css: Don't throw warnings on broken URL in image css

If a URL can't be loaded, we might end up with a NULL file. Handle that
case properly by creating an invalid image instead and don't crash or
complain to stderr when files are NULL.

This was broken since 0886ade182

A new reftest has been included. We need a reftest instead of a
CSS parser test, because the error only becomes visible when
compute()ing the actual image.

Fixes #4373


(cherry picked from commit ac8c4245b2)

gtk/gtkcssimageurl.c

@@ -42,6 +42,12 @@ gtk_css_image_url_load_image (GtkCssImageUrl  *url,
   if (url->loaded_image)
     return url->loaded_image;
 
+  if (url->file == NULL)
+    {
+      url->loaded_image = gtk_css_image_invalid_new ();
+      return url->loaded_image;
+    }
+
   /* We special case resources here so we can use gdk_texture_new_from_resource. */
   if (g_file_has_uri_scheme (url->file, "resource"))
     {
@@ -148,6 +154,10 @@ gtk_css_image_url_equal (GtkCssImage *image1,
 {
   GtkCssImageUrl *url1 = GTK_CSS_IMAGE_URL (image1);
   GtkCssImageUrl *url2 = GTK_CSS_IMAGE_URL (image2);
+  
+  /* FIXME: We don't save data: urls, so we can't compare them here */
+  if (url1->file == NULL || url2->file == NULL)
+    return FALSE;
 
   return g_file_equal (url1->file, url2->file);
 }

testsuite/reftests/css-image-broken-url.css

@@ -0,0 +1,4 @@
+#test {
+  /* The url should fail as early as possible */
+  background-image: url('data::::');
+}

testsuite/reftests/css-image-broken-url.ref.ui

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkWindow" id="window1">
+    <property name="width_request">10</property>
+    <property name="height_request">10</property>
+    <property name="decorated">0</property>
+  </object>
+</interface>

testsuite/reftests/css-image-broken-url.ui

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkWindow" id="window1">
+    <property name="width_request">10</property>
+    <property name="height_request">10</property>
+    <property name="decorated">0</property>
+    <property name="name">test</property>
+  </object>
+</interface>

testsuite/reftests/meson.build

@@ -191,6 +191,9 @@ testdata = [
   'css-image-aspect-ratio.css',
   'css-image-aspect-ratio.ui',
   'css-image-aspect-ratio.ref.ui',
+  'css-image-broken-url.css',
+  'css-image-broken-url.ref.ui',
+  'css-image-broken-url.ui',
   'css-image-color-aspect-ratio.css',
   'css-image-color-aspect-ratio.ui',
   'css-image-color-aspect-ratio.ref.ui',