archive/tar: make Reader error handling consistent

The tar.Reader guarantees stickiness of errors. Ensuring this property means
that the methods of Reader need to be consistent about whose responsibility it
is to actually ensure that errors are sticky.

In this CL, we make it only the responsibility of the exported methods
(Next and Read) to store tr.err. All other methods just return the error as is.

As part of this change, we also check the error value of mergePAX (and test
that it properly detects invalid PAX files). Since the value of mergePAX was
never used before, we change it such that it always returns ErrHeader instead
of strconv.SyntaxError. This keeps it consistent with other usages of strconv
in the same tar package.

Change-Id: Ia1c31da71f1de4c175da89a385dec665d3edd167
Reviewed-on: https://go-review.googlesource.com/28215
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

4 files changed, 80 insertions, 104 deletions

diff --git a/src/archive/tar/reader.go b/src/archive/tar/reader.go
index bdbb8804b0..b8b1652b2b 100644
--- a/src/archive/tar/reader.go
+++ b/src/archive/tar/reader.go
@@ -30,10 +30,14 @@ const maxNanoSecondIntSize = 9
 // and then it can be treated as an io.Reader to access the file's data.
 type Reader struct {
 	r    io.Reader
-	err  error
 	pad  int64          // amount of padding (ignored) after current file entry
 	curr numBytesReader // reader for current file entry
 	blk  block          // buffer to use as temporary local storage
+
+	// err is a persistent error.
+	// It is only the responsibility of every exported method of Reader to
+	// ensure that this error is sticky.
+	err error
 }
 
 type parser struct {
@@ -108,9 +112,12 @@ func (tr *Reader) Next() (*Header, error) {
 	if tr.err != nil {
 		return nil, tr.err
 	}
+	hdr, err := tr.next()
+	tr.err = err
+	return hdr, err
+}
 
-	var hdr *Header
-	var rawHdr *block
+func (tr *Reader) next() (*Header, error) {
 	var extHdrs map[string]string
 
 	// Externally, Next iterates through the tar archive as if it is a series of
@@ -120,34 +127,29 @@ func (tr *Reader) Next() (*Header, error) {
 	// one or more "header files" until it finds a "normal file".
 loop:
 	for {
-		tr.err = tr.skipUnread()
-		if tr.err != nil {
-			return nil, tr.err
+		if err := tr.skipUnread(); err != nil {
+			return nil, err
 		}
-
-		hdr, rawHdr = tr.readHeader()
-		if tr.err != nil {
-			return nil, tr.err
+		hdr, rawHdr, err := tr.readHeader()
+		if err != nil {
+			return nil, err
 		}
-
-		tr.err = tr.handleRegularFile(hdr)
-		if tr.err != nil {
-			return nil, tr.err
+		if err := tr.handleRegularFile(hdr); err != nil {
+			return nil, err
 		}
 
 		// Check for PAX/GNU special headers and files.
 		switch hdr.Typeflag {
 		case TypeXHeader:
-			extHdrs, tr.err = parsePAX(tr)
-			if tr.err != nil {
-				return nil, tr.err
+			extHdrs, err = parsePAX(tr)
+			if err != nil {
+				return nil, err
 			}
 			continue loop // This is a meta header affecting the next header
 		case TypeGNULongName, TypeGNULongLink:
-			var realname []byte
-			realname, tr.err = ioutil.ReadAll(tr)
-			if tr.err != nil {
-				return nil, tr.err
+			realname, err := ioutil.ReadAll(tr)
+			if err != nil {
+				return nil, err
 			}
 
 			// Convert GNU extensions to use PAX headers.
@@ -162,30 +164,28 @@ loop:
 				extHdrs[paxLinkpath] = p.parseString(realname)
 			}
 			if p.err != nil {
-				tr.err = p.err
-				return nil, tr.err
+				return nil, p.err
 			}
 			continue loop // This is a meta header affecting the next header
 		default:
 			// The old GNU sparse format is handled here since it is technically
 			// just a regular file with additional attributes.
 
