[PATCH] posix: Don't unnecessarily duplicate the parse tree in regcomp [BZ #20095]

[Florian Weimer]

* Collin Funk:

> Currently regcomp duplicates the the parse tree for each repetition
> operator. This patch avoids the duplication when the previous repetition
> operator is the same as the current operator. This prevents you from
> quickly exhausting the system's memory. Here is an example with GNU grep
> before and after this change:
>
>     $ (ulimit -v 10000000 \
>        && grep -Ec 'a++++++++++++++++++++++++' COPYING)
>     grep: Memory exhausted
>     $ (ulimit -v 10000000 \
>        && ./src/grep -Ec 'a++++++++++++++++++++++++' COPYING)
>     509

Does this change really make a difference?  It seems we still run out of
memory for ?+?+?+… and similar patterns.

Thanks,
Florian