On demand resources, started via on-demand tags

On-demand tickets need to be prioritized over the normal tickets
(priority queue), otherwise we risk that normal tickets take "on demand"
resources.

If we want to prefer one "on demand" pool over the other "on demand"
(e.g. spot AWS over normal instances), we need to sort them through a
priority queue based on tag priority.  The fallback for spawn failures
(e.g. if SPOT instances don't start) isn't resolved yet, though.

Fixes: #24

Makefile

@@ -6,6 +6,7 @@ SHELLTEST_OPTIONS :=
 SHELL_TESTS := \
 	basic.sh \
 	check.sh \
+	ondemand.sh \
 	reuse.sh
 
 TEST_PYTHONS   := python3

resallocserver/logic.py

@@ -20,7 +20,7 @@
 
 from resalloc.helpers import RState, TState
 from resallocserver import models
-from sqlalchemy.orm import Query
+from sqlalchemy.orm import Query, joinedload
 from sqlalchemy import or_
 
 
@@ -69,10 +69,11 @@ def ready(self):
         """
         Get ready resources, those which were never assigned or are released.
         The sandbox-assigned resources are sorted above others - so they can be
-        re-used first.
+        re-used first.  The query is already ordered by ID ASC.
         """
         return (self.up().filter(models.Resource.ticket_id.is_(None))
-                         .filter(models.Resource.check_failed_count==0))
+                         .filter(models.Resource.check_failed_count==0)
+                         .order_by(models.Resource.id.asc()))
 
     def taken(self):
         """
@@ -156,9 +157,15 @@ def kill(self, res_id):
 class QTickets(QObject):
     query = Query(models.Ticket)
 
-    def waiting(self):
-        return self.query.filter_by(resource_id=None)\
-                         .filter_by(state=TState.OPEN)
+    def waiting(self, preload_tags=False):
+        query = (
+            self.query.filter_by(resource_id=None)
+            .filter_by(state=TState.OPEN)
+        )
+        if preload_tags:
+            query = query.options(joinedload("tags"))
+        return query
+
 
     def not_closed(self):
         return self.query.filter(models.Ticket.state != TState.CLOSED)