Function for claiming multiple resources at once.

Like claim_for_update, the claim_many_for_update claims resources with
an expiring lock. This was written to allow the updating of multiple
application roles with a single cloud_id, since multiple application
roles will map to a single Azure Active Directory user.

atst/models/utils.py

@@ -1,3 +1,5 @@
+from typing import List
+
 from sqlalchemy import func, sql, Interval, and_, or_
 from contextlib import contextmanager
 
@@ -28,7 +30,7 @@ def claim_for_update(resource, minutes=30):
         .filter(
             and_(
                 Model.id == resource.id,
-                or_(Model.claimed_until == None, Model.claimed_until <= func.now()),
+                or_(Model.claimed_until.is_(None), Model.claimed_until <= func.now()),
             )
         )
         .update({"claimed_until": claim_until}, synchronize_session="fetch")
@@ -48,3 +50,51 @@ def claim_for_update(resource, minutes=30):
             Model.claimed_until != None
         ).update({"claimed_until": None}, synchronize_session="fetch")
         db.session.commit()
+
+
+@contextmanager
+def claim_many_for_update(resources: List, minutes=30):
+    """
+    Claim a mutually exclusive expiring hold on a group of resources.
+    Uses the database as a central source of time in case the server clocks have drifted.
+
+    Args:
+        resources:   A list of SQLAlchemy model instances with a `claimed_until` attribute.
+        minutes:    The maximum amount of time, in minutes, to hold the claim.
+    """
+    Model = resources[0].__class__
+
+    claim_until = func.now() + func.cast(
+        sql.functions.concat(minutes, " MINUTES"), Interval
+    )
+
+    ids = tuple(r.id for r in resources)
+
+    # Optimistically query for and update the resources in question. If they're
+    # already claimed, `rows_updated` will be 0 and we can give up.
+    rows_updated = (
+        db.session.query(Model)
+        .filter(
+            and_(
+                Model.id.in_(ids),
+                or_(Model.claimed_until.is_(None), Model.claimed_until <= func.now()),
+            )
+        )
+        .update({"claimed_until": claim_until}, synchronize_session="fetch")
+    )
+    if rows_updated < 1:
+        # TODO: Generalize this exception class so it can take multiple resources
+        raise ClaimFailedException(resources[0])
+
+    # Fetch the claimed resources
+    claimed = db.session.query(Model).filter(Model.id.in_(ids)).all()
+
+    try:
+        # Give the resource to the caller.
+        yield claimed
+    finally:
+        # Release the claim.
+        db.session.query(Model).filter(Model.id.in_(ids)).filter(
+            Model.claimed_until != None
+        ).update({"claimed_until": None}, synchronize_session="fetch")
+        db.session.commit()