Initial support for $HOME and some XDG variables inside the configuration file

This commit tries to address issues #263 and #476. It enables the path expansion of HOME, XDG_CONFIG_HOME, XDG_MUSIC_DIR, XDG_CACHE_HOME and XDG_RUNTIME_DIR by using the glue functions already available in MPD.

Signed-off-by: Joan Vilardaga <github-91yu@joanvc.cat>

doc/mpd.conf.5.rst

@@ -17,10 +17,19 @@ Each line in the configuration file contains a setting name and its value, e.g.:
 
 Lines starting with ``#`` are treated as comments and ignored.
 
-For settings which specify a filesystem path, the tilde is expanded:
+For settings that specify a file system path, the tilde ('~') is expanded to $HOME.
+In addition, the following path expansions are supported:
+
+- `$HOME`
+- `$XDG_CONFIG_HOME`
+- `$XDG_MUSIC_DIR`
+- `$XDG_CACHE_HOME`
+- `$XDG_RUNTIME_DIR`
 
 :code:`music_directory "~/Music"`
 
+:code:`db_file "$XDG_CONFIG_HOME/mpd/database"`
+
 Some of the settings are grouped in blocks with curly braces, e.g. per-plugin settings:
 
 .. code-block:: none

doc/mpdconf.example

@@ -10,6 +10,7 @@
 # be disabled and audio files will only be accepted over ipc socket (using
 # file:// protocol) or streaming files over an accepted protocol.
 #
+#music_directory		"$XDG_MUSIC_DIR"
 #music_directory		"~/music"
 #
 # This setting sets the MPD internal playlist directory. The purpose of this
@@ -17,6 +18,7 @@
 # playlist files not created by the server but only if they are in the MPD
 # format. This setting defaults to playlist saving being disabled.
 #
+#playlist_directory		"$XDG_CONFIG_HOME/mpd/playlists"
 #playlist_directory		"~/.mpd/playlists"
 #
 # This setting sets the location of the MPD database. This file is used to
@@ -25,6 +27,7 @@
 # MPD to accept files over ipc socket (using file:// protocol) or streaming
 # files over an accepted protocol.
 #
+#db_file			"$XDG_CACHE_HOME/mpd/database"
 #db_file			"~/.mpd/database"
 
 # These settings are the locations for the daemon log files for the daemon.
@@ -35,6 +38,7 @@
 # If you use systemd, do not configure a log_file.  With systemd, MPD
 # defaults to the systemd journal, which is fine.
 #
+#log_file			"$XDG_CACHE_HOME/mpd/log"
 #log_file			"~/.mpd/log"
 
 # This setting sets the location of the file which stores the process ID
@@ -43,6 +47,7 @@
 #
 # If you use systemd, do not configure a pid_file.
 #
+#pid_file			"$XDG_RUNTIME_DIR/mpd/mpd.pid"
 #pid_file			"~/.mpd/pid"
 
 # This setting sets the location of the file which contains information about
@@ -50,11 +55,13 @@
 # it was brought down. This setting is disabled by default and the server
 # state will be reset on server start up.
 #
+#state_file			"$XDG_RUNTIME_DIR/mpd/state"
 #state_file			"~/.mpd/state"
 #
 # The location of the sticker database.  This is a database which
 # manages dynamic information attached to songs.
 #
+#sticker_file			"$XDG_CACHE_HOME/sticker.sql"
 #sticker_file			"~/.mpd/sticker.sql"
 #
 ###############################################################################
@@ -85,6 +92,7 @@
 #bind_to_address		"any"
 #
 # And for Unix Socket
+#bind_to_address		"$XDG_RUNTIME_DIR/mpd/socket"
 #bind_to_address		"~/.mpd/socket"
 #
 # This setting is the TCP port that is desired for the daemon to get assigned

doc/user.rst

@@ -250,10 +250,18 @@ Each line in the configuration file contains a setting name and its value, e.g.:
 
 Lines starting with ``#`` are treated as comments and ignored.
 
