Re: Subtle bugs in interval code.

[David Kastrup <dak@gnu.org>]

storm@cua.dk (Kim F. Storm) writes:

> Studying the code in interval.c for merge_properties and
> intervals_equal, I noticed that they use Fmemq to search
> for a given property on a plist.
>
> That's not a safe way to do that;
>
> E.g. consider a plist like this:
>
> (setq p '(a b c d))
>
> Now,
>
>  (plist-get p 'a) => b
>  (plist-get p 'b) => nil
>
> whereas
>
>  (memq 'a p) => (a b c d)
>  (memq 'b p) => (b c d)   !=   nil
>
> So due to the use of Fmemq, the interval code may wrongly assume that
> the plist has a `b' member with a value of `c'.
>
> I'm not aware of any specific bugs related to this.
>
> Note that we cannot just use plist-get instead of memq, as we then
> cannot differentiate between "property is on list with nil value"
> and "property is not on list".

One could check whether the length of the rest sequence is even.

The cleanest option might be to add an optional DEFAULT argument to
plist-get, but that would require changes to all C level callers.
Maybe one should add an explicit plist-get-default function that takes
the default for non-existing elements from its third argument?

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum