database: add struct db_visitor

Use this struct for db_walk().

13 files changed, 363 insertions, 196 deletions

diff --git a/Makefile.am b/Makefile.am
index b883531f2..4b69a8d59 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -271,9 +271,11 @@ src_mpd_SOURCES = \
 	src/directory_save.c \
 	src/directory_print.c \
 	src/database.c \
+	src/db_error.h \
 	src/db_save.c src/db_save.h \
 	src/db_print.c src/db_print.h \
 	src/db_plugin.h \
+	src/db_visitor.h \
 	src/db/simple_db_plugin.c src/db/simple_db_plugin.h \
 	src/dirvec.c \
 	src/exclude.c \
diff --git a/src/command.c b/src/command.c
index 42d09cf8d..864b65354 100644
--- a/src/command.c
+++ b/src/command.c
@@ -43,6 +43,7 @@
 #include "output_print.h"
 #include "locate.h"
 #include "dbUtils.h"
+#include "db_error.h"
 #include "db_print.h"
 #include "tag.h"
 #include "client.h"
@@ -392,6 +393,13 @@ print_error(struct client *client, GError *error)
 		enum playlist_result result = error->code;
 		g_error_free(error);
 		return print_playlist_result(client, result);
+	} else if (error->domain == db_quark()) {
+		switch ((enum db_error)error->code) {
+		case DB_NOT_FOUND:
+			g_error_free(error);
+			command_error(client, ACK_ERROR_NO_EXIST, "Not found");
+			return COMMAND_RETURN_ERROR;
+		}
 	} else if (error->domain == g_file_error_quark()) {
 		command_error(client, ACK_ERROR_SYSTEM, "%s",
 			      g_strerror(error->code));
@@ -657,14 +665,10 @@ handle_add(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 		return print_playlist_result(client, result);
 	}
 
-	result = addAllIn(client->player_control, uri);
-	if (result == (enum playlist_result)-1) {
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
-		return COMMAND_RETURN_ERROR;
-	}
-
-	return print_playlist_result(client, result);
+	GError *error = NULL;
+	return addAllIn(client->player_control, uri, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 }
 
 static enum command_return
@@ -941,7 +945,6 @@ handle_playlistid(struct client *client, int argc, char *argv[])
 static enum command_return
 handle_find(struct client *client, int argc, char *argv[])
 {
-	int ret;
 	struct locate_item_list *list =
 		locate_item_list_parse(argv + 1, argc - 1);
 
@@ -953,10 +956,10 @@ handle_find(struct client *client, int argc, char *argv[])
 		return COMMAND_RETURN_ERROR;
 	}
 
-	ret = findSongsIn(client, NULL, list);
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
+	GError *error = NULL;
+	enum command_return ret = findSongsIn(client, NULL, list, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 
 	locate_item_list_free(list);
 
@@ -966,7 +969,6 @@ handle_find(struct client *client, int argc, char *argv[])
 static enum command_return
 handle_findadd(struct client *client, int argc, char *argv[])
 {
-    int ret;
     struct locate_item_list *list =
 	    locate_item_list_parse(argv + 1, argc - 1);
     if (list == NULL || list->length == 0) {
@@ -977,10 +979,11 @@ handle_findadd(struct client *client, int argc, char *argv[])
 	    return COMMAND_RETURN_ERROR;
     }
 
-    ret = findAddIn(client->player_control, NULL, list);
-    if (ret == -1)
-	    command_error(client, ACK_ERROR_NO_EXIST,
-			  "directory or file not found");
+    GError *error = NULL;
+    enum command_return ret =
+	    findAddIn(client->player_control, NULL, list, &error)
+	    ? COMMAND_RETURN_OK
+	    : print_error(client, error);
 
     locate_item_list_free(list);
 
@@ -990,7 +993,6 @@ handle_findadd(struct client *client, int argc, char *argv[])
 static enum command_return
 handle_search(struct client *client, int argc, char *argv[])
 {
-	int ret;
 	struct locate_item_list *list =
 		locate_item_list_parse(argv + 1, argc - 1);
 
@@ -1002,10 +1004,10 @@ handle_search(struct client *client, int argc, char *argv[])
 		return COMMAND_RETURN_ERROR;
 	}
 
