Don't use a data descriptor for symlinks so they can be stream-unzipped

This changes the behavior of createSymlink to not use a data descriptor.
Because symlinks are stored uncompressed, them not using a data descriptor
means that the correect size of the symlink must be stored up-front in the
local header of the file, which in turn means that the file can be
stream-unzipped because the unzipper can determine when the data of the symlink
ends without having random access to the zip.

This itself requires a small change to the behaviour of the lower level
function createHeaderRaw - it no longer set the data descriptor flag which
allows calling code, such as in createSymlink, to _not_ set it.

A very small extra benefit is that a few bytes are shaved off the resulting
file because symlinks no longer use a data descriptor.

This is to address the request at #48,
which in turn is inspired by the discussion at
uktrade/stream-unzip#105

(I suspect that all non-compressed files should maybe not use a data descriptor
and so would allow them to be stream-unzipped, but this is left to if/when)

archiver.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"context"
 	"fmt"
+	"hash/crc32"
 	"io"
 	"os"
 	"path/filepath"
@@ -228,12 +229,19 @@ func (a *Archiver) createSymlink(path string, fi os.FileInfo, hdr *zip.FileHeade
 	a.m.Lock()
 	defer a.m.Unlock()
 
-	w, err := a.createHeader(fi, hdr)
+	link, err := os.Readlink(path)
 	if err != nil {
 		return err
 	}
 
-	link, err := os.Readlink(path)
+	// Don't use a data descriptor to shave a few bytes and to make sure that the symlink can be stream-unzipped
+	hdr.Flags &= ^uint16(0x8)
+	hdr.Method = zip.Store
+	hdr.CompressedSize64 = uint64(len(link))
+	hdr.UncompressedSize64 = hdr.CompressedSize64
+	hdr.CRC32 = crc32.ChecksumIEEE([]byte(link))
+
+	w, err := a.createHeaderRaw(fi, hdr)
 	if err != nil {
 		return err
 	}
@@ -282,6 +290,7 @@ func (a *Archiver) compressFile(ctx context.Context, f *os.File, fi os.FileInfo,
 		return err
 	}
 
+	hdr.Flags |= 0x8
 	hdr.CompressedSize64 = tmp.Written()
 	// if compressed file is larger, use the uncompressed version.
 	if hdr.CompressedSize64 > hdr.UncompressedSize64 {
@@ -349,8 +358,6 @@ func (a *Archiver) createHeaderRaw(fi os.FileInfo, fh *zip.FileHeader) (io.Write
 		fh.Extra = append(fh.Extra, zipextra.NewExtendedTimestamp(fh.Modified).Encode()...)
 	}
 
-	fh.Flags |= 0x8
-
 	return a.createRaw(fi, fh)
 }