On Sunday, 25 September 2016 at 16:23:11 UTC, Matthias Klumpp
wrote:
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.
There is no way the GC scans memory allocated with malloc (unless
you tell it to) or used in the bindings.
A caveat is that if you are called from C (not your case), you
must initialize the runtime, and attach/detach threads.
The GC could well stop threads that are currently in the C code
if they were registered to the runtime.
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.
There can be multiple reasons.
- The GC is collecting some object that is unreachable from its
POV; when you are actually using it.
- The GC is calling destructors, that should not be called by
the GC. Performing illegal operations. usually this is solved by
using deterministic destruction instead and never relying on a
destructor called by the GC.
- The GC tries to stop threads that don't exist anymore or are
not interruptible
My advice is to have a fuly deterministic tree of objects, like a
C++ program, and Google for "GC-proof resource class" in case you
are using classes.
