Add interface to add a constraint to context

This adds methods to the Context to add, remove, and get a
constraint on an object to be checked before a change is
made to that object.

Related-Bug: #1493714
Change-Id: Id01c4dfb740f680e96349dc08b11b5bca6c59a74

neutron_lib/context.py

@@ -12,6 +12,7 @@
 
 """Context: context for security/db session."""
 
+import collections
 import copy
 import datetime
 
@@ -141,10 +142,15 @@ class ContextBaseWithSession(ContextBase):
     pass
 
 
+_TransactionConstraint = collections.namedtuple(
+    'TransactionConstraint', ['if_revision_match', 'resource', 'resource_id'])
+
+
 class Context(ContextBaseWithSession):
     def __init__(self, *args, **kwargs):
         super(Context, self).__init__(*args, **kwargs)
         self._session = None
+        self._txn_constraint = None
 
     @property
     def session(self):
@@ -157,6 +163,26 @@ class Context(ContextBaseWithSession):
 
         return self._session
 
+    def set_transaction_constraint(self, resource, resource_id, rev_number):
+        """Set a revision constraint to enfore before resource_id is changed.
+
+        :param resource: collection name of resource (e.g. ports or networks)
+        :param resource_id: the primary key ID of the individiual resource that
+                            should have its revision number matched before
+                            allowing the transaction to proceed.
+        :param rev_number: the revision_number that the resource should be at.
+        """
+
+        self._txn_constraint = _TransactionConstraint(
+            if_revision_match=rev_number, resource=resource,
+            resource_id=resource_id)
+
+    def clear_transaction_constraint(self):
+        self._txn_constraint = None
+
+    def get_transaction_constraint(self):
+        return self._txn_constraint
+
 
 def get_admin_context():
     return Context(user_id=None,

neutron_lib/tests/unit/test_context.py

@@ -213,3 +213,14 @@ class TestNeutronContext(_base.BaseTestCase):
         session1 = ctx.session
         session2 = ctx.session
         self.assertIs(session1, session2)
+
+    def test_add_get_remove_constraint(self):
+        ctx = context.Context('user_id', 'tenant_id')
+        self.assertIsNone(ctx.get_transaction_constraint())
+        ctx.set_transaction_constraint('networks', 'net_id', 44)
+        constraint = ctx.get_transaction_constraint()
+        self.assertEqual(44, constraint.if_revision_match)
+        self.assertEqual('networks', constraint.resource)
+        self.assertEqual('net_id', constraint.resource_id)
+        ctx.clear_transaction_constraint()
+        self.assertIsNone(ctx.get_transaction_constraint())

releasenotes/notes/transaction_constraint-d3f93c2ced4a74c6.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Contexts may now have transaction constraints set on them to be enforced
+    by the revision plugin in Neutron for generalized compare-and-swap updates.
+    Calling ``set_transaction_constraint`` on the context before performing a
+    resource mutation will setup the constraint. This is also exposed to users
+    via the HTTP API with ``if-match`` headers and the API layer sets the
+    constraint on the context.