monitor-config-store: Make parsing a bit more forgiving

Allow unknown XML elements inside <monitors>. This makes extending in
the future easier.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2030>

src/backends/meta-monitor-config-store.c

@@ -136,6 +136,7 @@ G_DEFINE_QUARK (meta-monitor-config-store-error-quark,
 typedef enum
 {
   STATE_INITIAL,
+  STATE_UNKNOWN,
   STATE_MONITORS,
   STATE_CONFIGURATION,
   STATE_MIGRATED,
@@ -180,12 +181,28 @@ typedef struct
   MetaLogicalMonitorConfig *current_logical_monitor_config;
   GList *current_disabled_monitor_specs;
 
+  ParserState unknown_state_root;
+  int unknown_level;
+
   MetaMonitorsConfigFlag extra_config_flags;
 } ConfigParser;
 
 G_DEFINE_TYPE (MetaMonitorConfigStore, meta_monitor_config_store,
                G_TYPE_OBJECT)
 
+static void
+enter_unknown_element (ConfigParser *parser,
+                       const char   *element_name,
+                       const char   *root_element_name,
+                       ParserState   root_state)
+{
+  parser->state = STATE_UNKNOWN;
+  parser->unknown_level = 1;
+  parser->unknown_state_root = root_state;
+  g_warning ("Unknown element <%s> under <%s>, ignoring",
+             element_name, root_element_name);
+}
+
 static void
 handle_start_element (GMarkupParseContext  *context,
                       const char           *element_name,
@@ -242,8 +259,8 @@ handle_start_element (GMarkupParseContext  *context,
       {
         if (!g_str_equal (element_name, "configuration"))
           {
-            g_set_error (error, G_MARKUP_ERROR, G_MARKUP_ERROR_UNKNOWN_ELEMENT,
+            enter_unknown_element (parser, element_name,
-                         "Invalid toplevel element '%s'", element_name);
+                                   "monitors", STATE_MONITORS);
             return;
           }
 
@@ -253,6 +270,13 @@ handle_start_element (GMarkupParseContext  *context,
         return;
       }
 
+    case STATE_UNKNOWN:
+      {
+        parser->unknown_level++;
+
+        return;
+      }
+
     case STATE_CONFIGURATION:
       {
         if (g_str_equal (element_name, "logicalmonitor"))
@@ -274,9 +298,8 @@ handle_start_element (GMarkupParseContext  *context,
           }
         else
           {
-            g_set_error (error, G_MARKUP_ERROR, G_MARKUP_ERROR_UNKNOWN_ELEMENT,
+            enter_unknown_element (parser, element_name,
-                         "Invalid configuration element '%s'", element_name);
+                                   "configuration", STATE_CONFIGURATION);
-            return;
           }
 
         return;
@@ -323,9 +346,8 @@ handle_start_element (GMarkupParseContext  *context,
           }
         else
           {
-            g_set_error (error, G_MARKUP_ERROR, G_MARKUP_ERROR_UNKNOWN_ELEMENT,
+            enter_unknown_element (parser, element_name,
-                         "Invalid monitor logicalmonitor element '%s'", element_name);
+                                   "logicalmonitor", STATE_LOGICAL_MONITOR);
-            return;
           }
 
         return;
@@ -793,6 +815,18 @@ handle_end_element (GMarkupParseContext  *context,
         return;
       }
 
+    case STATE_UNKNOWN:
+      {
+        parser->unknown_level--;
+        if (parser->unknown_level == 0)
+          {
+            g_assert (parser->unknown_state_root >= 0);
+            parser->state = parser->unknown_state_root;
+            parser->unknown_state_root = -1;
+          }
+        return;
+      }
+
     case STATE_MONITORS:
       {
         g_assert (g_str_equal (element_name, "monitors"));
@@ -912,6 +946,9 @@ handle_text (GMarkupParseContext *context,
 
   switch (parser->state)
     {
+    case STATE_UNKNOWN:
+      return;
+
     case STATE_INITIAL:
     case STATE_MONITORS:
     case STATE_CONFIGURATION:
@@ -1099,6 +1136,7 @@ read_config_file (MetaMonitorConfigStore  *config_store,
     .state = STATE_INITIAL,
     .config_store = config_store,
     .extra_config_flags = extra_config_flags,
+    .unknown_state_root = -1,
   };
 
   parse_context = g_markup_parse_context_new (&config_parser,

src/tests/monitor-configs/unknown-elements.xml

@@ -0,0 +1,31 @@
+<monitors version="2">
+  <unknownundermonitors>
+    <anotherlevel>text</anotherlevel>
+  </unknownundermonitors>
+  <configuration>
+    <unknownunderconfiguration>
+      <anotherlevel>text</anotherlevel>
+    </unknownunderconfiguration>
+    <logicalmonitor>
+      <unknownunderlogicalmonitor>
+	<anotherlevel>text</anotherlevel>
+      </unknownunderlogicalmonitor>
+      <x>0</x>
+      <y>0</y>
+      <primary>yes</primary>
+      <monitor>
+	<monitorspec>
+	  <connector>DP-1</connector>
+	  <vendor>MetaProduct&apos;s Inc.</vendor>
+	  <product>MetaMonitor</product>
+	  <serial>0x123456</serial>
+	</monitorspec>
+	<mode>
+	  <width>1920</width>
+	  <height>1080</height>
+	  <rate>60.000495910644531</rate>
+	</mode>
+      </monitor>
+    </logicalmonitor>
+  </configuration>
+</monitors>

src/tests/monitor-store-unit-tests.c

@@ -836,6 +836,60 @@ meta_test_monitor_store_interlaced (void)
   check_monitor_store_configurations (&expect);
 }
 
+static void
+meta_test_monitor_store_unknown_elements (void)
+{
+  MonitorStoreTestExpect expect = {
+    .configurations = {
+      {
+        .logical_monitors = {
+          {
+            .layout = {
+              .x = 0,
+              .y = 0,
+              .width = 1920,
+              .height = 1080
+            },
+            .scale = 1,
+            .is_primary = TRUE,
+            .is_presentation = FALSE,
+            .monitors = {
+              {
+                .connector = "DP-1",
+                .vendor = "MetaProduct's Inc.",
+                .product = "MetaMonitor",
+                .serial = "0x123456",
+                .mode = {
+                  .width = 1920,
+                  .height = 1080,
+                  .refresh_rate = 60.000495910644531
+                }
+              }
+            },
+            .n_monitors = 1,
+          }
+        },
+        .n_logical_monitors = 1
+      }
+    },
+    .n_configurations = 1
+  };
+
+  g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_WARNING,
+                         "Unknown element <unknownundermonitors> "
+                         "under <monitors>, ignoring");
+  g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_WARNING,
+                         "Unknown element <unknownunderconfiguration> "
+                         "under <configuration>, ignoring");
+  g_test_expect_message (G_LOG_DOMAIN, G_LOG_LEVEL_WARNING,
+                         "Unknown element <unknownunderlogicalmonitor> "
+                         "under <logicalmonitor>, ignoring");
+  set_custom_monitor_config ("unknown-elements.xml");
+  g_test_assert_expected_messages ();
+
+  check_monitor_store_configurations (&expect);
+}
+
 void
 init_monitor_store_tests (void)
 {
@@ -861,4 +915,6 @@ init_monitor_store_tests (void)
                    meta_test_monitor_store_second_rotated);
   g_test_add_func ("/backends/monitor-store/interlaced",
                    meta_test_monitor_store_interlaced);
+  g_test_add_func ("/backends/monitor-store/unknown-elements",
+                   meta_test_monitor_store_unknown_elements);
 }