zip: find preambles for zip files written by archive/zip

zip files written by `archive/zip`'s `Writer` correctly report the byte
offsets of their central directory file header and end of central
directory entries, but this means the offset of the zip data within the
file as calculated by `archive/zip`'s `Reader` will always be 0 - in
other words, `Reader` thinks that any non-zip data prepended to the file
is actually part of the first file's local file header. This prevents
any zip file written by `archive/zip` from being used as a source for
the `zip` command's `--preamble_from` option, because the non-zip data
doesn't appear to be non-zip data at all.

In cases where `Reader` identifies that the byte offset of the zip data
within the file is 0, check whether the byte offset of the header for
the first local file header is also 0. If it isn't, assume the byte
offset of the first local file header is the true starting position of
the zip data within the file, and that anything before it is in fact a
preamble.

zip/preamble.go

@@ -45,6 +45,14 @@ func Preamble(path string) (*PreambleReader, error) {
 	// when the Reader is created, so this is the fastest (and easiest) way to find out where the zip
 	// data begins in the underlying file.
 	zipOffset := reflect.ValueOf(zr).Elem().FieldByName("baseOffset").Int()
+	// zip files written by archive/zip's Writer correctly report the byte offsets of their CDFH and
+	// EOCD entries, but this means the baseOffset calculated by the Reader will always be 0, even when
+	// non-zip data is prepended. We can detect this based on the reported byte offset of the zip
+	// header for the first file in the archive - for files that truly contain only zip data, this
+	// should also be 0. If it isn't, assume everything before the first file header is the preamble.
+	if zipOffset == 0 && len(zr.File) != 0 {
+		zipOffset = reflect.ValueOf(zr.File[0]).Elem().FieldByName("headerOffset").Int()
+	}
 	log.Debugf("%s: zip data begins at byte offset %d", path, zipOffset)
 	zr.Close()
 	f, err := os.Open(path)

zip/preamble_test.go

@@ -30,6 +30,14 @@ func TestPreamble(t *testing.T) {
 			ZipFile:          "zip_preamble.zip",
 			PreambleChecksum: "038a57f3f807fa91bdd30239b9711fccf0d782fe2f036e03211852237e94d24c", // another.zip
 		},
+		{
+			ZipFile:          "arcat_preamble.zip",
+			PreambleChecksum: "46533b2dfa35ad537d3561ebee0c7af8941bc65363c1b188e1be6eaf79e9138c", // shebang_twolines.txt
+		},
+		{
+			ZipFile:          "empty.zip",
+			PreambleChecksum: "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855", // empty string
+		},
 	} {
 		t.Run(tc.ZipFile, func(t *testing.T) {
 			r := require.New(t)