-For settings which specify a filesystem path, the tilde is expanded:
+For settings that specify a file system path, the tilde ('~') is expanded to $HOME. In addition, the following path expansions are supported:
+
+- `$HOME`
+- `$XDG_CONFIG_HOME`
+- `$XDG_MUSIC_DIR`
+- `$XDG_CACHE_HOME`
+- `$XDG_RUNTIME_DIR`
 
 :code:`music_directory "~/Music"`
 
+:code:`db_file "$XDG_CONFIG_HOME/mpd/database"`
+
 Some of the settings are grouped in blocks with curly braces, e.g. per-plugin settings:
 
 .. code-block:: none
@@ -1210,8 +1218,11 @@ Mounting is only possible with the simple database plugin and a :code:`cache_dir
 
     database {
       plugin "simple"
-      path "~/.mpd/db"
-      cache_directory "~/.mpd/cache"
+      path "$XDG_CACHE_HOME/mpd/database"
+      cache_directory "$XDG_CACHE_HOME/mpd/"
+      # or you can also use relative or absolute paths
+      # path "~/.mpd/db"
+      # cache_directory "~/.mpd/cache"
     }
         
 This requires migrating from the old :code:`db_file` setting to a database section. The cache directory must exist, and :program:`MPD` will put one file per mount there, which will be reused when the same storage is used again later.

src/config/Net.cxx

@@ -5,13 +5,18 @@
 #include "event/ServerSocket.hxx"
 #include "Path.hxx"
 #include "fs/AllocatedPath.hxx"
+#include "fs/XDG.hxx"
 
 void
 ServerSocketAddGeneric(ServerSocket &server_socket, const char *address, unsigned int port)
 {
 	if (address == nullptr || 0 == strcmp(address, "any")) {
 		server_socket.AddPort(port);
+#ifdef USE_XDG
+	} else if (address[0] == '/' || address[0] == '~' || address[0] == '$') {
+#else
 	} else if (address[0] == '/' || address[0] == '~') {
+#endif
 		server_socket.AddPath(ParsePath(address));
 	} else if (address[0] == '@') {
 		server_socket.AddAbstract(address);

src/config/Path.cxx

@@ -5,6 +5,7 @@
 #include "Data.hxx"
 #include "fs/AllocatedPath.hxx"
 #include "fs/Traits.hxx"
+#include "fs/XDG.hxx"
 #include "fs/glue/StandardDirectory.hxx"
 #include "lib/fmt/RuntimeError.hxx"
 #include "util/StringSplit.hxx"
@@ -90,6 +91,29 @@ ParsePath(const char *path)
 			return GetHome(std::string{user}.c_str())
 				/ AllocatedPath::FromUTF8Throw(rest);
 		}
+#ifdef USE_XDG
+	} else if (path[0] == '$') {
+		++path;
+
+		const auto [env_var, rest] = Split(std::string_view{path}, '/');
+
+	        AllocatedPath xdg_path(nullptr);
+		if (env_var == "HOME") {
+			xdg_path = GetConfiguredHome();
+		} else if (env_var == "XDG_CONFIG_HOME") {
+			xdg_path = GetUserConfigDir();
+		} else if (env_var == "XDG_MUSIC_DIR") {
+			xdg_path = GetUserMusicDir();
+		} else if (env_var == "XDG_CACHE_HOME") {
+			xdg_path = GetUserCacheDir();
+		} else if (env_var == "XDG_RUNTIME_DIR") {
+			xdg_path = GetUserRuntimeDir();
+		} else {
+			throw FmtRuntimeError("environment variable not supported: {:?}", env_var);
+		}
+
+		return xdg_path / AllocatedPath::FromUTF8Throw(rest);
+#endif
 	} else if (!PathTraitsUTF8::IsAbsolute(path)) {
 		throw FmtRuntimeError("not an absolute path: {:?}", path);
 	} else {