-			// TODO(dsnet): We should handle errors reported by mergePAX.
-			mergePAX(hdr, extHdrs)
+			if err := mergePAX(hdr, extHdrs); err != nil {
+				return nil, err
+			}
 
 			// TODO(dsnet): The extended headers may have updated the size.
 			// Thus, we must setup the regFileReader again here.
 			//
 			// See golang.org/issue/15573
 
-			tr.err = tr.handleSparseFile(hdr, rawHdr, extHdrs)
-			if tr.err != nil {
-				return nil, tr.err
+			if err := tr.handleSparseFile(hdr, rawHdr, extHdrs); err != nil {
+				return nil, err
 			}
-			break loop // This is a file, so stop
+			return hdr, nil // This is a file, so stop
 		}
 	}
-	return hdr, nil
 }
 
 // handleRegularFile sets up the current file reader and padding such that it
@@ -217,9 +217,9 @@ func (tr *Reader) handleSparseFile(hdr *Header, rawHdr *block, extHdrs map[strin
 			return p.err
 		}
 
-		sp = tr.readOldGNUSparseMap(rawHdr)
-		if tr.err != nil {
-			return tr.err
+		sp, err = tr.readOldGNUSparseMap(rawHdr)
+		if err != nil {
+			return err
 		}
 	} else {
 		sp, err = tr.checkForGNUSparsePAXHeaders(hdr, extHdrs)
@@ -302,53 +302,32 @@ func (tr *Reader) checkForGNUSparsePAXHeaders(hdr *Header, headers map[string]st
 // 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) error {
+func mergePAX(hdr *Header, headers map[string]string) (err error) {
+	var id64 int64
 	for k, v := range headers {
 		switch k {
 		case paxPath:
 			hdr.Name = v
 		case paxLinkpath:
 			hdr.Linkname = v
-		case paxGname:
-			hdr.Gname = v
 		case paxUname:
 			hdr.Uname = v
+		case paxGname:
+			hdr.Gname = v
 		case paxUid:
-			uid, err := strconv.ParseInt(v, 10, 0)
-			if err != nil {
-				return err
-			}
-			hdr.Uid = int(uid)
+			id64, err = strconv.ParseInt(v, 10, 0)
+			hdr.Uid = int(id64)
 		case paxGid:
-			gid, err := strconv.ParseInt(v, 10, 0)
-			if err != nil {
-				return err
-			}
-			hdr.Gid = int(gid)
+			id64, err = strconv.ParseInt(v, 10, 0)
+			hdr.Gid = int(id64)
 		case paxAtime:
-			t, err := parsePAXTime(v)
-			if err != nil {
-				return err
-			}
-			hdr.AccessTime = t
+			hdr.AccessTime, err = parsePAXTime(v)
 		case paxMtime:
-			t, err := parsePAXTime(v)
-			if err != nil {
-				return err
-			}
-			hdr.ModTime = t
+			hdr.ModTime, err = parsePAXTime(v)
 		case paxCtime:
-			t, err := parsePAXTime(v)
-			if err != nil {
-				return err
-			}
-			hdr.ChangeTime = t
+			hdr.ChangeTime, err = parsePAXTime(v)
 		case paxSize:
-			size, err := strconv.ParseInt(v, 10, 0)
-			if err != nil {
-				return err
-			}
-			hdr.Size = size
+			hdr.Size, err = strconv.ParseInt(v, 10, 0)
 		default:
 			if strings.HasPrefix(k, paxXattr) {
 				if hdr.Xattrs == nil {
@@ -357,6 +336,9 @@ func mergePAX(hdr *Header, headers map[string]string) error {
 				hdr.Xattrs[k[len(paxXattr):]] = v
 			}
 		}
+		if err != nil {
+			return ErrHeader
+		}
 	}
 	return nil
 }
@@ -569,19 +551,17 @@ func (tr *Reader) skipUnread() error {
 			// Seek seems supported, so perform the real Seek.
 			pos2, err := sr.Seek(dataSkip-1, io.SeekCurrent)
 			if err != nil {
-				tr.err = err
-				return tr.err
+				return err
 			}
 			seekSkipped = pos2 - pos1
 		}
 	}
 
