libweston: Introduce a safer wayland signal emission

Wayland signals (wl_signal) do not reliably handle changes to the
notification list during signal emission. Such scenarios occasionally
come up and can be difficult to investigate and debug.

This commit introduces the weston_signal_emit_mutable() function which can be
used in place of wl_signal_emit() and safely implements the following
behavior regarding notification list changes:

1. Listeners deleted during a signal emission and which have not already been
   notified at the time of deletion are not notified by that emission.

2. Listeners added during signal emission are ignored by that emission.

The implementation of weston_signal_emit_mutable() is copied from the
wlr_signal_emit_safe() function of the wlroots project.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>

shared/meson.build

@@ -1,11 +1,12 @@
 srcs_libshared = [
 	'config-parser.c',
 	'option-parser.c',
+	'signal.c',
 	'file-util.c',
 	'os-compatibility.c',
 	'xalloc.c',
 ]
-deps_libshared = dep_wayland_client
+deps_libshared = [dep_wayland_client, dep_wayland_server]
 
 lib_libshared = static_library(
 	'shared',

shared/signal.c

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2018 Simon Ser
+ * Copyright 2021 Collabora, Ltd.
+ *
+ * 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 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.
+ */
+
+/* Implementation copied from wlroots util/signal.c file */
+
+#include "signal.h"
+
+static void
+handle_noop(struct wl_listener *listener, void *data)
+{
+	/* Do nothing */
+}
+
+void
+weston_signal_emit_mutable(struct wl_signal *signal, void *data)
+{
+	struct wl_listener cursor;
+	struct wl_listener end;
+
+	/* Add two special markers: one cursor and one end marker. This way, we
+	 * know that we've already called listeners on the left of the cursor
+	 * and that we don't want to call listeners on the right of the end
+	 * marker. The 'it' function can remove any element it wants from the
+	 * list without troubles.
+	 *
+	 * There was a previous attempt that used to steal the whole list of
+	 * listeners but then that broke wl_signal_get().
+	 *
+	 * 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. */
+	wl_list_insert(&signal->listener_list, &cursor.link);
+	cursor.notify = handle_noop;
+	wl_list_insert(signal->listener_list.prev, &end.link);
+	end.notify = handle_noop;
+
+	while (cursor.link.next != &end.link) {
+		struct wl_list *pos = cursor.link.next;
+		struct wl_listener *l = wl_container_of(pos, l, link);
+
+		wl_list_remove(&cursor.link);
+		wl_list_insert(pos, &cursor.link);
+
+		l->notify(l, data);
+	}
+
+	wl_list_remove(&cursor.link);
+	wl_list_remove(&end.link);
+}

shared/signal.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2018 Simon Ser
+ * Copyright 2021 Collabora, Ltd.
+ *
+ * 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 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.
+ */
+
+#ifndef WESTON_SIGNAL_H
+#define WESTON_SIGNAL_H
+
+#include <wayland-server-core.h>
+
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
+/* A safer version of wl_signal_emit() which can gracefully handle additions
+ * and deletions of any signal listener from within listener notification
+ * callbacks.
+ *
+ * Listeners deleted during a signal emission and which have not already been
+ * notified at the time of deletion are not notified by that emission.
+ *
+ * Listeners added (or readded) during signal emission are ignored by that
+ * emission.
+ *
+ * Note that repurposing a listener without explicitly removing it and readding
+ * it is not supported and can lead to unexpected behavior.
+ */
+
+void
+weston_signal_emit_mutable(struct wl_signal *signal, void *data);
+
+#ifdef  __cplusplus
+}
+#endif
+
+#endif /* WESTON_SIGNAL_H */