libgdm: Remove duplicate sessions once, after all sessions have been processed

We add sessions to a hash table keyed on the basename without extension,
and while we're adding them we de-duplicate based on the translated name
(the text that will be visible in the switcher).

This has a problem. In this situation:

```
laney@disco:~$ tree /usr/share/{wayland-,x}sessions/
/usr/share/wayland-sessions/
├── gnome.desktop
└── ubuntu-wayland.desktop
/usr/share/xsessions/
├── gnome.desktop -> gnome-xorg.desktop
├── gnome-xorg.desktop
└── ubuntu.desktop
```

We process the X sessions first, and then the Wayland sessions. The
deduplication might end up removing `xsessions/gnome-xorg` and leaving
`xsessions/gnome`. Then when we come to process the Wayland sessions, we
will clobber `xsessions/gnome` with `wayland-sessions/gnome`, as they
have the same ID.

When everything is working, it is actually *intentional* that
`xsessions/gnome` gets clobbered, so that you end up seeing "GNOME"
(wayland) and "GNOME on Xorg" in the switcher, and not (e.g.) "GNOME on
Xorg" twice, and you have the correct fallback behaviour in case you ever
enable/disable Wayland.

Instead of filtering while we add things, we can add all the sessions we
find (clobbering duplicate IDs as before), and then process the list
once at the end, removing sessions with duplicated visible names at that
point.

Closes: #473

1 files changed, 31 insertions, 16 deletions

diff --git a/libgdm/gdm-sessions.c b/libgdm/gdm-sessions.c
index 46713db5..9001c47b 100644
--- a/libgdm/gdm-sessions.c
+++ b/libgdm/gdm-sessions.c
@@ -111,14 +111,6 @@ key_file_is_relevant (GKeyFile     *key_file)
         return TRUE;
 }
 
-static gboolean
-find_session_with_same_name (const char     *id,
-                             GdmSessionFile *session,
-                             const char     *translated_name)
-{
-        return g_strcmp0 (session->translated_name, translated_name) == 0;
-}
-
 static void
 load_session_file (const char              *id,
                    const char              *path)
@@ -126,7 +118,7 @@ load_session_file (const char              *id,
         GKeyFile          *key_file;
         GError            *error;
         gboolean           res;
-        GdmSessionFile    *session, *session_with_same_name;
+        GdmSessionFile    *session;
 
         key_file = g_key_file_new ();
 
@@ -162,13 +154,6 @@ load_session_file (const char              *id,
         session->translated_name = g_key_file_get_locale_string (key_file, G_KEY_FILE_DESKTOP_GROUP, "Name", NULL, NULL);
         session->translated_comment = g_key_file_get_locale_string (key_file, G_KEY_FILE_DESKTOP_GROUP, "Comment", NULL, NULL);
 
-        session_with_same_name = g_hash_table_find (gdm_available_sessions_map,
-                                                    (GHRFunc) find_session_with_same_name,
-                                                    session->translated_name);
-
-        if (session_with_same_name != NULL)
-                g_hash_table_remove (gdm_available_sessions_map, session_with_same_name->id);
-
         g_hash_table_insert (gdm_available_sessions_map,
                              g_strdup (id),
                              session);
@@ -176,6 +161,29 @@ load_session_file (const char              *id,
         g_key_file_free (key_file);
 }
 
+static gboolean
+remove_duplicate_sessions (gpointer key,
+                           gpointer value,
+                           gpointer user_data)
+{
+        gboolean already_known;
+        const char *id;
+        GHashTable *names_seen_before;
+        GdmSessionFile *session;
+
+        id = (const char *) key;
+        names_seen_before = (GHashTable *) user_data;
+        session = (GdmSessionFile *) value;
+        already_known = !g_hash_table_add (names_seen_before, session->translated_name);
+
+        if (already_known)
+                g_debug ("GdmSession: Removing %s (%s) as we already have a session by this name",
+                         session->id,
+                         session->path);
+
+        return already_known;
+}
+
 static void
 collect_sessions_from_directory (const char *dirname)
 {
@@ -230,6 +238,7 @@ collect_sessions_from_directory (const char *dirname)
 static void
 collect_sessions (void)
 {
+        g_autoptr(GHashTable) names_seen_before = NULL;
         GArray     *xorg_search_array = NULL;
         GArray     *wayland_search_array = NULL;
         gchar      *session_dir = NULL;
@@ -241,6 +250,8 @@ collect_sessions (void)
                 DATADIR "/xsessions/",
         };
 
+        names_seen_before = g_hash_table_new (g_str_hash, g_str_equal);
+
         xorg_search_array = g_array_new (TRUE, TRUE, sizeof (char *));
 
         const gchar * const *system_data_dirs = g_get_system_data_dirs ();
@@ -291,6 +302,10 @@ collect_sessions (void)
         }
 
         g_array_free (wayland_search_array, TRUE);
+
+        g_hash_table_foreach_remove (gdm_available_sessions_map,
+                                     remove_duplicate_sessions,
+                                     names_seen_before);
 #endif
 }