-	ret = searchForSongsIn(client, NULL, list);
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
+	GError *error = NULL;
+	enum command_return ret = searchForSongsIn(client, NULL, list, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 
 	locate_item_list_free(list);
 
@@ -1015,7 +1017,6 @@ handle_search(struct client *client, int argc, char *argv[])
 static enum command_return
 handle_count(struct client *client, int argc, char *argv[])
 {
-	int ret;
 	struct locate_item_list *list =
 		locate_item_list_parse(argv + 1, argc - 1);
 
@@ -1027,10 +1028,11 @@ handle_count(struct client *client, int argc, char *argv[])
 		return COMMAND_RETURN_ERROR;
 	}
 
-	ret = searchStatsForSongsIn(client, NULL, list);
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
+	GError *error = NULL;
+	enum command_return ret =
+		searchStatsForSongsIn(client, NULL, list, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 
 	locate_item_list_free(list);
 
@@ -1259,17 +1261,14 @@ static enum command_return
 handle_listall(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 {
 	char *directory = NULL;
-	int ret;
 
 	if (argc == 2)
 		directory = argv[1];
 
-	ret = printAllIn(client, directory);
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
-
-	return ret;
+	GError *error = NULL;
+	return printAllIn(client, directory, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 }
 
 static enum command_return
@@ -1388,7 +1387,6 @@ handle_list(struct client *client, int argc, char *argv[])
 {
 	struct locate_item_list *conditionals;
 	int tagType = locate_parse_type(argv[1]);
-	int ret;
 
 	if (tagType < 0) {
 		command_error(client, ACK_ERROR_ARG, "\"%s\" is not known", argv[1]);
@@ -1425,14 +1423,14 @@ handle_list(struct client *client, int argc, char *argv[])
 		}
 	}
 
-	ret = listAllUniqueTags(client, tagType, conditionals);
+	GError *error = NULL;
+	enum command_return ret =
+		listAllUniqueTags(client, tagType, conditionals, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 
 	locate_item_list_free(conditionals);
 
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
-
 	return ret;
 }
 
@@ -1534,17 +1532,14 @@ static enum command_return
 handle_listallinfo(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 {
 	char *directory = NULL;
-	int ret;
 
 	if (argc == 2)
 		directory = argv[1];
 
-	ret = printInfoForAllIn(client, directory);
-	if (ret == -1)
-		command_error(client, ACK_ERROR_NO_EXIST,
-			      "directory or file not found");
-
-	return ret;
+	GError *error = NULL;
+	return printInfoForAllIn(client, directory, &error)
+		? COMMAND_RETURN_OK
+		: print_error(client, error);
 }
 
 static enum command_return
diff --git a/src/database.c b/src/database.c
index ea7f01eb1..6d7cc3e23 100644
--- a/src/database.c
+++ b/src/database.c
@@ -19,7 +19,9 @@
 
 #include "config.h"
 #include "database.h"
+#include "db_error.h"
 #include "db_save.h"
+#include "db_visitor.h"
 #include "db_plugin.h"
 #include "db/simple_db_plugin.h"
 #include "directory.h"
@@ -41,15 +43,6 @@
 static struct db *db;
 static bool db_is_open;
 
-/**
- * The quark used for GError.domain.
- */
-static inline GQuark
-db_quark(void)
-{
-	return g_quark_from_static_string("database");
-}
-
 bool
 db_init(const struct config_param *path, GError **error_r)
 {
@@ -114,25 +107,28 @@ db_get_song(const char *file)
 	return directory_lookup_song(music_root, file);
 }
 
-int
-db_walk(const char *name,
-	int (*forEachSong)(struct song *, void *),
-	int (*forEachDir)(struct directory *, void *), void *data)
+bool
+db_walk(const char *uri,
+	const struct db_visitor *visitor, void *ctx,
+	GError **error_r)
 {
 	struct directory *directory;
 
 	if (db == NULL)
 		return -1;
 
-	if ((directory = db_get_directory(name)) == NULL) {
+	if ((directory = db_get_directory(uri)) == NULL) {
 		struct song *song;
-		if ((song = db_get_song(name)) && forEachSong) {
-			return forEachSong(song, data);
-		}
-		return -1;
+		if (visitor->song != NULL &&
+		    (song = db_get_song(uri)) != NULL)
+			return visitor->song(song, ctx, error_r);
+
+		g_set_error(error_r, db_quark(), DB_NOT_FOUND,
+			    "No such directory: %s", uri);
+		return false;
 	}
 
