db: fix broken command "list ... group"

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77bdc47c954fd9a4b32ffc07fe6c4a8a49, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

1 files changed, 7 insertions, 10 deletions

diff --git a/src/command/DatabaseCommands.cxx b/src/command/DatabaseCommands.cxx
index 42f79b8e2..3f7d18324 100644
--- a/src/command/DatabaseCommands.cxx
+++ b/src/command/DatabaseCommands.cxx
@@ -191,7 +191,7 @@ handle_list(Client &client, Request args, Response &r)
 	}
 
 	std::unique_ptr<SongFilter> filter;
-	tag_mask_t group_mask = 0;
+	TagType group = TAG_NUM_OF_ITEM_TYPES;
 
 	if (args.size == 1) {
 		/* for compatibility with < 0.12.0 */
@@ -206,18 +206,16 @@ handle_list(Client &client, Request args, Response &r)
 					    args.shift()));
 	}
 
-	while (args.size >= 2 &&
-	       StringIsEqual(args[args.size - 2], "group")) {
+	if  (args.size >= 2 &&
+	     StringIsEqual(args[args.size - 2], "group")) {
 		const char *s = args[args.size - 1];
-		TagType gt = tag_name_parse_i(s);
-		if (gt == TAG_NUM_OF_ITEM_TYPES) {
+		group = tag_name_parse_i(s);
+		if (group == TAG_NUM_OF_ITEM_TYPES) {
 			r.FormatError(ACK_ERROR_ARG,
 				      "Unknown tag type: %s", s);
 			return CommandResult::ERROR;
 		}
 
-		group_mask |= tag_mask_t(1) << unsigned(gt);
-
 		args.pop_back();
 		args.pop_back();
 	}
@@ -230,14 +228,13 @@ handle_list(Client &client, Request args, Response &r)
 		}
 	}
 
-	if (tagType < TAG_NUM_OF_ITEM_TYPES &&
-	    group_mask & (tag_mask_t(1) << tagType)) {
+	if (tagType < TAG_NUM_OF_ITEM_TYPES && tagType == group) {
 		r.Error(ACK_ERROR_ARG, "Conflicting group");
 		return CommandResult::ERROR;
 	}
 
 	PrintUniqueTags(r, client.partition,
-			tagType, group_mask, filter.get());
+			tagType, group, filter.get());
 	return CommandResult::OK;
 }