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.IX Title "xparace 3"
.TH xparace 3 "June 7, 2012" "version 2.1.14" "SAORD Documentation"
.SH "NAME"
\&\fBXPA Race Conditions\fR
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
Potential \s-1XPA\s0 race conditions and how to avoid them.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Currently, there is only one known circumstance in which \s-1XPA\s0 can get
(temporarily) deadlocked in a race condition: if two or more \s-1XPA\s0
servers send messages to one another using an \s-1XPA\s0 client routine such
as \fIXPASet()\fR, they can deadlock while each waits for the other server
to respond. (This can happen if the servers call \fIXPAPoll()\fR with a
time limit, and send messages in between the polling call.) The
reason this happens is that both client routines send a string to the
other server to establish the handshake and then wait for the server
response. Since each client is waiting for a response, neither is able
to enter its event-handling loop and respond to the other's
request. This deadlock will continue until one of the timeout periods
expire, at which point an error condition will be triggered and the
timed-out server will return to its event loop.
.PP
Starting with version 2.1.6, this rare race condition can be
avoided by setting the \s-1XPA_IOCALLSXPA\s0 environment variable for servers
that will make client calls. Setting this variable causes all \s-1XPA\s0
socket \s-1IO\s0 calls to process outstanding \s-1XPA\s0 requests whenever the
primary socket is not ready for \s-1IO\s0. This means that a server making a
client call will (recursively) process incoming server requests while
waiting for client completion. It also means that a server callback
routine can handle incoming \s-1XPA\s0 messages if it makes its own \s-1XPA\s0 call.
The semi-public routine oldvalue=XPAIOCallsXPA(newvalue) can be used
to turn this behavior off and on temporarily. Passing a 0 will turn
off \s-1IO\s0 processing, 1 will turn it back on. The old value is returned
by the call.
.PP
By default, the \s-1XPA_IOCALLSXPA\s0 option is turned off, because we judge
that the added code complication and overhead involved will not be
justified by the amount of its use. Moreover, processing \s-1XPA\s0 requests
within socket \s-1IO\s0 can lead to non-intuitive results, since incoming
server requests will not necessarily be processed to completion in the
order in which they are received.
.PP
Aside from setting \s-1XPA_IOCALLSXPA\s0, the simplest way to avoid this race
condition is to multi\-process: when you want to send a client message,
simply start a separate process to call the client routine, so that
the server is not stopped. It probably is fastest and easiest to use
\&\fIfork()\fR and then have the child call the client routine and exit. But
you also can use either the \fIsystem()\fR or \fIpopen()\fR routine to start one
of the command line programs and do the same thing. Alternatively, you
can use \s-1XPA\s0's internal \fIlaunch()\fR routine instead of \fIsystem()\fR. Based on
\&\fIfork()\fR and \fIexec()\fR, this routine is more secure than \fIsystem()\fR because
it does not call /bin/sh.
.PP
Starting with version 2.1.5, you also can send an \fIXPAInfo()\fR message with
the mode string \*(L"ack=false\*(R". This will cause the client to send a message
to the server and then exit without waiting for any return message from
the server. This UDP-like behavior will avoid the server deadlock when
sending short XPAInfo messages.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
See xpa(n) for a list of \s-1XPA\s0 help pages