This is used when you're about to destroy an object that is
(potentially) the context parameter for some still-pending toplevel
callback. It causes callbacks.c to go through its pending list and
delete any callback records referring to that context parameter, so
that when you destroy the object those callbacks aren't still waiting
to cause stale-pointer dereferences.
I temporarily applied it as a means of testing the revised event loops
in r10040, and accidentally folded it into my final commit instead of
backing it out. Ahem.
[originally from svn r10042]
[r10040 == 5c4ce2fadf]
This change attempts to reinstate as a universal property something
which was sporadically true of the ad-hockery that came before
toplevel callbacks: that if there's a _very long_ queue of things to
be done through the callback mechanism, the doing of them will be
interleaved with re-checks of other event sources, which might (e.g.)
cause a flag to be set which makes the next callback decide not to do
anything after all.
[originally from svn r10040]
This is a little like schedule_timer, in that the callback you provide
will be run from the top-level message loop of whatever application
you're in; but unlike the timer mechanism, it will happen
_immediately_.
The aim is to provide a general way to avoid re-entrance of code, in
cases where just _doing_ the thing you want done is liable to trigger
a confusing recursive call to the function in which you came to the
decision to do it; instead, you just request a top-level callback at
the message loop's earliest convenience, and do it then.
[originally from svn r10019]
