BUG/MEDIUM: server: "proto" not working for dynamic servers

In 304672320e ("MINOR: server: support keyword proto in 'add server' cli")
improper use of conn_get_best_mux_entry() function was made:

First, server's proxy mode was directly passed as "proto_mode" argument
to conn_get_best_mux_entry(), but this is strictly invalid because while
there is some relationship between proto modes and proxy modes, they
don't use the same storage mechanism and cannot be used interchangeably.

Because of this bug, conn_get_best_mux_entry() would not work at all for
TCP because PR_MODE_TCP equals 0, where PROTO_MODE_TCP normally equals 1.

Then another, less sensitive bug, remains:
as its name and description implies, conn_get_best_mux_entry() will try
its best to return something to the user, only using keyword (mux_proto)
input as an hint to return the most relevant mux within the list of
mux that are compatibles with proto_side and proto_mode values.

This means that even if mux_proto cannot be found or is not available
with current proto_side and proto_mode values, conn_get_best_mux_entry()
will most probably fallback to a more generic mux.

However in cli_parse_add_server(), we directly check the result of
conn_get_best_mux_entry() and consider that it will return NULL if the
provided keyword hint for mux_proto cannot be found. This will result in
the function not raising errors as expected, because most of the times if
the expected proto cannot be found, then we'll silently switch to the
fallback one, despite the user providing an explicit proto.

To fix that, we store the result of conn_get_best_mux_entry() to compare
the returned mux proto name with the one we're expecting to get, as it
is originally performed in cfgparse during initial server keyword parsing.

This patch depends on
 - "MINOR: connection: add conn_pr_mode_to_proto_mode() helper func")

It must be backported up to 2.6.

src/server.c

@@ -5009,7 +5009,12 @@ static int cli_parse_add_server(char **args, char *payload, struct appctx *appct
 	srv->init_addr_methods = SRV_IADDR_NONE;
 
 	if (srv->mux_proto) {
-		if (!conn_get_best_mux_entry(srv->mux_proto->token, PROTO_SIDE_BE, be->mode)) {
+		int proto_mode = conn_pr_mode_to_proto_mode(be->mode);
+		const struct mux_proto_list *mux_ent;
+
+		mux_ent = conn_get_best_mux_entry(srv->mux_proto->token, PROTO_SIDE_BE, proto_mode);
+
+		if (!mux_ent || !isteq(mux_ent->token, srv->mux_proto->token)) {
 			ha_alert("MUX protocol is not usable for server.\n");
 			goto out;
 		}