Have transactions verify/assert safe usage

Have the transaction code verify that there is at most one active
child at the time and no one is using the parent while child is
active.  This is how the code is intended to be used and also the
code almost certainly does not work otherwise.

The new code does not cover commiting/rolling back a parent before a
child but that is already covered by the existing code (it will
trigger when child transaction is rolled back/committed or when
leaving the contextmanager from start_transaction).

This would have caught 7d758760d1
immediately with an assertion error.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/transaction.py

@@ -7,8 +7,16 @@ class MigrationTransactionState(object):
         self._is_rolled_back = False
         self._is_committed = False
         self._undo_items = []
+        self._pending_child = False
+        if self.parent_transaction:
+            # Transactions can only support one child transaction at a time
+            assert not self.parent_transaction._pending_child
+            self.parent_transaction._pending_child = True
 
     def add_undo_item(self, undo, updated_binaries):
+        # We do not accept any changes to this transaction while it has a child transaction
+        # (the undo code does not handle that case correctly)
+        assert not self._pending_child
         self._assert_open_transaction()
         self._undo_items.append((undo, updated_binaries))
 
@@ -33,6 +41,7 @@ class MigrationTransactionState(object):
         self._assert_open_transaction()
         self._is_committed = True
         if self.parent_transaction:
+            self.parent_transaction._pending_child = False
             for undo_item in self._undo_items:
                 self.parent_transaction.add_undo_item(*undo_item)
 
@@ -43,7 +52,6 @@ class MigrationTransactionState(object):
         """
 
         self._assert_open_transaction()
-
         self._is_rolled_back = True
         lundo = self._undo_items
         lundo.reverse()
@@ -108,6 +116,9 @@ class MigrationTransactionState(object):
                 else:
                     provides_t[arch][provided_pkg] = undo['virtual'][p]
 
+        if self.parent_transaction:
+            self.parent_transaction._pending_child = False
+
     @property
     def is_rolled_back(self):
         return self._is_rolled_back