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Re: Conservative GC isn't safe

From: Stefan Monnier
Subject: Re: Conservative GC isn't safe
Date: Mon, 28 Nov 2016 13:03:13 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux)

>>> How would you assert dynamically that if an interval is reachable, its
>>> owning string or buffer must be too?
>> You don't.  You check it statically (by a human).
>>> It's not enough for the variable holding the reference to the string
>>> or buffer to be in scope: you have to be sure that the reference
>>> isn't dead.
>> It should be: if it's in scope, it's not dead.
> That's not the case.

In general, no, but in this specific case I think it always will.
E.g. because in order to be in the process of working on the intervals of
a buffer, that buffer needs to be not just reachable but buffer-live-p,
so you'd have to mess with buffer-alist before the buffer can be
reclaimed, which is highly unlikely to happen within the functions that
manipulate intervals.

For strings, the argument might not be as strong.  Maybe code like
(ignore (propertize "foo" 'a 'b)) could lead to us working on an
unreachable string, so it could get GC'd while we manipulate its

So for those cases, I guess the main safety argument we have is that we
will not call the GC while we're in the middle of manipulating struct
interval objects.


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