pacman-key: rework and document holding keys in keyring

The HoldKey option was undocumented and was not suited for pacman.conf.
Instead use the file "/etc/pacman.d/gnupg/heldkeys" to contain a list
of keys not to be removed from the pacman keyring with the --populate
option.

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/pacman-key.8.txt

@@ -107,6 +107,10 @@ required to be signed (detached) by a trusted PGP key that the user must
 manually import to the pacman keyring. This prevents a potentially malicious
 repository adding keys to the pacman keyring without the users knowledge.
 
+A key being marked as revoked always takes priority over the key being added to
+the pacman keyring, regardless of the keyring it is provided in. To prevent a
+key from being revoked when using --populate, its ID can be listed in
++{sysconfdir}/pacman.d/gnupg/holdkeys+.
 
 See Also
 --------

scripts/pacman-key.sh.in

@@ -232,17 +232,15 @@ populate_keyring() {
 		fi
 	done
 
-	# List of keys that must be kept installed, even if in the list of keys to be removed
+	# Read list of keys that must be kept installed and remove them from the list
-	local HOLD_KEYS="$(get_from "$CONFIG" "HoldKeys")"
+	# of keys to be removed
-
+	if [[ -f "${PACMAN_KEYRING_DIR}/holdkeys" ]]; then
-	# Remove the keys that must be kept from the set of keys that should be removed
+		while read key; do
-	if [[ -n ${HOLD_KEYS} ]]; then
-		for key in ${HOLD_KEYS}; do
 			key_id="$("${GPG_PACMAN[@]}" --quiet --with-colons --list-key "${key}" | grep ^pub | cut -d: -f5)"
 			if [[ -n "${removed_ids[$key_id]}" ]]; then
 				unset removed_ids[$key_id]
 			fi
-		done
+		done < "${PACMAN_KEYRING_DIR}/holdkeys"
 	fi
 
 	# Remove the keys not marked to keep