bug#56108: 29.0.50; ASAN use-after-free in re_match_2_internal

[Stefan Monnier]

> Stefan, do you happen to know why some of the callers of
> compile_pattern don't call freeze_pattern to protect the new cache
> entry?  Is it just an omission or do we miss something here?

Before `freeze_pattern`, the design was that nothing could happen while
running the regexp matcher (no GC, no execution of Lisp code).

Commit 938d252d1c6c5e2027aa250c649deb024154f936 changed that so that
searching inside a *buffer* could end up running ELisp code (and hence
also GC).  AFAIK this still can't happen when searching in strings.
[ IIRC The need to run ELisp is so as to apply `syntax-table` text
  properties on demand via `syntax-propertize`.  ]

So I think freeze_pattern should be used in all cases where
`compile_pattern` is used to search inside a buffer, but it shouldn't be
necessary when searching within a string.

At least, that's my recollection.


        Stefan