core: keep empty groups from keyfile configs

When reading NetworkManager.conf and NetworkManager-intern.conf we might
need to know if a group is defined or not, even if it's empty. This is
the case, for example, for [global-dns]. If [global-dns] is defined in
NM.conf overwrites the config from NM-intern, and if it's defined in any
of them they overwrite the configs from connections.

Before this patch, defining it as an empty group was ignored:
```
[global-dns]
```

Instead, it was necessary to add at least one key-value to the group.
Otherwise the group was silently ignored.
```
[global-dns]
searches=
```

Keep empty groups so we can take better decissions about overwritting
configs from other sources.

(cherry picked from commit 4a46f454da)

src/core/nm-config.c

@@ -18,6 +18,7 @@
 #include "libnm-core-intern/nm-core-internal.h"
 #include "libnm-core-intern/nm-keyfile-internal.h"
 #include "libnm-core-intern/nm-keyfile-utils.h"
+#include "libnm-glib-aux/nm-keyfile-aux.h"
 
 #define DEFAULT_CONFIG_MAIN_FILE     NMCONFDIR "/NetworkManager.conf"
 #define DEFAULT_CONFIG_DIR           NMCONFDIR "/conf.d"
@@ -1046,6 +1047,10 @@ read_config(GKeyFile   *keyfile,
             /* internal groups cannot be set by user configuration. */
             continue;
         }
+
+        if (!g_key_file_has_group(keyfile, group))
+            nm_key_file_add_group(keyfile, group);
+
         keys = g_key_file_get_keys(kf, group, &nkeys, NULL);
         if (!keys)
             continue;
@@ -1639,6 +1644,12 @@ intern_config_read(const char        *filename,
                                  "");
         }
 
+        if (!g_key_file_has_group(keyfile_intern, group)) {
+            nm_key_file_add_group(keyfile_intern, group);
+            if (is_intern)
+                has_intern = TRUE;
+        }
+
         for (k = 0; keys[k]; k++) {
             gs_free char *value_set = NULL;
             const char   *key       = keys[k];
@@ -1823,6 +1834,9 @@ intern_config_write(const char        *filename,
             }
         }
 
+        if (!g_key_file_has_group(keyfile, group))
+            nm_key_file_add_group(keyfile, group);
+
         for (k = 0; keys[k]; k++) {
             const char   *key       = keys[k];
             gs_free char *value_set = NULL;

src/core/tests/config/global-dns-empty.conf

@@ -0,0 +1,3 @@
+# Good configuration, an empty global-dns section must be valid
+
+[global-dns]

src/core/tests/config/global-dns-not-set.conf

@@ -0,0 +1,5 @@
+# Good configuration, an empty [global-dns] must be implicitly assumed because a domain is defined
+
+[global-dns-domain-*]
+servers=4.5.6.7
+options=myoption1

src/core/tests/config/test-config.c

@@ -387,7 +387,27 @@ test_config_global_dns(void)
     g_assert(dns);
     g_object_unref(config);
 
-    /* Check that a file with a domain domain, but without a default one gives a NULL configuration */
+    /* Check that a file with an empty global-dns section gives a good configuration.
+     * Check also that searches and options are not NULL, as this is how we expose to
+     * D-Bus that global-dns is defined. */
+    config =
+        setup_config(NULL, TEST_DIR "/global-dns-empty.conf", "", NULL, "/no/such/dir", "", NULL);
+    dns = nm_config_data_get_global_dns_config(nm_config_get_data_orig(config));
+    g_assert(dns);
+    g_assert(nm_global_dns_config_get_searches(dns));
+    g_assert(nm_global_dns_config_get_options(dns));
+    g_object_unref(config);
+
+    /* Check that a file with a domain, but no global-dns, assumes an implicit empty global-dns */
+    config =
+        setup_config(NULL, TEST_DIR "/global-dns-not-set.conf", "", NULL, "/no/such/dir", "", NULL);
+    dns = nm_config_data_get_global_dns_config(nm_config_get_data_orig(config));
+    g_assert(dns);
+    g_assert(nm_global_dns_config_get_searches(dns));
+    g_assert(nm_global_dns_config_get_options(dns));
+    g_object_unref(config);
+
+    /* Check that a file with a domain, but without a default one, gives a NULL configuration */
     config =
         setup_config(NULL, TEST_DIR "/global-dns-invalid.conf", "", NULL, "/no/such/dir", "", NULL);
     dns = nm_config_data_get_global_dns_config(nm_config_get_data_orig(config));