Prevent multiple open sessions existing at once

The trigger used to prevent multiple open sessions from being created
was not reliable.

This commit removes it, and replaces it with a unique constraint on
session endtime, with NULLS NOT DISTINCT so only one row with a null
endtime can exist at any time.

Closes #309 on github.

Author: sde1000

quicktill/models.py

@@ -409,6 +409,11 @@ class Session(Base, Logged):
     date = Column('sessiondate', Date, nullable=False)
     accinfo = Column(String(), nullable=True, doc="Accounting system info")
 
+    __table_args__ = (
+        UniqueConstraint("endtime", postgresql_nulls_not_distinct=True,
+                         name="max_one_open_session"),
+    )
+
     tillweb_viewname = "tillweb-session"
     tillweb_argname = "sessionid"
     tillweb_index_description = "Sessions"
@@ -635,25 +640,6 @@ def previous(self):
         return self._prevsession
 
 
-add_ddl(Session.__table__, """
-CREATE OR REPLACE FUNCTION check_max_one_session_open() RETURNS trigger AS $$
-BEGIN
-  IF (SELECT count(*) FROM sessions WHERE endtime IS NULL)>1 THEN
-    RAISE EXCEPTION 'there is already an open session'
-          USING ERRCODE = 'integrity_constraint_violation';
-  END IF;
-  RETURN NULL;
-END;
-$$ LANGUAGE plpgsql;
-CREATE CONSTRAINT TRIGGER max_one_session_open
-  AFTER INSERT OR UPDATE ON sessions
-  FOR EACH ROW EXECUTE PROCEDURE check_max_one_session_open();
-""", """
-DROP TRIGGER max_one_session_open ON sessions;
-DROP FUNCTION check_max_one_session_open();
-""")
-
-
 class SessionMeta(Base):
     """Metadata on a session
 

quicktill/session.py

@@ -4,6 +4,7 @@
 from . import payment
 from . import config
 from .models import PayType, Session, SessionTotal, Transaction, zero
+import sqlalchemy.exc
 from sqlalchemy.orm import undefer
 from sqlalchemy.sql import select, func, desc
 from .plugins import InstancePluginMount
@@ -89,8 +90,14 @@ def key_enter(self):
             return
         self.dismiss()
         sc = Session(date=date)
-        td.s.add(sc)
-        td.s.flush()
+        try:
+            td.s.add(sc)
+            td.s.commit()
+        except sqlalchemy.exc.IntegrityError:
+            td.s.rollback()
+            ui.infopopup(["A new session has already been started."],
+                         title="Error")
+            return
         deferred = trans_restore()
         td.foodorder_reset()
         log.info("Started session number %d", sc.id)

release-notes.md

@@ -18,6 +18,13 @@ BEGIN;
 ALTER TABLE sessions
 	ALTER COLUMN starttime SET DEFAULT CURRENT_TIMESTAMP;
 
+DROP TRIGGER max_one_session_open ON sessions;
+
+DROP FUNCTION check_max_one_session_open();
+
+ALTER TABLE sessions
+        ADD CONSTRAINT max_one_open_session UNIQUE NULLS NOT DISTINCT (endtime);
+
 COMMIT;
 ```
 

setup.cfg

@@ -17,7 +17,7 @@ install_requires =
     psycopg2 ~= 2.9
     reportlab
     httplib2
-    sqlalchemy >= 2.0
+    sqlalchemy >= 2.0.16
     qrcode
     requests
     requests-oauthlib