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+.\" ========================================================================
+.\"
+.IX Title "xparace 3"
+.TH xparace 3 "July 23, 2013" "version 2.1.15" "SAORD Documentation"
+.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
+.\" way too many mistakes in technical documents.
+.if n .ad l
+.nh
+.SH "NAME"
+\&\fB\s-1XPA\s0 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