[Magick-bugs] About Imagemagik++ Problem

[emagick at magic.ms]

duc.sequere.aut.de.via.decede at imagemagick.org wrote:

> ImageMagick supports multi-threading and we have no current reports of threading problems.

Here you get one (again):

Here's a typical usage of DestroySemaphoreInfo (as of ImageMagick
6.4.1-3):

   wand_semaphore=DestroySemaphoreInfo(wand_semaphore);

Unfortunately, there is a time window during which the variable
wand_semaphore is still non-NULL but the object pointed to no
longer exists causing problems if another thread happens to call
AcquireSemaphoreInfo.

I think a level of indirection to DestroySemaphoreInfo's
parameter should be added (instead of returning a pointer) and
NULL should be stored to the object pointed to by that parameter
while semaphore_mutex is still held.

I have seen crashes due to that problem. Just run

     for (;;)
       {
         MagickWand *wand = NewMagickWand ();
         DestroyMagickWand (wand);
       }

in two threads (without having other MagickWand objects allocated).

Fixing DestroySemaphoreInfo as described above isn't enough for making
this code work, I also had to extend the critical section over the
AcquireWandId function (avoiding the double check pattern).