conf: add a "database" block

The new block overrides the "db_file" setting, and allows configuring
any database plugin.

doc/user.xml

@@ -164,6 +164,53 @@ systemctl start mpd.socket</programlisting>
       </para>
     </section>
 
+    <section>
+      <title>Configuring database plugins</title>
+
+      <para>
+        If a music directory is configured, one database plugin is
+        used.  To configure this plugin, add a
+        <varname>database</varname> block to
+        <filename>mpd.conf</filename>:
+      </para>
+
+      <programlisting>database {
+    plugin "simple"
+    path "/var/lib/mpd/db"
+}
+      </programlisting>
+
+      <para>
+        The following table lists the <varname>database</varname>
+        options valid for all plugins:
+      </para>
+
+      <informaltable>
+        <tgroup cols="2">
+          <thead>
+            <row>
+              <entry>
+                Name
+              </entry>
+              <entry>
+                Description
+              </entry>
+            </row>
+          </thead>
+          <tbody>
+            <row>
+              <entry>
+                <varname>plugin</varname>
+              </entry>
+              <entry>
+                The name of the plugin.
+              </entry>
+            </row>
+          </tbody>
+        </tgroup>
+      </informaltable>
+    </section>
+
     <section>
       <title>Configuring input plugins</title>
 
@@ -617,6 +664,78 @@ systemctl start mpd.socket</programlisting>
   <chapter>
     <title>Plugin reference</title>
 
+    <section>
+      <title>Database plugins</title>
+
+      <section>
+        <title><varname>simple</varname></title>
+
+        <para>
+          The default plugin.  Stores a copy of the database in
+          memory.  A file is used for permanent storage.
+        </para>
+
+        <informaltable>
+          <tgroup cols="2">
+            <thead>
+              <row>
+                <entry>Setting</entry>
+                <entry>Description</entry>
+              </row>
+            </thead>
+            <tbody>
+              <row>
+                <entry>
+                  <varname>path</varname>
+                </entry>
+                <entry>
+                  The path of the database file.
+                </entry>
+              </row>
+            </tbody>
+          </tgroup>
+        </informaltable>
+      </section>
+
+      <section>
+        <title><varname>proxy</varname></title>
+
+        <para>
+          Provides access to the database of another MPD instance
+          using <filename>libmpdclient</filename>.  Experimental!
+        </para>
+
+        <informaltable>
+          <tgroup cols="2">
+            <thead>
+              <row>
+                <entry>Setting</entry>
+                <entry>Description</entry>
+              </row>
+            </thead>
+            <tbody>
+              <row>
+                <entry>
+                  <varname>host</varname>
+                </entry>
+                <entry>
+                  The host name of the "master" MPD instance.
+                </entry>
+              </row>
+              <row>
+                <entry>
+                  <varname>port</varname>
+                </entry>
+                <entry>
+                  The port number of the "master" MPD instance.
+                </entry>
+              </row>
+            </tbody>
+          </tgroup>
+        </informaltable>
+      </section>
+    </section>
+
     <section>
       <title>Input plugins</title>
 

src/conf.c

@@ -102,6 +102,7 @@ static struct config_entry config_entries[] = {
 	{ .name = CONF_DESPOTIFY_PASSWORD, false, false},
 	{ .name = CONF_DESPOTIFY_HIGH_BITRATE, false, false },
 	{ .name = "filter", true, true },
+	{ .name = "database", false, true },
 };
 
 static bool

src/main.c

@@ -154,35 +154,47 @@ glue_mapper_init(GError **error_r)
 static bool
 glue_db_init_and_load(void)
 {
+	const struct config_param *param = config_get_param("database");
 	const struct config_param *path = config_get_param(CONF_DB_FILE);
 
+	if (param != NULL && path != NULL)
+		g_message("Found both 'database' and '" CONF_DB_FILE
+			  "' setting - ignoring the latter");
+
 	GError *error = NULL;
 	bool ret;
 
 	if (!mapper_has_music_directory()) {
+		if (param != NULL)
+			g_message("Found database setting without "
+				  CONF_MUSIC_DIR " - disabling database");
 		if (path != NULL)
 			g_message("Found " CONF_DB_FILE " setting without "
 				  CONF_MUSIC_DIR " - disabling database");
 		return true;
 	}
 
-	if (path == NULL)
+	struct config_param *allocated = NULL;
-		MPD_ERROR(CONF_DB_FILE " setting missing");
 
-	struct config_param *param = config_new_param("database", path->line);
+	if (param == NULL && path != NULL) {
-	config_add_block_param(param, "path", path->value, path->line);
+		allocated = config_new_param("database", path->line);
+		config_add_block_param(allocated, "path",
+				       path->value, path->line);
+		param = allocated;
+	}
 
 	if (!db_init(param, &error))
 		MPD_ERROR("%s", error->message);
 
-	config_param_free(param);
+	if (allocated != NULL)
+		config_param_free(allocated);
 
 	ret = db_load(&error);
 	if (!ret)
 		MPD_ERROR("%s", error->message);
 
 	/* run database update after daemonization? */
-	return db_exists();
+	return !db_is_simple() || db_exists();
 }
 
 /**