-	return directory_walk(directory, forEachSong, forEachDir, data);
+	return directory_walk(directory, visitor, ctx, error_r);
 }
 
 bool
diff --git a/src/database.h b/src/database.h
index 3e834ab84..ab9114f2d 100644
--- a/src/database.h
+++ b/src/database.h
@@ -27,6 +27,7 @@
 
 struct config_param;
 struct directory;
+struct db_visitor;
 
 /**
  * Initialize the database library.
@@ -52,9 +53,10 @@ db_get_directory(const char *name);
 struct song *
 db_get_song(const char *file);
 
-int db_walk(const char *name,
-	    int (*forEachSong)(struct song *, void *),
-	    int (*forEachDir)(struct directory *, void *), void *data);
+bool
+db_walk(const char *uri,
+	const struct db_visitor *visitor, void *ctx,
+	GError **error_r);
 
 bool
 db_check(GError **error_r);
diff --git a/src/dbUtils.c b/src/dbUtils.c
index e0d8c5379..827d0a0c1 100644
--- a/src/dbUtils.c
+++ b/src/dbUtils.c
@@ -21,52 +21,66 @@
 #include "dbUtils.h"
 #include "locate.h"
 #include "database.h"
+#include "db_visitor.h"
 #include "playlist.h"
 #include "stored_playlist.h"
 
 #include <glib.h>
 
-static int
-directoryAddSongToPlaylist(struct song *song, void *data)
+static bool
+add_to_queue_song(struct song *song, void *ctx, GError **error_r)
 {
-	struct player_control *pc = data;
+	struct player_control *pc = ctx;
+
+	enum playlist_result result =
+		playlist_append_song(&g_playlist, pc, song, NULL);
+	if (result != PLAYLIST_RESULT_SUCCESS) {
+		g_set_error(error_r, playlist_quark(), result,
+			    "Playlist error");
+		return false;
+	}
+
+	return true;
+}
+
+static const struct db_visitor add_to_queue_visitor = {
+	.song = add_to_queue_song,
+};
 
-	return playlist_append_song(&g_playlist, pc, song, NULL);
+bool
+addAllIn(struct player_control *pc, const char *uri, GError **error_r)
+{
+	return db_walk(uri, &add_to_queue_visitor, pc, error_r);
 }
 
 struct add_data {
 	const char *path;
-	GError **error_r;
 };
 
-static int
-directoryAddSongToStoredPlaylist(struct song *song, void *_data)
+static bool
+add_to_spl_song(struct song *song, void *ctx, GError **error_r)
 {
-	struct add_data *data = _data;
+	struct add_data *data = ctx;
 
-	if (!spl_append_song(data->path, song, data->error_r)) {
-		return -1;
-	}
+	if (!spl_append_song(data->path, song, error_r))
+		return false;
 
-	return 0;
+	return true;
 }
 
-int
-addAllIn(struct player_control *pc, const char *name)
-{
-	return db_walk(name, directoryAddSongToPlaylist, NULL, pc);
-}
+static const struct db_visitor add_to_spl_visitor = {
+	.song = add_to_spl_song,
+};
 
 bool
-addAllInToStoredPlaylist(const char *name, const char *utf8file,
+addAllInToStoredPlaylist(const char *uri_utf8, const char *path_utf8,
 			 GError **error_r)
 {
 	struct add_data data = {
-		.path = utf8file,
-		.error_r = error_r,
+		.path = path_utf8,
 	};
 
-	return db_walk(name, directoryAddSongToStoredPlaylist, NULL, &data) == 0;
+	return db_walk(uri_utf8, &add_to_spl_visitor, &data, error_r);
 }
 
 struct find_add_data {
@@ -74,26 +88,37 @@ struct find_add_data {
 	const struct locate_item_list *criteria;
 };
 
-static int
-findAddInDirectory(struct song *song, void *_data)
+static bool
+find_add_song(struct song *song, void *ctx, GError **error_r)
 {
-	struct find_add_data *data = _data;
-
-	if (locate_song_match(song, data->criteria))
-		return playlist_append_song(&g_playlist,
-					    data->pc,
-					    song, NULL);
+	struct find_add_data *data = ctx;
+
+	if (!locate_song_match(song, data->criteria))
+		return true;
+
+	enum playlist_result result =
+		playlist_append_song(&g_playlist, data->pc,
+				     song, NULL);
+	if (result != PLAYLIST_RESULT_SUCCESS) {
+		g_set_error(error_r, playlist_quark(), result,
+			    "Playlist error");
+		return false;
+	}
 
