic ordering.
 		var keys []string
 		for k := range paxHdrs {
@@ -160,11 +161,19 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 		}
 
 		// Write the extended header file.
+		var name string
+		var flag byte
+		if isGlobal {
+			name = "GlobalHead.0.0"
+			flag = TypeXGlobalHeader
+		} else {
 			dir, file := path.Split(realName)
-		name := path.Join(dir, "PaxHeaders.0", file)
+			name = path.Join(dir, "PaxHeaders.0", file)
+			flag = TypeXHeader
+		}
 		data := buf.String()
-		if err := tw.writeRawFile(name, data, TypeXHeader, FormatPAX); err != nil {
-			return err
+		if err := tw.writeRawFile(name, data, flag, FormatPAX); err != nil || isGlobal {
+			return err // Global headers return here
 		}
 	}
 

src/archive/tar/writer_test.go

@@ -247,6 +247,66 @@ func TestWriter(t *testing.T) {
 			}, nil},
 			testClose{nil},
 		},
+	}, {
+		// Craft a theoretically valid PAX archive with global headers.
+		// The GNU and BSD tar tools do not parse these the same way.
+		//
+		// BSD tar v3.1.2 parses and ignores all global headers;
+		// the behavior is verified by researching the source code.
+		//
+		//	$ bsdtar -tvf pax-global-records.tar
+		//	----------  0 0      0           0 Dec 31  1969 file1
+		//	----------  0 0      0           0 Dec 31  1969 file2
+		//	----------  0 0      0           0 Dec 31  1969 file3
+		//	----------  0 0      0           0 May 13  2014 file4
+		//
+		// GNU tar v1.27.1 applies global headers to subsequent records,
+		// but does not do the following properly:
+		//	* It does not treat an empty record as deletion.
+		//	* It does not use subsequent global headers to update previous ones.
+		//
+		//	$ gnutar -tvf pax-global-records.tar
+		//	---------- 0/0               0 2017-07-13 19:40 global1
+		//	---------- 0/0               0 2017-07-13 19:40 file2
+		//	gnutar: Substituting `.' for empty member name
+		//	---------- 0/0               0 1969-12-31 16:00
+		//	gnutar: Substituting `.' for empty member name
+		//	---------- 0/0               0 2014-05-13 09:53
+		//
+		// According to the PAX specification, this should have been the result:
+		//	---------- 0/0               0 2017-07-13 19:40 global1
+		//	---------- 0/0               0 2017-07-13 19:40 file2
+		//	---------- 0/0               0 2017-07-13 19:40 file3
+		//	---------- 0/0               0 2014-05-13 09:53 file4
+		file: "testdata/pax-global-records.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag:   TypeXGlobalHeader,
+				PAXRecords: map[string]string{"path": "global1", "mtime": "1500000000.0"},
+			}, nil},
+			testHeader{Header{
+				Typeflag: TypeReg, Name: "file1",
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeReg,
+				Name:       "file2",
+				PAXRecords: map[string]string{"path": "file2"},
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeXGlobalHeader,
+				PAXRecords: map[string]string{"path": ""}, // Should delete "path", but keep "mtime"
+			}, nil},
+			testHeader{Header{
+				Typeflag: TypeReg, Name: "file3",
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeReg,
+				Name:       "file4",
+				ModTime:    time.Unix(1400000000, 0),
+				PAXRecords: map[string]string{"mtime": "1400000000"},
+			}, nil},
+			testClose{nil},
+		},
 	}, {
 		file: "testdata/gnu-utf8.tar",
 		tests: []testFnc{