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Allison Karlitskaya authored
In the event that a GSettings object is being destroyed just as a change signal is being delivered, the destroying thread will race with the dconf worker thread for acquiring the lock on the GSettingsBackend. If the signalling thread gets there first then the destroying thread will block on the lock. The signalling thread adds a reference to the GSettings object that is being destroyed and releases the lock. The idea is that this should prevent the GSettings object from being destroyed and thus maintain its entry in the list. Unfortunately, the weak reference notify function is already running and as soon as we release the lock, the list entry is removed. The signalling thread crashes. This bug is indicative of a serious problem encountered in many situations where GObject instances are touched from multiple threads. Ideally, we will move to a place where g_object_ref() is not called at all on the GSettings object from the dconf worker thread and instead, a dispatch will be done without holding a reference (similar to how GAppInfoMonitor presently works). This would also prevent the unfortunate case of someone dropping what they assume to be the last reference on a GSettings object, only to have an already-pending signal delivered once they return to the mainloop, crashing their program. Making this change for GSettings (with multiple instances per thread, the possibility of multiple backends and each instance being interested in different events) is going to be extremely non-trivial, so it's not a change that makes sense at this point in the cycle. For now, we can do a relatively small and isolated tweak so that we never access the list except under a lock. We still perform the bad pattern of acquiring a ref in a foreign thread which means that we still risk delivering a signal to a GSettings object that the user has assumed is dead (unless they explicitly disconnect their signal handler). This is a problem that we already had, however. https://bugzilla.gnome.org/show_bug.cgi?id=710367
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