meson: Improve default firewall backend configuration

The current implementation requires users to configure the
preference as such:

  -Dfirewall_backend_default_1=iptables
  -Dfirewall_backend_default_2=nftables

In addition to being more verbose than one would hope, there
are several things that could go wrong.

First of all, meson performs no validation on the provided
values, so mistakes will only be caught by the compiler.
Additionally, it's entirely possible to provide nonsensical
combinations, such as repeating the same value twice.

Change things so that the preference can now be configured
as such:

  -Dfirewall_backend_priority=iptables,nftables

Checks have been added to prevent invalid values from being
accepted.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

meson.build

@@ -1635,15 +1635,17 @@ endif
 
 if not get_option('driver_network').disabled() and conf.has('WITH_LIBVIRTD')
   conf.set('WITH_NETWORK', 1)
-  firewall_backend_default_1 = get_option('firewall_backend_default_1')
-  firewall_backend_default_conf = firewall_backend_default_1
-  firewall_backend_default_1 = 'VIR_FIREWALL_BACKEND_' + firewall_backend_default_1.to_upper()
-  conf.set('FIREWALL_BACKEND_DEFAULT_1', firewall_backend_default_1)
 
-  firewall_backend_default_2 = get_option('firewall_backend_default_2')
+  firewall_backend_priority = get_option('firewall_backend_priority')
-  firewall_backend_default_2 = 'VIR_FIREWALL_BACKEND_' + firewall_backend_default_2.to_upper()
+  if (not firewall_backend_priority.contains('nftables') or
-  conf.set('FIREWALL_BACKEND_DEFAULT_2', firewall_backend_default_2)
+      not firewall_backend_priority.contains('iptables') or
+      firewall_backend_priority.length() != 2)
+    error('invalid value for firewall_backend_priority option')
+  endif
 
+  conf.set('FIREWALL_BACKEND_PRIORITY_0', 'VIR_FIREWALL_BACKEND_' + firewall_backend_priority[0].to_upper())
+  conf.set('FIREWALL_BACKEND_PRIORITY_1', 'VIR_FIREWALL_BACKEND_' + firewall_backend_priority[1].to_upper())
+  conf.set('FIREWALL_BACKEND_PRIORITY_NUM', firewall_backend_priority.length())
 elif get_option('driver_network').enabled()
   error('libvirtd must be enabled to build the network driver')
 endif

meson_options.txt

@@ -115,8 +115,7 @@ option('dtrace', type: 'feature', value: 'auto', description: 'use dtrace for st
 option('firewalld', type: 'feature', value: 'auto', description: 'firewalld support')
 # dep:firewalld
 option('firewalld_zone', type: 'feature', value: 'auto', description: 'whether to install firewalld libvirt zone')
-option('firewall_backend_default_1', type: 'string', value: 'nftables', description: 'first firewall backend to try  when none is specified')
+option('firewall_backend_priority', type: 'array', choices: ['nftables', 'iptables'], description: 'order in which to try firewall backends')
-option('firewall_backend_default_2', type: 'string', value: 'iptables', description: 'second firewall backend to try when none is specified (and first is unavailable)')
 option('host_validate', type: 'feature', value: 'auto', description: 'build virt-host-validate')
 option('init_script', type: 'combo', choices: ['systemd', 'openrc', 'check', 'none'], value: 'check', description: 'Style of init script to install')
 option('loader_nvram', type: 'string', value: '', description: 'Pass list of pairs of <loader>:<nvram> paths. Both pairs and list items are separated by a colon.')

src/network/bridge_driver_conf.c

@@ -67,8 +67,12 @@ virNetworkLoadDriverConfig(virNetworkDriverConfig *cfg G_GNUC_UNUSED,
     g_autofree char *fwBackendStr = NULL;
     bool fwBackendSelected = false;
     size_t i;
-    int fwBackends[] = { FIREWALL_BACKEND_DEFAULT_1, FIREWALL_BACKEND_DEFAULT_2 };
+    int fwBackends[] = {
+        FIREWALL_BACKEND_PRIORITY_0,
+        FIREWALL_BACKEND_PRIORITY_1,
+    };
     G_STATIC_ASSERT(G_N_ELEMENTS(fwBackends) == VIR_FIREWALL_BACKEND_LAST);
+    G_STATIC_ASSERT(G_N_ELEMENTS(fwBackends) == FIREWALL_BACKEND_PRIORITY_NUM);
     int nFwBackends = G_N_ELEMENTS(fwBackends);
 
     if (access(filename, R_OK) == 0) {

src/network/meson.build

@@ -51,7 +51,8 @@ if conf.has('WITH_NETWORK')
   }
 
   network_options_conf = configuration_data({
-    'FIREWALL_BACKEND': firewall_backend_default_conf,
+    'FIREWALL_BACKEND_PRIORITY': ', '.join(firewall_backend_priority),
+    'FIREWALL_BACKEND': firewall_backend_priority[0],
   })
 
   network_conf = configure_file(
@@ -61,7 +62,8 @@ if conf.has('WITH_NETWORK')
   )
 
   network_options_hack_conf = configuration_data({
-    'FIREWALL_BACKEND': firewall_backend_default_conf,
+    'FIREWALL_BACKEND_PRIORITY': ', '.join(firewall_backend_priority),
+    'FIREWALL_BACKEND': firewall_backend_priority[0],
     # This hack is necessary because the output file is going to be
     # used as input for another configure_file() call later, which
     # will take care of substituting @CONFIG@ with useful data

src/network/network.conf.in

@@ -12,12 +12,13 @@
 #     iptables - use iptables commands to construct the firewall
 #     nftables - use nft commands to construct the firewall
 #
-#   If firewall_backend isn't set in this file, libvirt will
+#   If firewall_backend isn't configured, libvirt will choose the
-#   prefer the @FIREWALL_BACKEND@ backend *if the necessary package.
+#   first available backend from the following list:
-#   binary is installed*, otherwise it will look for the package/binary
+#
-#   needed for the other backend and use that if available. If neither
+#     [@FIREWALL_BACKEND_PRIORITY@]
-#   is available on the host, then the network driver will fail to
+#
-#   start, and an error will be logged.
+#   If no backend is available on the host, then the network driver
+#   will fail to start, and an error will be logged.
 #
 #   (NB: switching from one backend to another while there are active
 #   virtual networks *is* supported. The change will take place the