server: add a safer signal type and port wl_display to it

wl_list_for_each_safe, which is used by wl_signal_emit is not really
safe. If a signal has two listeners, and the first one removes and
re-inits the second one, it would enter an infinite loop, which was hit
in weston on resource destruction, which emits a signal.
This commit adds a new version of wl_signal, called wl_priv_signal,
which is private in wayland-server.c and which does not have this problem.
The old wl_signal cannot be improved without breaking backwards compatibility.

Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

Makefile.am

@@ -159,6 +159,7 @@ built_test_programs =				\
 	socket-test				\
 	queue-test				\
 	signal-test				\
+	newsignal-test				\
 	resources-test				\
 	message-test				\
 	headers-test				\
@@ -226,6 +227,9 @@ queue_test_SOURCES = tests/queue-test.c
 queue_test_LDADD = libtest-runner.la
 signal_test_SOURCES = tests/signal-test.c
 signal_test_LDADD = libtest-runner.la
+# wayland-server.c is needed here to access wl_priv_* functions
+newsignal_test_SOURCES = tests/newsignal-test.c src/wayland-server.c
+newsignal_test_LDADD = libtest-runner.la
 resources_test_SOURCES = tests/resources-test.c
 resources_test_LDADD = libtest-runner.la
 message_test_SOURCES = tests/message-test.c

src/wayland-private.h

@@ -35,6 +35,7 @@
 #define WL_HIDE_DEPRECATED 1
 
 #include "wayland-util.h"
+#include "wayland-server-core.h"
 
 /* Invalid memory address */
 #define WL_ARRAY_POISON_PTR (void *) 4
@@ -233,4 +234,21 @@ zalloc(size_t s)
 	return calloc(1, s);
 }
 
+struct wl_priv_signal {
+	struct wl_list listener_list;
+	struct wl_list emit_list;
+};
+
+void
+wl_priv_signal_init(struct wl_priv_signal *signal);
+
+void
+wl_priv_signal_add(struct wl_priv_signal *signal, struct wl_listener *listener);
+
+struct wl_listener *
+wl_priv_signal_get(struct wl_priv_signal *signal, wl_notify_func_t notify);
+
+void
+wl_priv_signal_emit(struct wl_priv_signal *signal, void *data);
+
 #endif

src/wayland-server.c

@@ -97,8 +97,8 @@ struct wl_display {
 	struct wl_list client_list;
 	struct wl_list protocol_loggers;
 
-	struct wl_signal destroy_signal;
-	struct wl_signal create_client_signal;
+	struct wl_priv_signal destroy_signal;
+	struct wl_priv_signal create_client_signal;
 
 	struct wl_array additional_shm_formats;
 
@@ -489,7 +489,7 @@ wl_client_create(struct wl_display *display, int fd)
 
 	wl_list_insert(display->client_list.prev, &client->link);
 
-	wl_signal_emit(&display->create_client_signal, client);
+	wl_priv_signal_emit(&display->create_client_signal, client);
 
 	return client;
 
@@ -942,8 +942,8 @@ wl_display_create(void)
 	wl_list_init(&display->registry_resource_list);
 	wl_list_init(&display->protocol_loggers);
 
-	wl_signal_init(&display->destroy_signal);
-	wl_signal_init(&display->create_client_signal);
+	wl_priv_signal_init(&display->destroy_signal);
+	wl_priv_signal_init(&display->create_client_signal);
 
 	display->id = 1;
 	display->serial = 0;
@@ -1008,7 +1008,7 @@ wl_display_destroy(struct wl_display *display)
 	struct wl_socket *s, *next;
 	struct wl_global *global, *gnext;
 
-	wl_signal_emit(&display->destroy_signal, display);
+	wl_priv_signal_emit(&display->destroy_signal, display);
 
 	wl_list_for_each_safe(s, next, &display->socket_list, link) {
 		wl_socket_destroy(s);
@@ -1478,7 +1478,7 @@ WL_EXPORT void
 wl_display_add_destroy_listener(struct wl_display *display,
 				struct wl_listener *listener)
 {
-	wl_signal_add(&display->destroy_signal, listener);
+	wl_priv_signal_add(&display->destroy_signal, listener);
 }
 
