archive/tar: properly handle header-only "files" in Writer

Certain special type-flags, specifically 1, 2, 3, 4, 5, 6,
do not have a data section. Thus, regardless of what the size field
says, we should not attempt to write any data for these special types.

The relevant PAX and USTAR specification says:
<<<
If the typeflag field is set to specify a file to be of type 1 (a link)
or 2 (a symbolic link), the size field shall be specified as zero.
If the typeflag field is set to specify a file of type 5 (directory),
the size field shall be interpreted as described under the definition
of that record type. No data logical records are stored for types 1, 2, or 5.
If the typeflag field is set to 3 (character special file),
4 (block special file), or 6 (FIFO), the meaning of the size field is
unspecified by this volume of POSIX.1-2008, and no data logical records shall
be stored on the medium.
Additionally, for type 6, the size field shall be ignored when reading.
If the typeflag field is set to any other value, the number of logical
records written following the header shall be (size+511)/512, ignoring
any fraction in the result of the division.
>>>

Fixes #15565

Change-Id: Id11886b723b3b13deb15221dca51c25cd778a6b5
Reviewed-on: https://go-review.googlesource.com/55553Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/writer.go

@@ -94,7 +94,7 @@ func (tw *Writer) writeUSTARHeader(hdr *Header) error {
 	if f.err != nil {
 		return f.err // Should never happen since header is validated
 	}
-	return tw.writeRawHeader(blk, hdr.Size)
+	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
 }
 
 func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
@@ -133,7 +133,7 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 	if f.err != nil && len(paxHdrs) == 0 {
 		return f.err // Should never happen, otherwise PAX headers would be used
 	}
-	return tw.writeRawHeader(blk, hdr.Size)
+	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
 }
 
 func (tw *Writer) writeGNUHeader(hdr *Header) error {
@@ -151,7 +151,7 @@ func (tw *Writer) writeGNUHeader(hdr *Header) error {
 	if f.err != nil {
 		return f.err // Should never happen since header is validated
 	}
-	return tw.writeRawHeader(blk, hdr.Size)
+	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
 }
 
 type (
@@ -219,7 +219,7 @@ func (tw *Writer) writeRawFile(name, data string, flag byte, format int) error {
 	}
 
 	// Write the header and data.
-	if err := tw.writeRawHeader(&tw.blk, int64(len(data))); err != nil {
+	if err := tw.writeRawHeader(&tw.blk, int64(len(data)), flag); err != nil {
 		return err
 	}
 	_, err := io.WriteString(tw, data)
@@ -228,15 +228,17 @@ func (tw *Writer) writeRawFile(name, data string, flag byte, format int) error {
 
 // writeRawHeader writes the value of blk, regardless of its value.
 // It sets up the Writer such that it can accept a file of the given size.
-func (tw *Writer) writeRawHeader(blk *block, size int64) error {
+// If the flag is a special header-only flag, then the size is treated as zero.
+func (tw *Writer) writeRawHeader(blk *block, size int64, flag byte) error {
 	if err := tw.Flush(); err != nil {
 		return err
 	}
 	if _, err := tw.w.Write(blk[:]); err != nil {
 		return err
 	}
-	// TODO(dsnet): Set Size implicitly to zero for header-only entries.
-	// See https://golang.org/issue/15565
+	if isHeaderOnlyType(flag) {
+		size = 0
+	}
 	tw.nb = size
 	tw.pad = -size & (blockSize - 1) // blockSize is a power of two
 	return nil

src/archive/tar/writer_test.go

@@ -553,6 +553,25 @@ func TestValidTypeflagWithPAXHeader(t *testing.T) {
 	}
 }
 
+func TestWriteHeaderOnly(t *testing.T) {
+	tw := NewWriter(new(bytes.Buffer))
+	hdr := &Header{Name: "dir/", Typeflag: TypeDir}
+	if err := tw.WriteHeader(hdr); err != nil {
+		t.Fatalf("WriteHeader() = %v, want nil", err)
+	}
+	if _, err := tw.Write([]byte{0x00}); err != ErrWriteTooLong {
+		t.Fatalf("Write() = %v, want %v", err, ErrWriteTooLong)
+	}
+}
+
+func TestWriteNegativeSize(t *testing.T) {
+	tw := NewWriter(new(bytes.Buffer))
+	hdr := &Header{Name: "small.txt", Size: -1}
+	if err := tw.WriteHeader(hdr); err != ErrHeader {
+		t.Fatalf("WriteHeader() = nil, want %v", ErrHeader)
+	}
+}
+
 func TestWriteAfterClose(t *testing.T) {
 	var buffer bytes.Buffer
 	tw := NewWriter(&buffer)