makepkg: immutable bzr by hashing the checkout content

This feature makes bzr VCS build inputs immutable by adding support for
pinning a bzr checkout by a hash of its content using the deterministic
export functionality `bzr export`.

This feature allows to preserve security implications of immutable build inputs
using a trusted cryptographic hash function of the content.

Signed-off-by: Levente Polyak <anthraxx@archlinux.org>

scripts/libmakepkg/source/bzr.sh.in

@@ -109,3 +109,29 @@ extract_bzr() {
 
 	popd &>/dev/null
 }
+
+calc_checksum_bzr() {
+	local netfile=$1 integ=$2 ret=0 shellopts dir url fragment fragval sum
+
+	# this function requires pipefail - save current status to restore later
+	shellopts=$(shopt -p -o pipefail)
+	shopt -s -o pipefail
+
+	dir=$(get_filepath "$netfile")
+	url=$(get_url "$netfile")
+	fragment=$(get_uri_fragment "$url")
+
+	case ${fragment%%=*} in
+		revision)
+			fragval=${fragment##*=}
+			sum=$(bzr export --directory "$dir" --format tar --revision "$fragval" - | "${integ}sum" 2>&1) || ret=1
+			sum="${sum%% *}"
+			;;
+		*)
+			sum="SKIP"
+	esac
+
+	eval "$shellopts"
+	printf '%s' "$sum"
+	return $ret
+}