-	var copySkipped int64 // Number of bytes skipped via CopyN
-	copySkipped, tr.err = io.CopyN(ioutil.Discard, tr.r, totalSkip-seekSkipped)
-	if tr.err == io.EOF && seekSkipped+copySkipped < dataSkip {
-		tr.err = io.ErrUnexpectedEOF
+	copySkipped, err := io.CopyN(ioutil.Discard, tr.r, totalSkip-seekSkipped)
+	if err == io.EOF && seekSkipped+copySkipped < dataSkip {
+		err = io.ErrUnexpectedEOF
 	}
-	return tr.err
+	return err
 }
 
 // readHeader reads the next block header and assumes that the underlying reader
@@ -592,29 +572,25 @@ func (tr *Reader) skipUnread() error {
 //	* Exactly 0 bytes are read and EOF is hit.
 //	* Exactly 1 block of zeros is read and EOF is hit.
 //	* At least 2 blocks of zeros are read.
-func (tr *Reader) readHeader() (*Header, *block) {
-	if _, tr.err = io.ReadFull(tr.r, tr.blk[:]); tr.err != nil {
-		return nil, nil // io.EOF is okay here
-	}
-
+func (tr *Reader) readHeader() (*Header, *block, error) {
 	// Two blocks of zero bytes marks the end of the archive.
+	if _, err := io.ReadFull(tr.r, tr.blk[:]); err != nil {
+		return nil, nil, err // EOF is okay here; exactly 0 bytes read
+	}
 	if bytes.Equal(tr.blk[:], zeroBlock[:]) {
-		if _, tr.err = io.ReadFull(tr.r, tr.blk[:]); tr.err != nil {
-			return nil, nil // io.EOF is okay here
+		if _, err := io.ReadFull(tr.r, tr.blk[:]); err != nil {
+			return nil, nil, err // EOF is okay here; exactly 1 block of zeros read
 		}
 		if bytes.Equal(tr.blk[:], zeroBlock[:]) {
-			tr.err = io.EOF
-		} else {
-			tr.err = ErrHeader // zero block and then non-zero block
+			return nil, nil, io.EOF // normal EOF; exactly 2 block of zeros read
 		}
-		return nil, nil
+		return nil, nil, ErrHeader // Zero block and then non-zero block
 	}
 
 	// Verify the header matches a known format.
 	format := tr.blk.GetFormat()
 	if format == formatUnknown {
-		tr.err = ErrHeader
-		return nil, nil
+		return nil, nil, ErrHeader
 	}
 
 	var p parser
@@ -658,19 +634,13 @@ func (tr *Reader) readHeader() (*Header, *block) {
 			hdr.Name = prefix + "/" + hdr.Name
 		}
 	}
-
-	// Check for parsing errors.
-	if p.err != nil {
-		tr.err = p.err
-		return nil, nil
-	}
-	return hdr, &tr.blk
+	return hdr, &tr.blk, p.err
 }
 
