G'day list!

I've been working on writing an ffi interface to the GEOS library and
have hit a brick wall trying to track down a weird memory issue. The
interface for the library itself is available at https://github.com/dark-panda/ffi-geos
. I've posted a smaller example of the issue I'm seeing as a gist up
at https://gist.github.com/735620 , and I'm testing against the latest
ffi master from github and Ruby 1.9.2-p0 on OSX using the latest GEOS
svn trunk, although the same problem manifests itself in GEOS 3.2.2 as
well.

The GEOS library uses two callbacks for emitting notices and errors.
The callbacks are set in the initGEOS_r function, and both have a
prototype of "void (*)(const char *fmt, ...);".

In the code in the aforementioned gist, I am purposely passing bad
data to GEOSWKTReader_read_r to force the error_handler callback to
fire. The callback in this case does nothing, and GEOSWKTReader_read_r
will return a NULL pointer. However, when GC.start is called, we get a
segfault. From the gdb backtrace:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: 13 at address: 0x0000000000000000
0x0000000100106932 in st_lookup (table=0x100043780, key=5912,
value=0x7fff5fbfc808) at st.c:333
333	            if ((st_data_t)table->bins[i*2] == key) {

However, if you uncomment out the print statement in the error_handler
callback, the segfault disappears. The print statement can contain any
non-empty string and it will prevent the segfault.

Other strangeness: moving the wkt_ptr lines inside of the 10.times
loop produces one of several different errors at random...

- sometimes a segfault occurs due to an EXC_BAD_ACCESS in
callback_with_gvl in Function.c, line 558 ("Type* returnType = cbInfo-

>returnType;").

- other times the code won't segfault but an exception will be raised.
The exceptions will be one of the following:

* undefined method `call' for "ParseException: Unknown type:
'GIBBERISH'":String
* undefined method `call' for #<Array:0x00000102019278>
* undefined method `call' for "%s":String
* method `call' called on hidden T_ARRAY object (0x00000101812980
flags=0xa007 klass=0x0)

The only real solution I've found so far is to modify the GEOS library
code to allow the error and notice handlers to be set to NULL pointers
and to do checks in the library to prevent the handlers from being
called in such cases. This allows you to set the handlers to nil in
Ruby code and all is well.

Anyone have any ideas as to what's going on? I'm at my wits end at
this point. Modifying GEOS itself might not be an option, but if
there's something I can do in Ruby that would be awesome.

For anyone interested, I've also started a thread on the geos-devel
list here: http://lists.osgeo.org/pipermail/geos-devel/2010-December/005043.html
.

Cheers and thanks for reading list!

J