BUG/MEDIUM: mux-fcgi: Be sure to never set EOM flag on an empty HTX message

This bug was already fixed at many places (stats, promex, lua) but the FCGI
multiplexer is also affected. When there is no content-length specified in
the response and when the END_REQUEST record is delayed, the response may be
truncated because an abort is erroneously detected. If the connection is not
closed because "keep-conn" option is set, the response is aborted at the end
of the server timeout.

This bug is a design issue with the HTX. It should be addressed. But it will
probably not be possible to backport them as far as 2.4. So, for now, the
only solution is to explicitly add an EOT block with the EOM flag in this
case.

This patch should fix the issue #1682. It must be backported as far as 2.4.

(cherry picked from commit 2db904e86c5f19e12d30b8746a622bcb072d914a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f5214f134529cd537d7897cd6befee762921e521)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index dd82346..d916496 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -3442,6 +3442,8 @@
 			if (!(h1m->flags & H1_MF_XFER_LEN) && fstrm->state != FCGI_SS_ERROR &&
 			    (fstrm->flags & FCGI_SF_ES_RCVD) && b_data(&fstrm->rxbuf) == total) {
 				TRACE_DEVEL("end of data", FCGI_EV_RSP_DATA, fconn->conn, fstrm);
+				if (htx_is_empty(htx) && !htx_add_endof(htx, HTX_BLK_EOT))
+					break;
 				htx->flags |= HTX_FL_EOM;
 				h1m->state = H1_MSG_DONE;
 				TRACE_USER("H1 response fully rcvd", FCGI_EV_RSP_DATA|FCGI_EV_RSP_EOM, fconn->conn, fstrm, htx);