-	return 0;
+	return true;
 }
 
-int
+static const struct db_visitor find_add_visitor = {
+	.song = find_add_song,
+};
+
+bool
 findAddIn(struct player_control *pc, const char *name,
-	  const struct locate_item_list *criteria)
+	  const struct locate_item_list *criteria, GError **error_r)
 {
 	struct find_add_data data;
 	data.pc = pc;
 	data.criteria = criteria;
 
-	return db_walk(name, findAddInDirectory, NULL, &data);
+	return db_walk(name, &find_add_visitor, &data, error_r);
 }
diff --git a/src/dbUtils.h b/src/dbUtils.h
index 65349a32f..f693a7793 100644
--- a/src/dbUtils.h
+++ b/src/dbUtils.h
@@ -26,16 +26,16 @@
 struct locate_item_list;
 struct player_control;
 
-int
-addAllIn(struct player_control *pc, const char *name);
+bool
+addAllIn(struct player_control *pc, const char *uri, GError **error_r);
 
 bool
 addAllInToStoredPlaylist(const char *uri_utf8, const char *path_utf8,
 			 GError **error_r);
 
-int
+bool
 findAddIn(struct player_control *pc, const char *name,
-	  const struct locate_item_list *criteria);
+	  const struct locate_item_list *criteria, GError **error_r);
 
 unsigned long sumSongTimesIn(const char *name);
 
diff --git a/src/db_error.h b/src/db_error.h
new file mode 100644
index 000000000..35051c40c
--- /dev/null
+++ b/src/db_error.h
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2003-2011 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_DB_ERROR_H
+#define MPD_DB_ERROR_H
+
+#include <glib.h>
+
+enum db_error {
+	DB_NOT_FOUND,
+};
+
+/**
+ * Quark for GError.domain; the code is an enum #db_error.
+ */
+G_GNUC_CONST
+static inline GQuark
+db_quark(void)
+{
+	return g_quark_from_static_string("db");
+}
+
+#endif
diff --git a/src/db_print.c b/src/db_print.c
index 629832a3d..c4086a387 100644
--- a/src/db_print.c
+++ b/src/db_print.c
@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "db_print.h"
+#include "db_visitor.h"
 #include "locate.h"
 #include "directory.h"
 #include "database.h"
@@ -41,46 +42,72 @@ typedef struct _SearchStats {
 	unsigned long playTime;
 } SearchStats;
 
-static int
-printDirectoryInDirectory(struct directory *directory, void *data)
+static bool
+print_visitor_directory(const struct directory *directory, void *data,
+			G_GNUC_UNUSED GError **error_r)
 {
 	struct client *client = data;
 
 	if (!directory_is_root(directory))
 		client_printf(client, "directory: %s\n", directory_get_path(directory));
 
-	return 0;
+	return true;
 }
 
-static int
-printSongInDirectory(struct song *song, G_GNUC_UNUSED void *data)
+static bool
+print_visitor_song(struct song *song, void *data,
+		   G_GNUC_UNUSED GError **error_r)
 {
 	struct client *client = data;
 	song_print_uri(client, song);
-	return 0;
+	return true;
 }
 
+static bool
+print_visitor_song_info(struct song *song, void *data,
+			G_GNUC_UNUSED GError **error_r)
+{
+	struct client *client = data;
+	song_print_info(client, song);
+	return true;
+}
+
+static const struct db_visitor print_visitor = {
+	.directory = print_visitor_directory,
+	.song = print_visitor_song,
+};
+
+static const struct db_visitor print_info_visitor = {
+	.directory = print_visitor_directory,
+	.song = print_visitor_song_info,
+};
+
 struct search_data {
 	struct client *client;
 	const struct locate_item_list *criteria;
 };
 
-static int
-searchInDirectory(struct song *song, void *_data)
+static bool
+search_visitor_song(struct song *song, void *_data,
+		    G_GNUC_UNUSED GError **error_r)
 {
 	struct search_data *data = _data;
 
 	if (locate_song_search(song, data->criteria))
 		song_print_info(data->client, song);
 
