[Mesa-dev] OSMesa multiple context problem?

[Randall Frank]

Hello,
    I have been working around an issue with the OSMesa interface a bit 
and was curious if others see this as a bug.  An example that triggers 
the issue is:

#include <GL/gl.h>
#include <GL/osmesa.h>
#include <stdlib.h>
#include <malloc.h>

int main(int iargs, const char *argv[])
{
     void *pBuf0Ptr, *pBuf1Ptr, *pBuf2Ptr;
     GLsizei iBuf0Width = 720, iBuf0Height = 480;
     GLsizei iBuf1Width = 720, iBuf1Height = 480;
     GLsizei iBuf2Width = 120, iBuf2Height = 120;
     OSMesaContext ctx0, ctx1;
     GLboolean bOk;

     pBuf0Ptr = memalign(iBuf0Width*iBuf0Height*4, 4);
     pBuf1Ptr = memalign(iBuf1Width*iBuf1Height*4, 4);
     pBuf2Ptr = memalign(iBuf2Width*iBuf2Height*4, 4);

     ctx0 = OSMesaCreateContext(GL_RGBA, NULL);
     ctx1 = OSMesaCreateContext(GL_RGBA, NULL);

     bOk = OSMesaMakeCurrent(ctx0, pBuf0Ptr, GL_UNSIGNED_BYTE, 
iBuf0Width, iBuf0Height);
     glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
     glFlush();

     bOk = OSMesaMakeCurrent(ctx1, pBuf1Ptr, GL_UNSIGNED_BYTE, 
iBuf1Width, iBuf1Height);
     glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
     glFlush();

     bOk = OSMesaMakeCurrent(ctx1, pBuf2Ptr, GL_UNSIGNED_BYTE, 
iBuf2Width, iBuf2Height);
     glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
     glFlush();

     bOk = OSMesaMakeCurrent(ctx0, pBuf0Ptr, GL_UNSIGNED_BYTE, 
iBuf0Width, iBuf0Height);
     glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
     glFlush();

     exit(0);
}

When I link this to an gallium enabled Mesa build I get a crash during 
the flush to the previous context in osmesa_st_framebuffer_flush_front().

I looked into it a bit and I believe there may be an issue with 
osmesa_find_buffer(). Since the two contexts initially use the same size 
buffers, this function returns the same osmesa_buffer for each context.  
This causes a problem when we change the size of one of the contexts 
during an OSMesaMakeCurrent() and it changes the buffer fields that are 
effectively (incorrectly?) shared.  The flush on the change of context 
then crashes...

At least that is my read of the issue. Is this interpretation correct?  
If so is there a patch or should we put one together?

-- 
Thanks
rjf.

-------------------------------------------------------
rjfrank at ensight.com voice:919-363-0883 fax:919-363-0833
Computational Engineering International www.ensight.com
2166 North Salem St,  Suite 101,  Apex,  NC  27523-6456