xwayland: do not weston_log() after fork()

Between fork() and exec() in the child process it is only safe to use
async-signal-safe functions. weston_log() definitely is not one, it
allocates memory and does whatnot.

weston_log() is also inappropriate for other reasons: the child process
has its own stream buffers and flight-recorder. No-one looks into the
child process' flight recorder, so messages would be lost there. The
logging machinery might also attempt to write into debug streams,
meaning both parent and child could be writing simultaneously.

It seems that the best we can do is to pre-bake an error message and
only write() it out if exec() fails. There is no mention that even
strerror_r() might be safe to call, so we don't.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

compositor/xwayland.c

@@ -36,6 +36,7 @@
 #include <libweston/xwayland-api.h>
 #include "shared/helpers.h"
 #include "shared/os-compatibility.h"
+#include "shared/string-helpers.h"
 
 #ifdef HAVE_XWAYLAND_LISTENFD
 #  define LISTEN_STR "-listenfd"
@@ -113,6 +114,7 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 	struct weston_config *config = wet_get_config(wxw->compositor);
 	struct weston_config_section *section;
 	struct wl_event_loop *loop;
+	char *exec_failure_msg;
 	bool ret;
 
 	if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, sv) < 0) {
@@ -143,6 +145,8 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 	section = weston_config_get_section(config, "xwayland", NULL, NULL);
 	weston_config_section_get_string(section, "path",
 					 &xserver, XSERVER_PATH);
+	str_printf(&exec_failure_msg,
+		   "Error: executing Xwayland as '%s' failed.\n", xserver);
 
 	pid = fork();
 	switch (pid) {
@@ -170,12 +174,10 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 			  "-wm", wm_fd_str,
 			  "-terminate",
 			  NULL) < 0) {
-			weston_log("exec of '%s %s -rootless "
-				   LISTEN_STR " %s " LISTEN_STR " %s "
-				   "-wm %s -terminate' failed: %s\n",
-				   xserver, display,
-				   abstract_fd_str, unix_fd_str, wm_fd_str,
-				   strerror(errno));
+			if (exec_failure_msg) {
+				write(STDERR_FILENO, exec_failure_msg,
+				      strlen(exec_failure_msg));
+			}
 		}
 		_exit(EXIT_FAILURE);
 
@@ -205,6 +207,7 @@ spawn_xserver(void *user_data, const char *display, int abstract_fd, int unix_fd
 		break;
 	}
 
+	free(exec_failure_msg);
 	free(xserver);
 
 	return pid;