bug#18361: New 'sort' implementation can crash Emacs

[Paul Eggert]

Dmitry Antipov wrote:
> If an undefined behavior doesn't cause crash,

Unfortunately undefined behavior in qsort can cause a crash (or an 
infinite loop, etc., etc.).  It's platform-dependent, and on many 
platforms the problem happens only in unusual cases, so I'm not 
surprised your tests didn't find it.  But it definitely can happen. 
See, for example,

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42157

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51297

These crash reports are for Solaris qsort, but today I found similar 
issues in the latest glibc qsort by code inspection (e.g., the path 
qsort takes when memory is low).  These issues are not qsort bugs, since 
the qsort spec requires a total-order comparison function.  It's a bug 
in the Emacs trunk.