Re: LUA + musl, garbage collection issue?

[Rich Felker <dalias@libc.org>]

On Sat, Sep 20, 2014 at 04:41:14PM -1000, Scott Valentine wrote:
> I noticed that in order to free memory, it basically calls realloc
> with 0 as the new size. Is this something musl doesn't handle well?
> 
> I'm trying a rebuild with a check for n == 0 in musl's realloc
> function to just free the pointer, and I'll report back.
> 
> What is "the right thing to do" to fix this? Should lua not be using
> realloc to free memory, or should musl handle the case better, if,
> in fact this is the problem?

This is a bug in lua; it's depending on a bug in glibc. POSIX attempts
to allow the glibc behavior (see the text here:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/realloc.html)
but this allowance is invalid since it conflicts with the requirements
of ISO C (and, as you can see in the above link, "The functionality
described on this reference page is aligned with the ISO C standard.
Any conflict between the requirements described here and the ISO C
standard is unintentional. This volume of POSIX.1-2008 defers to the
ISO C standard.") The relevant ISO C requirement is:

"The realloc function returns a pointer to the new object (which may
have the same value as a pointer to the old object), or a null pointer
if the new object could not be allocated."

In particular, the only way realloc is permitted to return 0 is if the
operation failed, in which case the old pointer is still valid (not
freed).

But even if the glibc behavior weren't conflicting with the C
standard, it's still not valid for an application to assume it, and
it's still undesirable behavior (because it's inconsistent with
glibc's malloc, which returns a non-null, unique pointer for each
malloc(0), and because it makes it impossible for applications to
reliably determine whether the operation succeeded or failed).

The whole situation is a big enough mess that I feel I can safely say
that any application that ever passes 0 as the size argument to
realloc has potentially-serious bugs. So if lua wants to free memory,
it just needs to call free.

Rich