Multithreading with GM

I was finally able to get pthreads to work with GM by recompiling it
with OpenMP turned off. I'm on OSX, and gcc 4.2, 4.4 and 4.5 would
link GM but I'm using Qt with many threads, and any Magick call
outside the main thread would crash.  This seems to be well documented-ish
online (In terms of OpenMP+pThreads), and I'm not entirely sure I
need OpenMP for my application.

I'm creating an image server and I can spawn new threads on new tcp
connections just fine, and serve images from separate threads...

Each row of the image is sent as a separate request, and in my initial
implementation each thread (which represents a distinct tcp connection)
is dispatching those rows in a single thread.

As an upgrade I'm trying to process each row (which wants to be
transformed into other colorspaces and compressed) in a separate thread
before transmitting to the client.  So, I have spawned a thread for the tcp
connection, which in turn spawns many threads to process each scanline.

The problem I'm having is that no combination of Mutexes seems to
protect the image cache from flushing unpredictably.  As an example, there
may be 8 or more threads trying to call something like:

Pixels view(img);
const PixelPacket *pixels = view.getConst(0,row,img.columns(),1);

concurrently.  It works fine for a while but eventually it crashes.
Running with MAGIC_DEBUG=all reveals:

destroy cache [path]
Destroy blob, image=0x10282b200, filename="[path]"

are reliably the culprits, and:

Assertion failed: (image->signature == MagickSignature), function
OpenCacheView, file magick/pixel_cache.c, line 3193.

is usually thrown (but not always)

This makes sense... anyone know of a way to control the flushing, or create
a mutex of some kind so I can grant permission for the cache to do its thing?
I can't seem to find anything in the documentation like this...


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