parse: fix redefinition of gateway(4|6)

Options gateway4 and gateway6 were not being properly redefined when a
merge happens. Redefinition was causing a crash due to an assert in the
set_str_if_null macro.

While this assertion helped identifying the merge issue, I believe it's
not the best place to keep it so I'm removing it from the macro and
making the macro do only what it name suggests it does.

This macro is used in other places but this problem seems to not happen
there as far as I can see.

Add a couple of unit test to check the merge is working.

src/parse.c

@@ -49,9 +49,7 @@
 #define vxlan_offset(field) GUINT_TO_POINTER(offsetof(NetplanVxlan, field))
 
 /* convenience macro to avoid strdup'ing a string into a field if it's already set. */
-#define set_str_if_null(dst, src) { if (dst) {\
-    g_assert_cmpstr(src, ==, dst); \
-} else { \
+#define set_str_if_null(dst, src) { if (dst == NULL) {\
     dst = g_strdup(src); \
 } }
 
@@ -1455,6 +1453,10 @@ handle_gateway4(NetplanParser* npp, yaml_node_t* node, __unused const void* _, G
 {
     if (!is_ip4_address(scalar(node)))
         return yaml_error(npp, node, error, "invalid IPv4 address '%s'", scalar(node));
+    if (npp->current.netdef->gateway4) {
+        g_free(npp->current.netdef->gateway4);
+        npp->current.netdef->gateway4 = NULL;
+    }
     set_str_if_null(npp->current.netdef->gateway4, scalar(node));
     mark_data_as_dirty(npp, &npp->current.netdef->gateway4);
     g_warning("`gateway4` has been deprecated, use default routes instead.\n"
@@ -1467,6 +1469,10 @@ handle_gateway6(NetplanParser* npp, yaml_node_t* node, __unused const void* _, G
 {
     if (!is_ip6_address(scalar(node)))
         return yaml_error(npp, node, error, "invalid IPv6 address '%s'", scalar(node));
+    if (npp->current.netdef->gateway6) {
+        g_free(npp->current.netdef->gateway6);
+        npp->current.netdef->gateway6 = NULL;
+    }
     set_str_if_null(npp->current.netdef->gateway6, scalar(node));
     mark_data_as_dirty(npp, &npp->current.netdef->gateway6);
     g_warning("`gateway6` has been deprecated, use default routes instead.\n"

tests/generator/test_common.py

@@ -1825,3 +1825,43 @@ def test_wifi_access_points_overwriting(self):
   psk="bbbbbbbb"
 }
 """)
+
+    def test_gateway6_redefinition_works(self):
+        self.generate('''network:
+  version: 2
+  ethernets:
+    engreen:
+      addresses: [2001:FFfe::1/62]
+      gateway6: 2001:FFfe::2''', confs={'b': '''network:
+  ethernets:
+    engreen:
+      gateway6: 2001:FFfe::34'''}, expect_fail=False)
+
+        self.assert_networkd({'engreen.network': '''[Match]
+Name=engreen
+
+[Network]
+LinkLocalAddressing=ipv6
+Address=2001:FFfe::1/62
+Gateway=2001:FFfe::34
+'''})
+
+    def test_gateway4_redefinition_works(self):
+        self.generate('''network:
+  version: 2
+  ethernets:
+    engreen:
+      addresses: [192.168.0.1/24]
+      gateway4: 192.168.0.123''', confs={'b': '''network:
+  ethernets:
+    engreen:
+      gateway4: 192.168.0.254'''}, expect_fail=False)
+
+        self.assert_networkd({'engreen.network': '''[Match]
+Name=engreen
+
+[Network]
+LinkLocalAddressing=ipv6
+Address=192.168.0.1/24
+Gateway=192.168.0.254
+'''})