http: Retry request (once) on error receiving response

A Juniper server has been encountered in the wild which sends an initial
302 redirect without Connection:close, but then just closes the connection
when it receives the next request.

This happens only for the first redirect to /dana-na/auth/… and not for
subsequent redirects through cookie-check and realm stuff. So instead of
a preemptive hack to avoid connection reuse for *all* redirects in NC,
just cope with it when it happens.

Since rq_retry is only set when the connection is already open, it won't
get set again the second time round, thus avoiding endless retries.

Fixes: #96

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

http.c

@@ -1085,6 +1085,15 @@ int do_https_request(struct openconnect_info *vpninfo, const char *method,
 
 	result = process_http_response(vpninfo, 0, http_auth_hdrs, buf);
 	if (result < 0) {
+		if (rq_retry) {
+			openconnect_close_https(vpninfo, 0);
+			vpn_progress(vpninfo, PRG_INFO,
+				     _("Retrying failed %s request on new connection\n"),
+				     method);
+			/* All the way back to 'redirected' since we need to rebuild
+			 * the request in 'buf' from scratch. */
+			goto redirected;
+		}
 		goto out;
 	}
 	if (vpninfo->dump_http_traffic && buf->pos)