Hi,
On 02/04/2011 12:27 PM, Alon Levy wrote:
On Fri, Feb 04, 2011 at 10:11:59AM +0100, Hans de Goede wrote:
Hi,
On 02/04/2011 09:39 AM, Alon Levy wrote:
<snip>
Hmm,
This seems error prone. What if the other side is sending a success state for
some
other reason? Either error->code == VSC_SUCCESS can only be send for the
adding a reader case and keeping track of num_pending_reader_inserts is
not necessary or this seems racy to me. Maybe add a VCS_READER_ADD_SUCCESS
error code ?
Ok, I admit this whole counter was a way to quickly fix the problem that I was
getting a second VSC_SUCCESS. The idea was to have a single outstanding request
that needs an answer, thereby removing ambiguity. But the implementation
doesn't do that, hence the hack of _num_pending_reader_inserts. To keep the
protocol as is (btw I just *removed* such a message because I wanted to
simplify the protocol - ReaderAddResponse. equivalent to the suggested error
code) I could just serialize to:
client: request reader add
client: wait for response
server: send VSCError.code==VSC_SUCCESS
client: report card insert
cient: wait for response
That is still racy. What if a VSCError.code==VSC_SUCCESS is already in the pipe
from server to client for some other request send by the client earlier. Then
the client will report the card insert, when it receives the unrelated
VSCError.code==VSC_SUCCESS.
another way to state my previous response: the VSC_Error is only sent by the
server when the client is expected to wait for it. it is only a response to one
of:
card insert (atr)
card remove
reader insert
reader remove
I sort of understand. Let me put it this way looking at it from a pure
data communications pov. You can have 2 things:
1) A stop and wait flow control kind of situation, if this is the case you know
a success report belongs to your last command, but then I see no need for
num_pending_reader_inserts magic
2) A flow control situation where there is a window, iow there can be multiple
outstanding requests, your num_pending_reader_inserts magic gives me the
feeling (I don't know the details of the smartcard stuff). This is the
case here. In which case you somehow need to know which request a success
message acks -> id's
Given that you're getting 2 success messages, this feels like a multiple
outstanding requests case, and the num_pending_reader_inserts magic feels like
the wrong solution, as it assumes the first success message is the one you're
waiting for.
While nitpicking I would also rename VCSError to VCSStatus, as it also reports
successes.
Regards,
Hans
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