-	return 0;
+	return true;
 }
 
-int
+static const struct db_visitor search_visitor = {
+	.song = search_visitor_song,
+};
+
+bool
 searchForSongsIn(struct client *client, const char *name,
-		 const struct locate_item_list *criteria)
+		 const struct locate_item_list *criteria,
+		 GError **error_r)
 {
-	int ret;
 	struct locate_item_list *new_list
 		= locate_item_list_casefold(criteria);
 	struct search_data data;
@@ -88,34 +115,40 @@ searchForSongsIn(struct client *client, const char *name,
 	data.client = client;
 	data.criteria = new_list;
 
-	ret = db_walk(name, searchInDirectory, NULL, &data);
+	bool success = db_walk(name, &search_visitor, &data, error_r);
 
 	locate_item_list_free(new_list);
 
-	return ret;
+	return success;
 }
 
-static int
-findInDirectory(struct song *song, void *_data)
+static bool
+find_visitor_song(struct song *song, void *_data,
+		  G_GNUC_UNUSED GError **error_r)
 {
 	struct search_data *data = _data;
 
 	if (locate_song_match(song, data->criteria))
 		song_print_info(data->client, song);
 
-	return 0;
+	return true;
 }
 
-int
+static const struct db_visitor find_visitor = {
+	.song = find_visitor_song,
+};
+
+bool
 findSongsIn(struct client *client, const char *name,
-	    const struct locate_item_list *criteria)
+	    const struct locate_item_list *criteria,
+	    GError **error_r)
 {
 	struct search_data data;
 
 	data.client = client;
 	data.criteria = criteria;
 
-	return db_walk(name, findInDirectory, NULL, &data);
+	return db_walk(name, &find_visitor, &data, error_r);
 }
 
 static void printSearchStats(struct client *client, SearchStats *stats)
@@ -124,8 +157,9 @@ static void printSearchStats(struct client *client, SearchStats *stats)
 	client_printf(client, "playtime: %li\n", stats->playTime);
 }
 
-static int
-searchStatsInDirectory(struct song *song, void *data)
+static bool
+stats_visitor_song(struct song *song, void *data,
+		   G_GNUC_UNUSED GError **error_r)
 {
 	SearchStats *stats = data;
 
@@ -134,45 +168,42 @@ searchStatsInDirectory(struct song *song, void *data)
 		stats->playTime += song_get_duration(song);
 	}
 
-	return 0;
+	return true;
 }
 
-int
+static const struct db_visitor stats_visitor = {
+	.song = stats_visitor_song,
+};
+
+bool
 searchStatsForSongsIn(struct client *client, const char *name,
-		      const struct locate_item_list *criteria)
+		      const struct locate_item_list *criteria,
+		      GError **error_r)
 {
 	SearchStats stats;
-	int ret;
 
 	stats.criteria = criteria;
 	stats.numberOfSongs = 0;
 	stats.playTime = 0;
 
-	ret = db_walk(name, searchStatsInDirectory, NULL, &stats);
-	if (ret == 0)
-		printSearchStats(client, &stats);
+	if (!db_walk(name, &stats_visitor, &stats, error_r))
+		return false;
 
-	return ret;
+	printSearchStats(client, &stats);
+	return true;
 }
 
-int printAllIn(struct client *client, const char *name)
+bool
+printAllIn(struct client *client, const char *uri_utf8, GError **error_r)
 {
-	return db_walk(name, printSongInDirectory,
-		       printDirectoryInDirectory, client);
+	return db_walk(uri_utf8, &print_visitor, client, error_r);
 }
 
-static int
-directoryPrintSongInfo(struct song *song, void *data)
+bool
+printInfoForAllIn(struct client *client, const char *uri_utf8,
+		  GError **error_r)
 {
-	struct client *client = data;
-	song_print_info(client, song);
-	return 0;
-}
-
-int printInfoForAllIn(struct client *client, const char *name)
-{
-	return db_walk(name, directoryPrintSongInfo,
-			     printDirectoryInDirectory, client);
+	return db_walk(uri_utf8, &print_info_visitor, client, error_r);
 }
 
 static ListCommandItem *
@@ -223,8 +254,9 @@ struct list_tags_data {
 	struct strset *set;
 };
 
