From 2c7deddc06374700c967a7a1f72b913d0f0d9bb0 Mon Sep 17 00:00:00 2001
From: Willy Tarreau <w@1wt.eu>
Date: Thu, 24 Jan 2019 18:22:19 +0100
Subject: [PATCH] BUG/MEDIUM: backend: never try to attach to a mux having no
 more stream available

The code dealing with idle connections used to check the number of streams
available on the connection only to unlink the connection from the idle
list. But this still resulted in too many streams reusing the same connection
when they were already attached to it.

We must detect that there is no more room and refrain from using this
connection at all, and instead fall back to the no-reuse case. Ideally
we should try to search among other idle connections, but for a backport
let's stay safe.

This must be backported to 1.9.
---
 src/backend.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/src/backend.c b/src/backend.c
index aef58c2b9..59324538d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1297,26 +1297,40 @@ int connect_server(struct stream *s)
 		}
 	}
 
-	if (!reuse) {
-		srv_conn = conn_new();
-		srv_cs = NULL;
-	} else {
+	if (reuse) {
 		/* We already created a cs earlier when using http_proxy, so
 		 * only create a new one if we don't have one already.
 		 */
 		if (!srv_cs) {
-			if (srv_conn->mux->avail_streams(srv_conn) <= 1) {
+			int avail = srv_conn->mux->avail_streams(srv_conn);
+
+			if (avail <= 1) {
 				/* No more streams available, remove it from the list */
 				LIST_DEL(&srv_conn->list);
 				LIST_INIT(&srv_conn->list);
 			}
-			srv_cs = srv_conn->mux->attach(srv_conn, s->sess);
-			if (srv_cs)
-				si_attach_cs(&s->si[1], srv_cs);
+
+			if (avail >= 1) {
+				srv_cs = srv_conn->mux->attach(srv_conn, s->sess);
+				if (srv_cs)
+					si_attach_cs(&s->si[1], srv_cs);
+				else
+					srv_conn = NULL;
+			}
 			else
 				srv_conn = NULL;
 		}
+		/* otherwise srv_conn is left intact */
 	}
+	else
+		srv_conn = NULL;
+
+	/* no reuse or failed to reuse the connection above, pick a new one */
+	if (!srv_conn) {
+		srv_conn = conn_new();
+		srv_cs = NULL;
+	}
+
 	if (srv_conn && old_conn != srv_conn) {
 		if (srv_conn->owner)
 			session_unown_conn(srv_conn->owner, srv_conn);