Clean up workaround for sqlite opening errors

Now that we know the real cause, we can be a bit stricter with our workaround.

qutebrowser/misc/sql.py

@@ -89,12 +89,13 @@ def raise_sqlite_error(msg, error):
     """Raise either a SqlBugError or SqlEnvironmentError."""
     error_code = error.nativeErrorCode()
     database_text = error.databaseText()
+    driver_text = error.driverText()
 
     log.sql.debug("SQL error:")
     log.sql.debug("type: {}".format(
         debug.qenum_key(QSqlError, error.type())))
     log.sql.debug("database text: {}".format(database_text))
-    log.sql.debug("driver text: {}".format(error.driverText()))
+    log.sql.debug("driver text: {}".format(driver_text))
     log.sql.debug("error code: {}".format(error_code))
 
     environmental_errors = [
@@ -105,18 +106,15 @@ def raise_sqlite_error(msg, error):
         SqliteErrorCode.FULL,
         SqliteErrorCode.CANTOPEN,
     ]
-    # At least in init(), we can get errors like this:
-    # > type: ConnectionError
-    # > database text: out of memory
-    # > driver text: Error opening database
-    # > error code: -1
-    environmental_strings = [
-        "out of memory",
-    ]
 
-    if (error_code in environmental_errors or
-            (error_code == SqliteErrorCode.UNKNOWN and
-             database_text in environmental_strings)):
+    # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-70506
+    # We don't know what the actual error was, but let's assume it's not us to
+    # blame... Usually this is something like an unreadable database file.
+    qtbug_70506 = (error_code == SqliteErrorCode.UNKNOWN and
+                   driver_text == "Error opening database" and
+                   database_text == "out of memory")
+
+    if error_code in environmental_errors or qtbug_70506:
         raise SqlEnvironmentError(msg, error)
     else:
         raise SqlBugError(msg, error)

tests/unit/misc/test_sql.py

@@ -49,9 +49,13 @@ class TestSqlError:
         with pytest.raises(exception):
             sql.raise_sqlite_error("Message", sql_err)
 
-    def test_out_of_memory(self):
-        """On out of memory error, we get an error code '-1' from Qt."""
-        sql_err = QSqlError("driver text",
+    def test_qtbug_70506(self):
+        """Test Qt's wrong handling of errors while opening the database.
+
+        Due to https://bugreports.qt.io/browse/QTBUG-70506 we get an error with
+        "out of memory" as string and -1 as error code.
+        """
+        sql_err = QSqlError("Error opening database",
                             "out of memory",
                             QSqlError.UnknownError,
                             sql.SqliteErrorCode.UNKNOWN)