 /** Registers a listener for the client connection signal.
@@ -1496,14 +1496,14 @@ WL_EXPORT void
 wl_display_add_client_created_listener(struct wl_display *display,
 					struct wl_listener *listener)
 {
-	wl_signal_add(&display->create_client_signal, listener);
+	wl_priv_signal_add(&display->create_client_signal, listener);
 }
 
 WL_EXPORT struct wl_listener *
 wl_display_get_destroy_listener(struct wl_display *display,
 				wl_notify_func_t notify)
 {
-	return wl_signal_get(&display->destroy_signal, notify);
+	return wl_priv_signal_get(&display->destroy_signal, notify);
 }
 
 WL_EXPORT void
@@ -1812,6 +1812,97 @@ wl_client_for_each_resource(struct wl_client *client,
 	wl_map_for_each(&client->objects, resource_iterator_helper, &context);
 }
 
+/** Initialize a wl_priv_signal object
+ *
+ * wl_priv_signal is a safer implementation of a signal type, with the same API
+ * as wl_signal, but kept as a private utility of libwayland-server.
+ * It is safer because listeners can be removed from within wl_priv_signal_emit()
+ * without corrupting the signal's list.
+ *
+ * Before passing a wl_priv_signal object to any other function it must be
+ * initialized by useing wl_priv_signal_init().
+ *
+ * \memberof wl_priv_signal
+ */
+void
+wl_priv_signal_init(struct wl_priv_signal *signal)
+{
+	wl_list_init(&signal->listener_list);
+	wl_list_init(&signal->emit_list);
+}
+
+/** Add a listener to a signal
+ *
+ * The new listener will be called when calling wl_signal_emit(). If a listener is
+ * added to the signal while wl_signal_emit() is running it will be called from
+ * the next time wl_priv_signal_emit() is called.
+ * To remove a listener call wl_list_remove() on its link member.
+ *
+ * \memberof wl_priv_signal
+ */
+void
+wl_priv_signal_add(struct wl_priv_signal *signal, struct wl_listener *listener)
+{
+	wl_list_insert(signal->listener_list.prev, &listener->link);
+}
+
+/** Get a listener added to a signal
+ *
+ * Returns the listener added to the given \a signal and with the given
+ * \a notify function, or NULL if there isn't any.
+ * Calling this function from withing wl_priv_signal_emit() is safe and will
+ * return the correct value.
+ *
+ * \memberof wl_priv_signal
+ */
+struct wl_listener *
+wl_priv_signal_get(struct wl_priv_signal *signal, wl_notify_func_t notify)
+{
+	struct wl_listener *l;
+
+	wl_list_for_each(l, &signal->listener_list, link)
+		if (l->notify == notify)
+			return l;
+	wl_list_for_each(l, &signal->emit_list, link)
+		if (l->notify == notify)
+			return l;
+
+	return NULL;
+}
+
+/** Emit the signal, calling all the installed listeners
+ *
+ * Iterate over all the listeners added to this \a signal and call
+ * their \a notify function pointer, passing on the given \a data.
+ * Removing or adding a listener from within wl_priv_signal_emit()
+ * is safe.
+ */
+void
+wl_priv_signal_emit(struct wl_priv_signal *signal, void *data)
+{
+	struct wl_listener *l;
+	struct wl_list *pos;
+
+	wl_list_insert_list(&signal->emit_list, &signal->listener_list);
+	wl_list_init(&signal->listener_list);
+
+	/* Take every element out of the list and put them in a temporary list.
+	 * This way, the 'it' func can remove any element it wants from the list
+	 * without troubles, because we always get the first element, not the
+	 * one after the current, which may be invalid.
+	 * wl_list_for_each_safe tries to be safe but it fails: it works fine
+	 * if the current item is removed, but not if the next one is. */
+	while (!wl_list_empty(&signal->emit_list)) {
+		pos = signal->emit_list.next;
+		l = wl_container_of(pos, l, link);
+
+		wl_list_remove(pos);
+		wl_list_insert(&signal->listener_list, pos);
+
+		l->notify(l, data);
+	}
+}
+
 /** \cond */ /* Deprecated functions below. */
 
