[Xcb] Number of iovecs for xcb_send_request()

[Uli Schlachter]

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Hi,

recently I stumbled upon a PDF which evince could not display. It would always
die with an XIO error. Through some debugging, I noticed that this was due to
some magic that cairo does to avoid having to copy data around:

http://cgit.freedesktop.org/cairo/tree/src/cairo-xcb-connection-core.c#n162

I wrote a test case for cairo which reproduces this problem. However, for this
report, I also wrote an xcb-only test case. See the attached program.

The problem is that xcb tries to write out the huge number of struct iovs in a
single call to writev(). However, there is an upper limit on the number of
iovs that are accepted. This limit causes writev() to fail with EINVAL.

So my question now is: Where is the bug? If cairo is doing something wrong, it
would have to split the PutImage up into multiple individual requests. I'd
like to avoid this, if possible (And in this case, someone should extend xcb's
documentation to explain this limit and how it should be handled). If this
problem should be handled in xcb, then... I'm not really sure how this should
be handled.

What do others think?

Cheers,
Uli
- -- 
Q: Because it reverses the logical flow of conversation.
A: Why is putting a reply at the top of the message frowned upon?
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