Instead of passing tag and group, pass an array of tags. To support a
nested return value, return a nested std::map of std::maps. Each key
specifies the tag value, and each value may be another nesting level.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/408
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 ae178c77bd, 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.
This commit is similar to 788e3b31e1,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's). GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.
Closes#58
