[Xcb] mesa +xcb = assertion death

[Jeremy Kolb]

Jamey Sharp wrote:
> On Fri, 2005-04-29 at 04:44 -0400, Jeremy Kolb wrote:
> 
>>Okay.
>>
>>In line 250 of xcb_in_read_packet we have:
>>
>>c->in.request_read = (lastread & 0xffff0000) | genrep.sequence;
>>
>>For some reason when it fails genrep has a response_type of 5, a 
>>sequence of 0 and a length of 1.  This is when it should be pixing up 
>>sequence 12 I believe.  Up until this point I get perfectly sane values 
>>for this.
>>
>>I don't know why it's getting this value for genrep.  But it causes the 
>>next read to be 65536 since 0 & 0xffff = 65536.  And that's why the 
>>assertion fires.
> 
> 
> (For those less familiar with the code base, I'll mention that of
> course, '0 & x == 0'. But yes, immediately after the line you quoted
> we'll add 0x10000 to it in this case.)
> 
> Right. Wow. I suppose there's *some* way that could be my fault, but I'm
> having trouble imagining how.
> 
> It sounds like you can very reliably reproduce this bug. Would you
> capture a protocol trace using either 'xscope -v4' or some network
> sniffer like Ethereal, and post a URL for it? I want to see if your X
> server is really sending you a busted sequence number stream. Perhaps
> XCB is getting misaligned somewhere (but I still can't imagine how).
> 

http://people.brandeis.edu/~jkolb/glx.dump is the file.  It's in 
tcpdump's format.

I should have used ethereal earlier but I completely forgot about it. grrr.

> 
>>However I have no idea why the generic reply is returning 0 as a 
>>sequence number, nor do I know what response_type 5 is.
> 
> 
> Response type 5 is a ButtonRelease event. (0 is an error, 1 is a reply,
> and everything else is an event; you can look up core events in
> xproto.xml, xproto.h, or your friendly neighborhood X protocol
> reference.)
> 

I saw that but disregarded it since no button has been hit at this 
point, indeed there isn't a window and this happens in both glxgears 
(eventually spawns a window when this problem doesn't occur) and glxinfo 
(no window ever).

> 
>>Maybe Jamey has an answer?
> 
> 
> Not yet, but you're still doing a great job of troubleshooting.
> 
> One cool effect of this problem is that it's forcing me to write down a
> lot of what I know about debugging these sorts of problems. :-) On the
> other hand, it's a little disheartening to realize that I haven't found
> many tricks that actually help. :-/
> 
> --Jamey
> 
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