Check if existing database files are writable for us

src/common/database.c

@@ -4185,6 +4185,32 @@ dt_database_t *dt_database_init(const char *alternative, const gboolean load_dat
   else
     snprintf(dbfilename_data, sizeof(dbfilename_data), ":memory:");
 
+  // It may happen that we will not have write access to the database restored
+  // from a backup or snapshot. Running darktable with a database that cannot
+  // be written to may result in incorrect operation, the cause of which will
+  // be difficult to diagnose. Let's check if we can continue.
+  char *readonly_db_text = g_markup_printf_escaped(
+    _("you do not have write access to at least one of the darktable databases:\n"
+      "\n"
+      "<span style='italic'>%s</span>\n"
+      "<span style='italic'>%s</span>\n"
+      "\n"
+      "please fix this and then run darktable again"),
+    dbfilename_library,
+    dbfilename_data);
+  if((access(dbfilename_library, F_OK) == 0 && access(dbfilename_library, W_OK) != 0)
+     || (access(dbfilename_data, F_OK) == 0 && access(dbfilename_data, W_OK) != 0))
+  {
+    dt_print(DT_DEBUG_ALWAYS, "at least one of the dt databases (%s, %s) is not writeable",
+                               dbfilename_library, dbfilename_data);
+    if(has_gui)
+      dt_gui_show_standalone_yes_no_dialog(_("darktable - read-only database detected"),
+                                           readonly_db_text,
+                                           _("_quit darktable"),
+                                           NULL);
+    exit(1);
+  }
+
   /* create database */
   dt_database_t *db = g_malloc0(sizeof(dt_database_t));
   db->dbfilename_data = g_strdup(dbfilename_data);