We (abrt project) use syncronous dbus calls in a single-threaded application.
static GDBusProxy *get_dbus_proxy(void)
GError *error = NULL;
GDBusProxy *proxy = g_dbus_proxy_new_for_bus_sync(G_BUS_TYPE_SYSTEM,
if (error) ...
GDBusProxy *proxy = get_dbus_proxy();
GError *error = NULL;
GVariant *result = g_dbus_proxy_call_sync(proxy,
We discovered that sometime after we do such calls (and they work successfully, no problem), our application hangs after fork().
The hanging code looks like this:
pid_t pid = fork();
if (pid < 0) /* error */...
if (pid == 0) /* child */
struct run_event_state *run_state = new_run_event_state();
int r = run_event_on_dir_name(run_state, dirname, "open-gui");
int no_such_event = (r == 0 && run_state->children_count == 0);
/* Default: launch graphical tool */
execlp("report-gtk", "report-gtk", "--", (char *)dirname, NULL);
perror_msg_and_die("Can't execute %s", "report-gtk");
Child process gets stuck on a futex() call soon after fork() - we do not reach run_event_on_dir_name() and execlp().
Even when parent process is gone, the still-living child remains visible in ps output.
This bug disappears under strace and when debug logging is added. Basically, it looks like a non-deterministic inconsistent lock state in the child.
After investigating the problem we discovered that our program is not single-threaded anymore (!!!) - glib created a thread in g_dbus_proxy_new_for_bus_sync() or in g_dbus_proxy_call_sync() function, and this thread persists for the lifetime of the program. I see it in ps -AT output.
Stracing shows a fair amount of futex ops in this thread. So, it is capable of locking and unlocking locks while we fork() - classic cause of PITA when threading and fork() are mixed.
We wrote the program as a single-threaded one exactly because we wnated to avoid going there.
When we run the program so that it does not use dbus ("abrt-gui -D /var/spool/abrt"), this thread is not created, and the buggy behavior does not trigger.
Can you guys please make *syncronous* glib dbus API to not create threads? Or at least nuke these threads when they are done?
If it is a performance hit to repeatedle create and tear them down, consider exposing a nuke_stupid_thread_I_did_not_ask_for()-esque function in the API so that we can still enjoy the simplicity of single-threaded life?
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