ui: allow to delete old events of the in-memory db

Not deleting events from in-memory db can lead to a high mem usage under
certain scenarios.

Previous attempt to solve this issue wrote events to disk in a temporal
file (when using file::memory:?cache=shared).

Related issues: #844 #857

Closes: #1030

ui/opensnitch/database/__init__.py

@@ -7,7 +7,7 @@
 class Database:
     db = None
     __instance = None
-    DB_IN_MEMORY   = ":memory:"
+    DB_IN_MEMORY   = "file::memory:"
     DB_TYPE_MEMORY = 0
     DB_TYPE_FILE   = 1
     DB_JRNL_WAL    = False
@@ -50,6 +50,8 @@ def initialize(self, dbtype=DB_TYPE_MEMORY, dbfile=DB_IN_MEMORY, dbjrnl_wal=DB_J
 
         self.db = QSqlDatabase.addDatabase("QSQLITE", self.db_name)
         self.db.setDatabaseName(self.db_file)
+        if dbtype == Database.DB_TYPE_MEMORY:
+            self.db.setConnectOptions("QSQLITE_OPEN_URI;QSQLITE_ENABLE_SHARED_CACHE")
         if not self.db.open():
             print("\n ** Error opening DB: SQLite driver not loaded. DB name: %s\n" % self.db_file)
             print("\n    Available drivers: ", QSqlDatabase.drivers())
@@ -103,6 +105,7 @@ def get_db_name(self):
         return self.db_name
 
     def _create_tables(self):
+        # https://www.sqlite.org/wal.html
         if self.db_file == Database.DB_IN_MEMORY:
             self.set_schema_version(self.DB_VERSION)
             # Disable journal (default)

ui/opensnitch/dialogs/preferences.py

@@ -73,6 +73,7 @@ def __init__(self, parent=None, appicon=None):
         self.setupUi(self)
         self.setWindowIcon(appicon)
 
+        self.checkDBMaxDays.setEnabled(True)
         self.dbFileButton.setVisible(False)
         self.dbLabel.setVisible(False)
         self.dbType = None
@@ -773,6 +774,7 @@ def _reset_status_message(self):
         self._hide_status_label()
 
     def _enable_db_cleaner_options(self, enable, db_max_days):
+        self.checkDBMaxDays.setChecked(enable)
         self.spinDBMaxDays.setEnabled(enable)
         self.spinDBPurgeInterval.setEnabled(enable)
         self.labelDBPurgeInterval.setEnabled(enable)
@@ -823,8 +825,7 @@ def _cb_db_type_changed(self):
         isDBMem = self.comboDBType.currentIndex() == Database.DB_TYPE_MEMORY
         self.dbFileButton.setVisible(not isDBMem)
         self.dbLabel.setVisible(not isDBMem)
-        self.checkDBMaxDays.setEnabled(not isDBMem)
-        self.checkDBMaxDays.setChecked(not isDBMem)
+        self.checkDBMaxDays.setChecked(self._cfg.getBool(Config.DEFAULT_DB_PURGE_OLDEST))
         self.checkDBJrnlWal.setEnabled(not isDBMem)
         self.checkDBJrnlWal.setChecked(False)