storage: Forbid subtransactions.

...when connection is used as a context manager (which is the expected
coding style anyway).
If silently ignored, rollback may be incomplete if a subtransaction already
committed.
And if commit only happen at outmost transaction, no-undo changes could
come undone if outer transaction aborts.

This is currently observed in the code, so no other change is needed.

caucase/storage.py

@@ -31,6 +31,19 @@ __all__ = ('SQLite3Storage', )
 
 DAY_IN_SECONDS = 60 * 60 * 24
 
+class NoReentryConnection(sqlite3.Connection):
+  __entered = False
+
+  def __enter__(self):
+    if self.__entered:
+      raise Exception('Subtransactions are not supported')
+    self.__entered = True
+    return super(NoReentryConnection, self).__enter__()
+
+  def __exit__(self, exc_type, exc_value, traceback):
+    self.__entered = False
+    return super(NoReentryConnection, self).__exit__(exc_type, exc_value, traceback)
+
 class SQLite3Storage(local):
   """
   CA data storage.
@@ -80,7 +93,7 @@ class SQLite3Storage(local):
       os.O_CREAT | os.O_RDONLY,
       mode,
     ))
-    self._db = db = sqlite3.connect(db_path)
+    self._db = db = sqlite3.connect(db_path, factory=NoReentryConnection)
     self._table_prefix = table_prefix
     db.row_factory = sqlite3.Row
     self._max_csr_amount = max_csr_amount