mmdebstrap: unpack essential packages with proxy script

try unpack package with dpkg-deb and fallback to manual extraction

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>

debextract

@@ -0,0 +1,50 @@
+#!/bin/sh
+set -ef
+
+# assume that both data.tar and control.tar are compressed with same compressor
+# if dpkg-deb can handle control.tar.* then it will handle data.tar.* too
+if dpkg-deb --ctrl-tarfile "$1" >/dev/null 2>&1 ; then
+	exec dpkg-deb --fsys-tarfile "$1"
+fi
+
+me=${0##*/}
+log() { echo "${me}: $*" 1>&2 ; }
+
+# here we're unpacking data.tar.* manually
+
+# naive test for .deb file
+# also check `ar' presense
+ar t "$1" >/dev/null
+
+data_tar=
+while read -r pkg_member ; do
+	[ -n "${pkg_member}" ] || continue
+
+	if [ -n "${data_tar}" ] ; then
+		log "extra data tarball '${pkg_member}' in package $1 - bailing out"
+		exit 1
+	fi
+
+	data_tar="${pkg_member}"
+done <<-EOF
+$(ar t "$1" | grep -E '^data\.tar')
+EOF
+
+decomp=
+case "${data_tar}" in
+data.tar.gz)  decomp='gzip' ;;
+data.tar.xz)  decomp='xz'   ;;
+data.tar.zst) decomp='zstd' ;;
+esac
+
+if [ -z "${decomp}" ] ; then
+	log "unable to handle '${data_tar}' from $1 - compression type isn't known to script"
+	exit 1
+fi
+
+if ! command -v ${decomp} >/dev/null ; then
+	log "enable to find '${decomp}'"
+	exit 1
+fi
+
+ar p "$1" "${data_tar}" | ${decomp} -d

mmdebstrap

@@ -2502,9 +2502,9 @@ sub run_extract() {
                 close($filter_writer)
                   or error "cannot close filter_writer: $!";
             }
-            debug("running dpkg-deb --fsys-tarfile $options->{root}/$deb");
+            debug("running debextract $options->{root}/$deb");
             eval { Devel::Cover::set_coverage("none") } if $is_covering;
-            exec 'dpkg-deb', '--fsys-tarfile', "$options->{root}/$deb";
+            exec 'debextract', "$options->{root}/$deb";
         }
         my $pid2;
         if (scalar @tarfilterargs > 0) {
@@ -2544,7 +2544,7 @@ sub run_extract() {
             close($filter_writer) or error "cannot close filter_writer: $!";
         }
         waitpid($pid1, 0);
-        $? == 0 or error "dpkg-deb --fsys-tarfile failed: $?";
+        $? == 0 or error "debextract failed: $?";
         if (scalar @tarfilterargs > 0) {
             waitpid($pid2, 0);
             $? == 0 or error "tarfilter failed: $?";