[plt-scheme] Using Scheme_Object * from another thread

> Hans Oesterholt-Dijkema wrote:
> > I thought Hans Boehms GC was thread safe. Are you sure?

I think we set the compilation flags so that the GC doesn't support
threads. You could change that when compiling your own binary, of
course.

> Suppose, I would like to build up an array of arrays of Scheme_Object *
> in an other OS thread then the mzscheme main OS thread, using 
> scheme_malloc(), etc.
> and scheme_make_utf8_string(), while the calling thread in mzscheme is
> waiting on mzscheme_block_until().
>
> Will this cause problems with mzscheme? (e.g. because schheme_malloc()/
> scheme_make_utf8_string() is not OS thread safe).

When allocating a string through MzScheme, there's a little bit shared
state other than the GC-private state. Specifically, there's a global
function pointer that is used to escape when the GC runs out of memory
trying to allocate the string. (This is one of the few places where
MzScheme attempts to catch an out-of-memory condition and raise an
exception.) See scheme_malloc_fail_ok() in "salloc.c".

This race condition is unlikely to cause a problem in practice, because
you won't allocate large strings. But I may have overlooked some other
shared state (I didn't expect the one I found), so I recommend against
using any scheme_...() functions from the non-MzScheme thread, even if
you ensure that the GC itself is thread-safe.

Matthew