9af2d7e4c8
The normal order of operations when iterating the GLib main loop is:
- prepare() -> sd_event_prepare()
- poll the file descriptors
- check() -> sd_event_wait()
- dispatch() -> sd_event_dispatch()
GLib does not guarantee that check() is always called between two
consecutive prepare() invocations. When another thread attaches or
removes a source with poll fds while the main thread is inside
poll(), g_main_context_check_unlocked() returns immediately without
calling any source's check() callback.
Since the sd_event GSource adapter maps prepare/check/dispatch to
sd_event_prepare/sd_event_wait/sd_event_dispatch, a skipped check()
leaves sd_event in ARMED state. The next prepare() then hits
assertion "e->state == SD_EVENT_INITIAL".
The following program reproduces the faulty scenario by creating a
thread that attaches a new fd during the poll phase:
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <glib.h>
#include <systemd/sd-event.h>
#include <unistd.h>
/* from glib-event-glue.c */
extern GSource *g_sd_event_create_source(sd_event *event);
static gpointer thread_func(gpointer user_data) {
GSource *source;
GPollFD pollfd;
int fd;
g_usleep(G_USEC_PER_SEC / 10);
fd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
if (fd < 0)
abort();
source = g_source_new(&(GSourceFuncs){0}, sizeof(GSource));
pollfd.fd = fd;
pollfd.events = G_IO_IN;
pollfd.revents = 0;
g_source_add_poll(source, &pollfd);
g_source_attach(source, NULL);
g_source_destroy(source);
g_source_unref(source);
close(fd);
return NULL;
}
int main(int argc, char *argv[]) {
sd_event *event = NULL;
GSource *source;
GThread *thread;
int i, r;
r = sd_event_default(&event);
if (r < 0)
return 1;
source = g_sd_event_create_source(event);
if (!source)
return 1;
g_source_attach(source, NULL);
for (i = 0; i < 50; i++) {
thread = g_thread_new("check-skip", thread_func, NULL);
g_main_context_iteration(NULL, TRUE);
g_thread_join(thread);
g_main_context_iteration(NULL, FALSE);
}
g_source_destroy(source);
g_source_unref(source);
sd_event_unref(event);
return 0;
}
When the program uses the current glue code, it crashes with:
Assertion 'e->state == SD_EVENT_INITIAL' failed at src/libsystemd/sd-event/sd-event.c:4560, function sd_event_prepare(). Aborting.
Fix the problem by skipping sd_event_prepare() if the event is
already ARMED: it was already prepared in a previous iteration and
it can be polled immediately.
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