Detect UStar tar archives (#308)

* Detect UStar tar archives

UStar tar archives have a `magic` header field at byte offset 257 in
each entry whose value begins with the string `ustar`. Identify them
with the MIME type `application/x-tar`.

Also add test cases for a number of UStar-compatible formats, created by
GNU tar 1.29 (with `--format=<format-name>`):

* `tar.gnu.tar`
* `tar.oldgnu.tar`
* `tar.posix.tar`
* `tar.ustar.tar`

as well as `tar.star.tar` (created by star 1.6) and, for completeness,
`tar.v7-gnu.tar` (a v7 tar archive created by GNU tar 1.29).

Fixes #307.

internal/magic/archive.go

@@ -74,13 +74,21 @@ func CRX(raw []byte, limit uint32) bool {
 }
 
 // Tar matches a (t)ape (ar)chive file.
-//
-// Signature source: https://www.nationalarchives.gov.uk/PRONOM/Format/proFormatSearch.aspx?status=detailReport&id=385&strPageToDisplay=signatures
 func Tar(raw []byte, _ uint32) bool {
+ // The "magic" header field for files in in UStar (POSIX IEEE P1003.1) archives
+ // has the prefix "ustar". The values of the remaining bytes in this field vary
+ // by archiver implementation.
+ if len(raw) >= 512 && bytes.HasPrefix(raw[257:], []byte{0x75, 0x73, 0x74, 0x61, 0x72}) {
+ return true
+ }
+
  if len(raw) < 256 {
  return false
  }
 
+ // The older v7 format has no "magic" field, and therefore must be identified
+ // with heuristics based on legal ranges of values for other header fields:
+ // https://www.nationalarchives.gov.uk/PRONOM/Format/proFormatSearch.aspx?status=detailReport&id=385&strPageToDisplay=signatures
  rules := []struct {
  min, max uint8
  i int

mimetype_test.go

@@ -180,12 +180,20 @@ var files = map[string]string{
  // the timestamps.
  "not.srt.txt": "text/plain; charset=utf-8",
  // not.srt.2.txt does not specify milliseconds.
- "not.srt.2.txt": "text/plain; charset=utf-8",
- "svg.1.svg": "image/svg+xml",
- "svg.svg": "image/svg+xml",
- "swf.swf": "application/x-shockwave-flash",
- "tar.tar": "application/x-tar",
- "tar.v7.tar": "application/x-tar",
+ "not.srt.2.txt": "text/plain; charset=utf-8",
+ "svg.1.svg": "image/svg+xml",
+ "svg.svg": "image/svg+xml",
+ "swf.swf": "application/x-shockwave-flash",
+ "tar.tar": "application/x-tar",
+ "tar.gnu.tar": "application/x-tar",
+ "tar.oldgnu.tar": "application/x-tar",
+ "tar.posix.tar": "application/x-tar",
+ // tar.star.tar was generated with star 1.6.
+ "tar.star.tar": "application/x-tar",
+ "tar.ustar.tar": "application/x-tar",
+ "tar.v7.tar": "application/x-tar",
+ // tar.v7-gnu.tar is a v7 tar archive generated with GNU tar 1.29.
+ "tar.v7-gnu.tar": "application/x-tar",
  "tcl.tcl": "text/x-tcl",
  "tcx.tcx": "application/vnd.garmin.tcx+xml",
  "tiff.tiff": "image/tiff",