 uint32_t

tests/newsignal-test.c

@@ -0,0 +1,337 @@
+/*
+ * Copyright © 2013 Marek Chalupa
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the
+ * next paragraph) shall be included in all copies or substantial
+ * portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include <assert.h>
+
+#include "test-runner.h"
+#include "wayland-private.h"
+
+static void
+signal_notify(struct wl_listener *listener, void *data)
+{
+	/* only increase counter*/
+	++(*((int *) data));
+}
+
+TEST(signal_init)
+{
+	struct wl_priv_signal signal;
+
+	wl_priv_signal_init(&signal);
+
+	/* Test if listeners' list is initialized */
+	assert(&signal.listener_list == signal.listener_list.next
+		&& "Maybe wl_priv_signal implementation changed?");
+	assert(signal.listener_list.next == signal.listener_list.prev
+		&& "Maybe wl_priv_signal implementation changed?");
+}
+
+TEST(signal_add_get)
+{
+	struct wl_priv_signal signal;
+
+	/* we just need different values of notify */
+	struct wl_listener l1 = {.notify = (wl_notify_func_t) 0x1};
+	struct wl_listener l2 = {.notify = (wl_notify_func_t) 0x2};
+	struct wl_listener l3 = {.notify = (wl_notify_func_t) 0x3};
+	/* one real, why not */
+	struct wl_listener l4 = {.notify = signal_notify};
+
+	wl_priv_signal_init(&signal);
+
+	wl_priv_signal_add(&signal, &l1);
+	wl_priv_signal_add(&signal, &l2);
+	wl_priv_signal_add(&signal, &l3);
+	wl_priv_signal_add(&signal, &l4);
+
+	assert(wl_priv_signal_get(&signal, signal_notify) == &l4);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x3) == &l3);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x2) == &l2);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x1) == &l1);
+
+	/* get should not be destructive */
+	assert(wl_priv_signal_get(&signal, signal_notify) == &l4);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x3) == &l3);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x2) == &l2);
+	assert(wl_priv_signal_get(&signal, (wl_notify_func_t) 0x1) == &l1);
+}
+
+TEST(signal_emit_to_one_listener)
+{
+	int count = 0;
+	int counter;
+
+	struct wl_priv_signal signal;
+	struct wl_listener l1 = {.notify = signal_notify};
+
+	wl_priv_signal_init(&signal);
+	wl_priv_signal_add(&signal, &l1);
+
+	for (counter = 0; counter < 100; counter++)
+		wl_priv_signal_emit(&signal, &count);
+
+	assert(counter == count);
+}
+
+TEST(signal_emit_to_more_listeners)
+{
+	int count = 0;
+	int counter;
+
+	struct wl_priv_signal signal;
+	struct wl_listener l1 = {.notify = signal_notify};
+	struct wl_listener l2 = {.notify = signal_notify};
+	struct wl_listener l3 = {.notify = signal_notify};
+
+	wl_priv_signal_init(&signal);
+	wl_priv_signal_add(&signal, &l1);
+	wl_priv_signal_add(&signal, &l2);
+	wl_priv_signal_add(&signal, &l3);
+
+	for (counter = 0; counter < 100; counter++)
+		wl_priv_signal_emit(&signal, &count);
+
+	assert(3 * counter == count);
+}
+
+struct signal
+{
+	struct wl_priv_signal signal;
+	struct wl_listener l1, l2, l3;
+	int count;
+	struct wl_listener *current;
+};
+
+static void notify_remove(struct wl_listener *l, void *data)
+{
+	struct signal *sig = data;
+	wl_list_remove(&sig->current->link);
+	wl_list_init(&sig->current->link);
+	sig->count++;
+}
+
+#define INIT \
+	wl_priv_signal_init(&signal.signal); \
+	wl_list_init(&signal.l1.link); \
+	wl_list_init(&signal.l2.link); \
+	wl_list_init(&signal.l3.link);
+#define CHECK_EMIT(expected) \
+	signal.count = 0; \
+	wl_priv_signal_emit(&signal.signal, &signal); \
+	assert(signal.count == expected);
+
+TEST(signal_remove_listener)
+{
+	test_set_timeout(4);
+
+	struct signal signal;
+
+	signal.l1.notify = notify_remove;
+	signal.l2.notify = notify_remove;
+	signal.l3.notify = notify_remove;
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+
+	signal.current = &signal.l1;
