Disconnect swaybg instead of killing it

This is much more reliable. This also fixes race conditions when killing swaybg
while it's doing a wl_display_roundtrip.

include/sway/output.h

@@ -21,6 +21,7 @@ struct sway_output {
 	struct sway_node node;
 	struct wlr_output *wlr_output;
 	struct sway_server *server;
+	struct wl_list link;
 
 	struct wl_list layers[4]; // sway_layer_surface::link
 	struct wlr_box usable_area;
@@ -36,6 +37,8 @@ struct sway_output {
 
 	struct sway_output_state current;
 
+	struct wl_client *swaybg_client;
+
 	struct wl_listener destroy;
 	struct wl_listener mode;
 	struct wl_listener transform;
@@ -43,10 +46,7 @@ struct sway_output {
 	struct wl_listener present;
 	struct wl_listener damage_destroy;
 	struct wl_listener damage_frame;
-
-	struct wl_list link;
-
-	pid_t bg_pid;
+	struct wl_listener swaybg_client_destroy;
 
 	struct {
 		struct wl_signal destroy;

sway/config/output.c

@@ -1,16 +1,17 @@
 #define _POSIX_C_SOURCE 200809L
 #include <assert.h>
+#include <fcntl.h>
 #include <stdbool.h>
 #include <string.h>
-#include <signal.h>
+#include <sys/socket.h>
 #include <sys/wait.h>
 #include <unistd.h>
-#include <wlr/types/wlr_output.h>
 #include <wlr/types/wlr_output_layout.h>
+#include <wlr/types/wlr_output.h>
+#include "log.h"
 #include "sway/config.h"
 #include "sway/output.h"
 #include "sway/tree/root.h"
-#include "log.h"
 
 int output_name_cmp(const void *item, const void *data) {
 	const struct output_config *output = item;
@@ -165,14 +166,71 @@ static bool set_mode(struct wlr_output *output, int width, int height,
 	return wlr_output_set_mode(output, best);
 }
 
-void terminate_swaybg(pid_t pid) {
-	int ret = kill(pid, SIGTERM);
-	if (ret != 0) {
-		sway_log(SWAY_ERROR, "Unable to terminate swaybg [pid: %d]", pid);
-	} else {
-		int status;
-		waitpid(pid, &status, 0);
+static void handle_swaybg_client_destroy(struct wl_listener *listener,
+		void *data) {
+	struct sway_output *output =
+		wl_container_of(listener, output, swaybg_client_destroy);
+	wl_list_remove(&output->swaybg_client_destroy.link);
+	wl_list_init(&output->swaybg_client_destroy.link);
+	output->swaybg_client = NULL;
+}
+
+static bool spawn_swaybg(struct sway_output *output, char *const cmd[]) {
+	int sockets[2];
+	if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, sockets) != 0) {
+		sway_log_errno(SWAY_ERROR, "socketpair failed");
+		return false;
 	}
+
+	output->swaybg_client = wl_client_create(server.wl_display, sockets[0]);
+	if (output->swaybg_client == NULL) {
+		sway_log_errno(SWAY_ERROR, "wl_client_create failed");
+		return false;
+	}
+
+	output->swaybg_client_destroy.notify = handle_swaybg_client_destroy;
+	wl_client_add_destroy_listener(output->swaybg_client,
+		&output->swaybg_client_destroy);
+
+	pid_t pid = fork();
+	if (pid < 0) {
+		sway_log_errno(SWAY_ERROR, "fork failed");
+		return false;
+	} else if (pid == 0) {
+		pid = fork();
+		if (pid < 0) {
+			sway_log_errno(SWAY_ERROR, "fork failed");
+			exit(EXIT_FAILURE);
+		} else if (pid == 0) {
+			// Remove the CLOEXEC flag
+			int flags = fcntl(sockets[1], F_GETFD);
+			if (flags == -1) {
+				sway_log_errno(SWAY_ERROR, "fcntl() failed");
+				exit(EXIT_FAILURE);
+			}
+			if (fcntl(sockets[1], F_SETFD, flags & ~FD_CLOEXEC) == -1) {
+				sway_log_errno(SWAY_ERROR, "fcntl() failed");
+				exit(EXIT_FAILURE);
+			}
+
+			char wayland_socket_str[16];
+			snprintf(wayland_socket_str, sizeof(wayland_socket_str),
+				"%d", sockets[1]);
+			setenv("WAYLAND_SOCKET", wayland_socket_str, true);
+
+			execvp(cmd[0], cmd);
+			sway_log_errno(SWAY_ERROR, "execvp failed");
+			exit(EXIT_FAILURE);
+		}
+		exit(EXIT_SUCCESS);
+	}
+
+	if (waitpid(pid, NULL, 0) < 0) {
+		sway_log_errno(SWAY_ERROR, "waitpid failed");
+		return false;
+	}
+
+	return true;
 }
 
 bool apply_output_config(struct output_config *oc, struct sway_output *output) {
@@ -243,8 +301,8 @@ bool apply_output_config(struct output_config *oc, struct sway_output *output) {
 		wlr_output_layout_add_auto(root->output_layout, wlr_output);
 	}
 
-	if (output->bg_pid != 0) {
-		terminate_swaybg(output->bg_pid);
+	if (output->swaybg_client != NULL) {
+		wl_client_destroy(output->swaybg_client);
 	}
 	if (oc && oc->background && config->swaybg_command) {
 		sway_log(SWAY_DEBUG, "Setting background for output %s to %s",
@@ -258,13 +316,8 @@ bool apply_output_config(struct output_config *oc, struct sway_output *output) {
 			oc->background_fallback ? oc->background_fallback : NULL,
 			NULL,
 		};
-
-		output->bg_pid = fork();
-		if (output->bg_pid < 0) {
-			sway_log_errno(SWAY_ERROR, "fork failed");
-		} else if (output->bg_pid == 0) {
-			execvp(cmd[0], cmd);
-			sway_log_errno(SWAY_ERROR, "Failed to execute swaybg");
+		if (!spawn_swaybg(output, cmd)) {
+			return false;
 		}
 	}
 

sway/desktop/output.c

@@ -506,6 +506,7 @@ static void handle_destroy(struct wl_listener *listener, void *data) {
 	wl_list_remove(&output->present.link);
 	wl_list_remove(&output->damage_destroy.link);
 	wl_list_remove(&output->damage_frame.link);
+	wl_list_remove(&output->swaybg_client_destroy.link);
 
 	transaction_commit_dirty();
 }
@@ -612,6 +613,7 @@ void handle_new_output(struct wl_listener *listener, void *data) {
 	output->damage_frame.notify = damage_handle_frame;
 	wl_signal_add(&output->damage->events.destroy, &output->damage_destroy);
 	output->damage_destroy.notify = damage_handle_destroy;
+	wl_list_init(&output->swaybg_client_destroy.link);
 
 	struct output_config *oc = output_find_config(output);
 	if (!oc || oc->enabled) {

sway/tree/output.c

@@ -233,9 +233,8 @@ void output_disable(struct sway_output *output) {
 
 	root_for_each_container(untrack_output, output);
 
-	if (output->bg_pid) {
-		terminate_swaybg(output->bg_pid);
-		output->bg_pid = 0;
+	if (output->swaybg_client != NULL) {
+		wl_client_destroy(output->swaybg_client);
 	}
 
 	int index = list_find(root->outputs, output);