BinpkgExtractorAsync: remove trailer when decompressing binary packages

xz and zstd don't support trailing data
and fail upon encountering xpak trailer

Bug: 630132
Closes: #210

pym/_emerge/BinpkgExtractorAsync.py

@@ -74,20 +74,22 @@ def _start(self):
 				self._async_wait()
 				return
 
-		# Add -q to decomp_cmd opts, in order to avoid "trailing garbage
-		# after EOF ignored" warning messages due to xpak trailer.
+		pkg_xpak = portage.xpak.tbz2(self.pkg_path)
+		pkg_xpak.scan()
+
 		# SIGPIPE handling (128 + SIGPIPE) should be compatible with
 		# assert_sigpipe_ok() that's used by the ebuild unpack() helper.
 		self.args = [self._shell_binary, "-c",
-			("%s -cq -- %s | tar -xp %s -C %s -f - ; " + \
+			("head -c-%d -- %s | %s | tar -xp %s -C %s -f - ; " + \
 			"p=(${PIPESTATUS[@]}) ; " + \
 			"if [[ ${p[0]} != 0 && ${p[0]} != %d ]] ; then " % (128 + signal.SIGPIPE) + \
 			"echo bzip2 failed with status ${p[0]} ; exit ${p[0]} ; fi ; " + \
 			"if [ ${p[1]} != 0 ] ; then " + \
 			"echo tar failed with status ${p[1]} ; exit ${p[1]} ; fi ; " + \
 			"exit 0 ;") % \
-			(decomp_cmd,
+			(pkg_xpak.xpaksize,
 			portage._shell_quote(self.pkg_path),
+			decomp_cmd,
 			tar_options,
 			portage._shell_quote(self.image_dir))]