 // readOldGNUSparseMap reads the sparse map as stored in the old GNU sparse format.
 // The sparse map is stored in the tar header if it's small enough. If it's larger than four entries,
 // then one or more extension headers are used to store the rest of the sparse map.
-func (tr *Reader) readOldGNUSparseMap(blk *block) []sparseEntry {
+func (tr *Reader) readOldGNUSparseMap(blk *block) ([]sparseEntry, error) {
 	var p parser
 	var s sparseArray = blk.GNU().Sparse()
 	var sp = make([]sparseEntry, 0, s.MaxEntries())
@@ -678,8 +648,7 @@ func (tr *Reader) readOldGNUSparseMap(blk *block) []sparseEntry {
 		offset := p.parseOctal(s.Entry(i).Offset())
 		numBytes := p.parseOctal(s.Entry(i).NumBytes())
 		if p.err != nil {
-			tr.err = p.err
-			return nil
+			return nil, p.err
 		}
 		if offset == 0 && numBytes == 0 {
 			break
@@ -690,8 +659,8 @@ func (tr *Reader) readOldGNUSparseMap(blk *block) []sparseEntry {
 	for s.IsExtended()[0] > 0 {
 		// There are more entries. Read an extension header and parse its entries.
 		var blk block
-		if _, tr.err = io.ReadFull(tr.r, blk[:]); tr.err != nil {
-			return nil
+		if _, err := io.ReadFull(tr.r, blk[:]); err != nil {
+			return nil, err
 		}
 		s = blk.Sparse()
 
@@ -699,8 +668,7 @@ func (tr *Reader) readOldGNUSparseMap(blk *block) []sparseEntry {
 			offset := p.parseOctal(s.Entry(i).Offset())
 			numBytes := p.parseOctal(s.Entry(i).NumBytes())
 			if p.err != nil {
-				tr.err = p.err
-				return nil
+				return nil, p.err
 			}
 			if offset == 0 && numBytes == 0 {
 				break
@@ -708,7 +676,7 @@ func (tr *Reader) readOldGNUSparseMap(blk *block) []sparseEntry {
 			sp = append(sp, sparseEntry{offset: offset, numBytes: numBytes})
 		}
 	}
-	return sp
+	return sp, nil
 }
 
 // readGNUSparseMap1x0 reads the sparse map as stored in GNU's PAX sparse format
@@ -836,7 +804,7 @@ func (tr *Reader) numBytes() int64 {
 // Calling Read on special types like TypeLink, TypeSymLink, TypeChar,
 // TypeBlock, TypeDir, and TypeFifo returns 0, io.EOF regardless of what
 // the Header.Size claims.
-func (tr *Reader) Read(b []byte) (n int, err error) {
+func (tr *Reader) Read(b []byte) (int, error) {
 	if tr.err != nil {
 		return 0, tr.err
 	}
@@ -844,11 +812,11 @@ func (tr *Reader) Read(b []byte) (n int, err error) {
 		return 0, io.EOF
 	}
 
-	n, err = tr.curr.Read(b)
+	n, err := tr.curr.Read(b)
 	if err != nil && err != io.EOF {
 		tr.err = err
 	}
-	return
+	return n, err
 }
 
 func (rfr *regFileReader) Read(b []byte) (n int, err error) {
diff --git a/src/archive/tar/reader_test.go b/src/archive/tar/reader_test.go
index 7b148b5122..3de5299bac 100644
--- a/src/archive/tar/reader_test.go
+++ b/src/archive/tar/reader_test.go
@@ -230,6 +230,14 @@ var untarTests = []*untarTest{
 		},
 	},
 	{
+		file: "testdata/pax-bad-hdr-file.tar",
+		err:  ErrHeader,
+	},
+	{
+		file: "testdata/pax-bad-mtime-file.tar",
+		err:  ErrHeader,
+	},
+	{
 		file: "testdata/nil-uid.tar", // golang.org/issue/5290
 		headers: []*Header{
 			{
diff --git a/src/archive/tar/testdata/pax-bad-hdr-file.tar b/src/archive/tar/testdata/pax-bad-hdr-file.tar
new file mode 100644
index 0000000000..b97cc981f2
--- /dev/null
+++ b/src/archive/tar/testdata/pax-bad-hdr-file.tar
diff --git a/src/archive/tar/testdata/pax-bad-mtime-file.tar b/src/archive/tar/testdata/pax-bad-mtime-file.tar
new file mode 100644
index 0000000000..9b22f7e8d9
--- /dev/null
+++ b/src/archive/tar/testdata/pax-bad-mtime-file.tar