+	CHECK_EMIT(1)
+	CHECK_EMIT(0)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	CHECK_EMIT(2)
+	CHECK_EMIT(1)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.current = &signal.l2;
+	CHECK_EMIT(1)
+	CHECK_EMIT(1)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+	wl_priv_signal_add(&signal.signal, &signal.l3);
+
+	signal.current = &signal.l1;
+	CHECK_EMIT(3)
+	CHECK_EMIT(2)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+	wl_priv_signal_add(&signal.signal, &signal.l3);
+
+	signal.current = &signal.l2;
+	CHECK_EMIT(2)
+	CHECK_EMIT(2)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+	wl_priv_signal_add(&signal.signal, &signal.l3);
+
+	signal.current = &signal.l3;
+	CHECK_EMIT(2)
+	CHECK_EMIT(2)
+}
+
+static void notify_readd(struct wl_listener *l, void *data)
+{
+	struct signal *signal = data;
+	if (signal->current) {
+		wl_list_remove(&signal->current->link);
+		wl_list_init(&signal->current->link);
+		wl_priv_signal_add(&signal->signal, signal->current);
+	}
+	signal->count++;
+}
+
+static void notify_empty(struct wl_listener *l, void *data)
+{
+	struct signal *signal = data;
+	signal->count++;
+}
+
+TEST(signal_readd_listener)
+{
+	/* Readding a listener is supported, that is it doesn't trigger an
+	 * infinite loop or other weird things, but if in a listener you
+	 * readd another listener, that will not be fired in the current
+	 * signal emission. */
+
+	test_set_timeout(4);
+
+	struct signal signal;
+
+	signal.l1.notify = notify_readd;
+	signal.l2.notify = notify_readd;
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+
+	signal.current = &signal.l1;
+	CHECK_EMIT(1)
+	CHECK_EMIT(1)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+
+	signal.current = &signal.l2;
+	CHECK_EMIT(1)
+	signal.current = NULL;
+	CHECK_EMIT(2)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.current = &signal.l1;
+	CHECK_EMIT(1)
+	/* l2 was added before l1, so l2 is fired first, which by readding l1
+	 * removes it from the current list that is being fired, so 1 is correct */
+	CHECK_EMIT(1)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.l1.notify = notify_empty;
+	signal.current = &signal.l2;
+	CHECK_EMIT(2)
+	CHECK_EMIT(2)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.l1.notify = notify_empty;
+	signal.current = &signal.l1;
+	CHECK_EMIT(2)
+	/* same as before, by readding l1 in the first emit, it now is fired
+	 * after l2, so on the second emit it is not fired at all. */
+	CHECK_EMIT(1)
+}
+
+static void notify_addandget(struct wl_listener *l, void *data)
+{
+	struct signal *signal = data;
+	wl_list_remove(&signal->current->link);
+	wl_list_init(&signal->current->link);
+	wl_priv_signal_add(&signal->signal, signal->current);
+
+	assert(wl_priv_signal_get(&signal->signal, signal->current->notify) != NULL);
+
+	signal->count++;
+}
+
+static void notify_get(struct wl_listener *l, void *data)
+{
+	struct signal *signal = data;
+	assert(wl_priv_signal_get(&signal->signal, signal->current->notify) == signal->current);
+	signal->count++;
+}
+
+TEST(signal_get_listener)
+{
+	test_set_timeout(4);
+
+	struct signal signal;
+
+	signal.l1.notify = notify_addandget;
+	signal.l2.notify = notify_get;
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+
+	signal.current = &signal.l2;
+	CHECK_EMIT(1)
+
+	INIT
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.current = &signal.l2;
+	CHECK_EMIT(1)
+
+	INIT
+	signal.l1.notify = notify_get;
+	signal.l2.notify = notify_empty;
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	CHECK_EMIT(2)
+
+	INIT
+	signal.l1.notify = notify_empty;
+	signal.l2.notify = notify_get;
+	wl_priv_signal_add(&signal.signal, &signal.l1);
+	wl_priv_signal_add(&signal.signal, &signal.l2);
+
+	signal.current = &signal.l1;
+	CHECK_EMIT(2)
+}