sd-bus: do not connect to dbus-1 socket when kdbus is available

We should not fall back to dbus-1 and connect to the proxy when kdbus
returns an error that indicates that kdbus is running but just does not
accept new connections because of quota limits or something similar.

Based on a patch by Kay.

Author: David Herrmann

src/libsystemd/sd-bus/sd-bus.c

@@ -1002,6 +1002,8 @@ static int bus_parse_next_address(sd_bus *b) {
 }
 
 static int bus_start_address(sd_bus *b) {
+        bool container_kdbus_available = false;
+        bool kdbus_available = false;
         int r;
 
         assert(b);
@@ -1011,15 +1013,29 @@ static int bus_start_address(sd_bus *b) {
 
                 bus_close_fds(b);
 
+                /*
+                 * Usually, if you provide multiple different bus-addresses, we
+                 * try all of them in order. We use the first one that
+                 * succeeds. However, if you mix kernel and unix addresses, we
+                 * never try unix-addresses if a previous kernel address was
+                 * tried and kdbus was available. This is required to prevent
+                 * clients to fallback to the bus-proxy if kdbus is available
+                 * but failed (eg., too many connections).
+                 */
+
                 if (b->exec_path)
                         r = bus_socket_exec(b);
-                else if ((b->nspid > 0 || b->machine) && b->kernel)
+                else if ((b->nspid > 0 || b->machine) && b->kernel) {
                         r = bus_container_connect_kernel(b);
-                else if ((b->nspid > 0 || b->machine) && b->sockaddr.sa.sa_family != AF_UNSPEC)
+                        if (r < 0 && !IN_SET(r, -ENOENT, -ESOCKTNOSUPPORT))
+                                container_kdbus_available = true;
+                } else if (!container_kdbus_available && (b->nspid > 0 || b->machine) && b->sockaddr.sa.sa_family != AF_UNSPEC)
                         r = bus_container_connect_socket(b);
-                else if (b->kernel)
+                else if (b->kernel) {
                         r = bus_kernel_connect(b);
-                else if (b->sockaddr.sa.sa_family != AF_UNSPEC)
+                        if (r < 0 && !IN_SET(r, -ENOENT, -ESOCKTNOSUPPORT))
+                                kdbus_available = true;
+                } else if (!kdbus_available && b->sockaddr.sa.sa_family != AF_UNSPEC)
                         r = bus_socket_connect(b);
                 else
                         skipped = true;