How to debug (potential) GC bugs?

[Matthias Klumpp via Digitalmars-d-learn]

Hello!
I am working together with others on the D-based 
appstream-generator[1] project, which is generating software 
metadata for "software centers" and other package-manager 
functionality on Linux distributions, and is used by default on 
Debian, Ubuntu and Arch Linux.

For Ubuntu, some modifications on the code were needed, and 
apparently for them the code is currently crashing in the GC 
collection thread: http://paste.debian.net/840490/

The project is running a lot of stuff in parallel and is using 
the GC (if the extraction is a few seconds slower due to the GC 
being active, it doesn't matter much).

We also link against a lot of 3rd-party libraries and use a big 
amount of existing C code in the project.

So, I would like to know the following things:

1) Is there any caveat when linking to C libraries and using the 
GC in a project? So far, it seems to be working well, but there 
have been a few cases where I was suspicious about the GC 
actually doing something to malloc'ed stuff or C structs present 
in the bindings.

2) How can one debug issues like the one mentioned above 
properly? Since it seems to happen in the GC and doesn't give me 
information on where to start searching for the issue, I am a bit 
lost.

3) The tool seems to leak memory somewhere and OOMs pretty 
quickly on some machines. All the stuff using C code frees 
resources properly though, and using Valgrind on the project is a 
pain due to large amounts of data being mmapped. I worked around 
this a while back, but then the GC interfered with Valgrind, 
making information less useful. Is there any information on how 
to find memory leaks, or e.g. large structs the GC cannot free 
because something is still having a needless reference on it?

Unfortunately I can't reproduce the crash from 2) myself, it only 
seems to happen at Ubuntu (but Ubuntu is using some different 
codepaths too).

Any insights would be highly appreciated!
Cheers,
    Matthias

[1[: https://github.com/ximion/appstream-generator