On 07.11.2013 13:24, Keith Packard wrote:
Uli Schlachter <psyc...@znc.in> writes:
Having thought about this some more in the shower, I would suggest to turn
xcb_send_fd() into a private _send_fd() and instead add
xcb_send_request_with_fds() (or something like that).
This isn't necessary; as I explained, the X server just holds onto FDs
that the client sends until a request consumes them. Yes, it has to deal
with clients sending unexpected FDs, there's nothing special about this.
Ok, but then xcb needs to do the same (see below).
[...]
- Does the new sendmsg() call work correctly on non-unix sockets when no FDs are
transmitted?
You'll note that sendmsg is *not* called when we have no file
descriptors to send.
Ah, sorry, I did indeed miss that part.
- Should we have some API to test if sending FDs works at all or if xcb just
ignores the FDs?
I'm not entirely sure how you'd do that without actually trying to send
an FD, given that XCB can be handed a pre-connected file descriptor.
- What happens with partial reads? Let's say we send 4KiB of requests to the
server in one sendmsg() call and the last request has FDs attached. For whatever
reason, the server only reads 2KiB of data from the socket. Will the FDs be
handled correctly in this case? What if the FDs are for the first request
instead of the last one?
The FDs are effectively attached to the first byte of the write; read
that bytes and you get all of the FDs sent there. So, if you perform
a 4kB write with 10 file descriptors, and read only 2kB, then you'll get
all 10 file descriptors as you've read the relevant byte.
Ok, seems like the server handles this correctly then.
However, now I wonder about patch 5/8 "Add support for receiving fds" which does
not do this.
If I understood the code correctly, _xcb_in_read() will close the connection
with an error if a partial read happens and thus we get FDs before the
corresponding event/reply.
In other words, the server handles partial reads correctly, xcb doesn't
Oh and I just noticed:
If HAVE_SENDMSG is not defined, shouldn't xcb_send_fd() close() the FD
that it gets?
The X server won't advertise any requests which receive or send file
descriptors on a system which doesn't support SENDMSG, so I'm not sure
how interesting the precise semantics of this error case are. And, I
assume the BSD people will shortly figure out how to make this work on
their operating systems, which means that pretty much anything with
local applications will have working xcb FD passing.
I've seen lots of code which just uses an extension without checking for its
availability. I am sure people will be able to get this wrong, too. At least I
hope that this would then cause an error from the server and thus gets noticed...
Anyway, I guess I ran out of doubts for this patch (and now have new doubts
about patch 5/8), so:
Reviewed-By: Uli Schlachter <psyc...@znc.in>
Uli
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