-static int
-listUniqueTagsInDirectory(struct song *song, void *_data)
+static bool
+unique_tags_visitor_song(struct song *song, void *_data,
+			 G_GNUC_UNUSED GError **error_r)
 {
 	struct list_tags_data *data = _data;
 	ListCommandItem *item = data->item;
@@ -232,13 +264,18 @@ listUniqueTagsInDirectory(struct song *song, void *_data)
 	if (locate_song_match(song, item->criteria))
 		visitTag(data->client, data->set, song, item->tagType);
 
-	return 0;
+	return true;
 }
 
-int listAllUniqueTags(struct client *client, int type,
-		      const struct locate_item_list *criteria)
+static const struct db_visitor unique_tags_visitor = {
+	.song = unique_tags_visitor_song,
+};
+
+bool
+listAllUniqueTags(struct client *client, int type,
+		  const struct locate_item_list *criteria,
+		  GError **error_r)
 {
-	int ret;
 	ListCommandItem *item = newListCommandItem(type, criteria);
 	struct list_tags_data data = {
 		.client = client,
@@ -249,7 +286,10 @@ int listAllUniqueTags(struct client *client, int type,
 		data.set = strset_new();
 	}
 
-	ret = db_walk(NULL, listUniqueTagsInDirectory, NULL, &data);
+	if (!db_walk(NULL, &unique_tags_visitor, &data, error_r)) {
+		freeListCommandItem(item);
+		return false;
+	}
 
 	if (type >= 0 && type <= TAG_NUM_OF_ITEM_TYPES) {
 		const char *value;
@@ -266,5 +306,5 @@ int listAllUniqueTags(struct client *client, int type,
 
 	freeListCommandItem(item);
 
-	return ret;
+	return true;
 }
diff --git a/src/db_print.h b/src/db_print.h
index a02e7e3d8..7d6309517 100644
--- a/src/db_print.h
+++ b/src/db_print.h
@@ -20,27 +20,38 @@
 #ifndef MPD_DB_PRINT_H
 #define MPD_DB_PRINT_H
 
+#include <glib.h>
+#include <stdbool.h>
+
 struct client;
 struct locate_item_list;
+struct db_visitor;
 
-int printAllIn(struct client *client, const char *name);
+bool
+printAllIn(struct client *client, const char *uri_utf8, GError **error_r);
 
-int printInfoForAllIn(struct client *client, const char *name);
+bool
+printInfoForAllIn(struct client *client, const char *uri_utf8,
+		  GError **error_r);
 
-int
+bool
 searchForSongsIn(struct client *client, const char *name,
-		 const struct locate_item_list *criteria);
+		 const struct locate_item_list *criteria,
+		 GError **error_r);
 
-int
+bool
 findSongsIn(struct client *client, const char *name,
-	    const struct locate_item_list *criteria);
+	    const struct locate_item_list *criteria,
+	    GError **error_r);
 
-int
+bool
 searchStatsForSongsIn(struct client *client, const char *name,
-		      const struct locate_item_list *criteria);
+		      const struct locate_item_list *criteria,
+		      GError **error_r);
 
-int
+bool
 listAllUniqueTags(struct client *client, int type,
-		  const struct locate_item_list *criteria);
+		  const struct locate_item_list *criteria,
+		  GError **error_r);
 
 #endif
diff --git a/src/db_visitor.h b/src/db_visitor.h
new file mode 100644
index 000000000..3eb215c86
--- /dev/null
+++ b/src/db_visitor.h
@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2003-2011 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_DB_VISITOR_H
+#define MPD_DB_VISITOR_H
+
+struct directory;
+struct song;
+
+struct db_visitor {
+	/**
+	 * Visit a directory.  Optional method.
+	 *
+	 * @return true to continue the operation, false on error (set error_r)
+	 */
+	bool (*directory)(const struct directory *directory, void *ctx,
+			  GError **error_r);
+
+	/**
+	 * Visit a song.  Optional method.
+	 *
+	 * @return true to continue the operation, false on error (set error_r)
+	 */
+	bool (*song)(struct song *song, void *ctx, GError **error_r);
+};
+
+#endif
diff --git a/src/directory.c b/src/directory.c
index d8b241118..eb076d94d 100644
--- a/src/directory.c
+++ b/src/directory.c
@@ -21,6 +21,7 @@
 #include "directory.h"
 #include "song.h"
 #include "path.h"
+#include "db_visitor.h"
 
