MINOR: connection: define mux flag for reverse support

Add a new MUX flag MX_FL_REVERSABLE. This value is used to indicate that
MUX instance supports connection reversal. For the moment, only HTTP/2
multiplexer is flagged with it.

This allows to dynamically check if reversal can be completed during MUX
installation. This will allow to relax requirement on config writing for
'tcp-request session attach-srv' which currently cannot be used mixed
with non-http/2 listener instances, even if used conditionnally with an
ACL.

include/haproxy/connection-t.h

@@ -310,6 +310,7 @@ enum {
 	MX_FL_HOL_RISK    = 0x00000002, /* set if the protocol is subject the to head-of-line blocking on server */
 	MX_FL_NO_UPG      = 0x00000004, /* set if mux does not support any upgrade */
 	MX_FL_FRAMED      = 0x00000008, /* mux working on top of a framed transport layer (QUIC) */
+	MX_FL_REVERSABLE  = 0x00000010, /* mux supports connection reversal */
 };
 
 /* PROTO token registration */

src/connection.c

@@ -282,6 +282,15 @@ int conn_install_mux_fe(struct connection *conn, void *ctx)
 		if (!mux_ops)
 			return -1;
 	}
+
+	/* Ensure a valid protocol is selected if connection is targetted by a
+	 * tcp-request session attach-srv rule.
+	 */
+	if (conn->reverse.target && !(mux_ops->flags & MX_FL_REVERSABLE)) {
+		conn->err_code = CO_ER_REVERSE;
+		return -1;
+	}
+
 	return conn_install_mux(conn, mux_ops, ctx, bind_conf->frontend, conn->owner);
 }
 

src/mux_h2.c

@@ -7158,7 +7158,7 @@ static const struct mux_ops h2_ops = {
 	.show_fd = h2_show_fd,
 	.show_sd = h2_show_sd,
 	.takeover = h2_takeover,
-	.flags = MX_FL_HTX|MX_FL_HOL_RISK|MX_FL_NO_UPG,
+	.flags = MX_FL_HTX|MX_FL_HOL_RISK|MX_FL_NO_UPG|MX_FL_REVERSABLE,
 	.name = "H2",
 };