 #include <glib.h>
 
@@ -167,28 +168,33 @@ directory_sort(struct directory *directory)
 		directory_sort(dv->base[i]);
 }
 
-int
+bool
 directory_walk(struct directory *directory,
-	       int (*forEachSong)(struct song *, void *),
-	       int (*forEachDir)(struct directory *, void *),
-	       void *data)
+	       const struct db_visitor *visitor, void *ctx,
+	       GError **error_r)
 {
-	struct dirvec *dv = &directory->children;
-	int err = 0;
-	size_t j;
-
-	if (forEachDir && (err = forEachDir(directory, data)) < 0)
-		return err;
-
-	if (forEachSong) {
-		err = songvec_for_each(&directory->songs, forEachSong, data);
-		if (err < 0)
-			return err;
+	assert(directory != NULL);
+	assert(visitor != NULL);
+	assert(error_r == NULL || *error_r == NULL);
+
+	if (visitor->directory != NULL &&
+	    !visitor->directory(directory, ctx, error_r))
+		return false;
+
+	if (visitor->song != NULL) {
+		struct songvec *sv = &directory->songs;
+		for (size_t i = 0; i < sv->nr; ++i)
+			if (!visitor->song(sv->base[i], ctx, error_r))
+				return false;
 	}
 
-	for (j = 0; err >= 0 && j < dv->nr; ++j)
-		err = directory_walk(dv->base[j], forEachSong,
-						forEachDir, data);
+	if (visitor->directory != NULL) {
+		const struct dirvec *dv = &directory->children;
+		for (size_t i = 0; i < dv->nr; ++i)
+			if (!directory_walk(dv->base[i], visitor, ctx,
+					    error_r))
+				return false;
+	}
 
-	return err;
+	return true;
 }
diff --git a/src/directory.h b/src/directory.h
index a3ad0ffa3..3359a2e99 100644
--- a/src/directory.h
+++ b/src/directory.h
@@ -25,6 +25,7 @@
 #include "songvec.h"
 #include "playlist_vector.h"
 
+#include <glib.h>
 #include <stdbool.h>
 #include <sys/types.h>
 
@@ -33,6 +34,8 @@
 #define DEVICE_INARCHIVE (dev_t)(-1)
 #define DEVICE_CONTAINER (dev_t)(-2)
 
+struct db_visitor;
+
 struct directory {
 	struct dirvec children;
 	struct songvec songs;
@@ -127,10 +130,9 @@ directory_lookup_song(struct directory *directory, const char *uri);
 void
 directory_sort(struct directory *directory);
 
-int
+bool
 directory_walk(struct directory *directory,
-	       int (*forEachSong)(struct song *, void *),
-	       int (*forEachDir)(struct directory *, void *),
-	       void *data);
+	       const struct db_visitor *visitor, void *ctx,
+	       GError **error_r);
 
 #endif
diff --git a/src/stats.c b/src/stats.c
index 8f9ef0dbf..fe0c2a476 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -20,6 +20,7 @@
 #include "config.h"
 #include "stats.h"
 #include "database.h"
+#include "db_visitor.h"
 #include "tag.h"
 #include "song.h"
 #include "client.h"
@@ -68,8 +69,9 @@ visit_tag(struct visit_data *data, const struct tag *tag)
 	}
 }
 
-static int
-stats_collect_song(struct song *song, void *_data)
+static bool
+collect_stats_song(struct song *song, void *_data,
+		   G_GNUC_UNUSED GError **error_r)
 {
 	struct visit_data *data = _data;
 
@@ -78,9 +80,13 @@ stats_collect_song(struct song *song, void *_data)
 	if (song->tag != NULL)
 		visit_tag(data, song->tag);
 
-	return 0;
+	return true;
 }
 
+static const struct db_visitor collect_stats_visitor = {
+	.song = collect_stats_song,
+};
+
 void stats_update(void)
 {
 	struct visit_data data;
@@ -92,7 +98,7 @@ void stats_update(void)
 	data.artists = strset_new();
 	data.albums = strset_new();
 
-	db_walk(NULL, stats_collect_song, NULL, &data);
+	db_walk(NULL, &collect_stats_visitor, &data, NULL);
 
 	stats.artist_count = strset_size(data.artists);
 	stats.album_count = strset_size(data.albums);