React immediately when a user moves to a new terminal

Use database notifications to close a user's session immediately when
they move to a different terminal.

quicktill/models.py

@@ -863,6 +863,31 @@ def logtext(self):
         return self.fullname
 
 
+# When the 'transid' or 'register' column of a user is changed, send
+# notification 'user_register' with the user ID as payload. This
+# enables registers to detect the user arriving at a different
+# register, or the user's transaction being taken over by another
+# user.
+add_ddl(User.__table__, """
+CREATE OR REPLACE FUNCTION notify_user_change() RETURNS trigger AS $$
+DECLARE
+BEGIN
+  IF NEW.transid IS DISTINCT FROM OLD.transid
+    OR NEW.register IS DISTINCT FROM OLD.register THEN
+    PERFORM pg_notify('user_register', CAST(NEW.id AS text));
+  END IF;
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+CREATE TRIGGER user_changed
+  AFTER UPDATE ON users
+  FOR EACH ROW EXECUTE PROCEDURE notify_user_change();
+""", """
+DROP TRIGGER user_changed ON users;
+DROP FUNCTION notify_user_change();
+""")
+
+
 class UserToken(Base, Logged):
     """A token used by a till user to identify themselves
 

quicktill/register.py

@@ -37,6 +37,7 @@
 from . import user
 from . import plugins
 from . import config
+from . import listen
 import logging
 import datetime
 from .models import Transline, Transaction, Session, StockOut, penny
@@ -823,6 +824,8 @@ def __init__(self, user, hotkeys, autolock=None, timeout=300):
                 self.user.dbuser.message = None
         td.s.flush()
         self._redraw()
+        self._notify_user_register_listener = listen.listener.listen_for(
+            "user_register", self._notify_user_register)
         self.hook("new_page")
         self._resume_pending_payment()
 
@@ -3178,6 +3181,7 @@ def entry(self):
             ui.infopopup([self.user.dbuser.message],
                          title="Transaction information")
             self._clear()
+            self._redraw()
             self.user.dbuser.message = None
             td.s.flush()
             return False
@@ -3188,6 +3192,7 @@ def entry(self):
             ui.infopopup(["Your transaction has gone away."],
                          title="Transaction gone")
             self._clear()
+            self._redraw()
             td.s.flush()
             return False
 
@@ -3217,11 +3222,30 @@ def entry_noninteractive(self):
         if self.transid and not self.user.dbuser.transaction:
             # Transaction has been taken by another user
             return False
-        if self.transid != self.user.dbuser.transaction.id:
+        if self.transid and self.transid != self.user.dbuser.transaction.id:
             # This _should_ never happen but would be very bad if it did!
             return False
         return True
 
+    def _notify_user_register(self, userid_string):
+        # This is called whenever a user_register notification is sent
+        # by the database, which will happen whenever a user's transid
+        # or register column is changed. If the userid_string
+        # corresponds to our user, run entry_noninteractive() to check
+        # that the user hasn't moved to another register and that
+        # their transaction hasn't been taken over by someone else.
+        if not self.selected:
+            return
+        log.debug(f"{self=} got user_register for {userid_string}")
+        try:
+            userid = int(userid_string)
+        except Exception:
+            return
+        if userid == self.user.userid:
+            with td.orm_session():
+                if not self.entry_noninteractive():
+                    self.deselect()
+
     def hook(self, action, *args, **kwargs):
         """Check with plugins before performing an action
 
@@ -3352,6 +3376,9 @@ def deselect(self):
         if self._timeout_handle:
             self._timeout_handle.cancel()
             self._timeout_handle = None
+        if self._notify_user_register_listener:
+            self._notify_user_register_listener.cancel()
+            self._notify_user_register_listener = None
 
     def alarm(self):
         # The timeout has passed.  If the scrollable has the input

quicktill/ui.py

@@ -334,8 +334,14 @@ def __init__(self):
     def pagename(self):
         return "Basic page"
 
+    @property
+    def selected(self):
+        """Is this the current page?
+        """
+        return self._basepage == self
+
     def select(self):
-        if basicpage._basepage == self:
+        if self.selected:
             return  # Nothing to do
         # Tell the current page we're switching away
         if basicpage._basepage:

release-notes.md

@@ -14,6 +14,10 @@ What's new:
  * The Twitter integration has been removed. A stub remains so that
    till configurations that refer to it will still load.
 
+ * When a user moves to another terminal their session on the terminal
+   they left closes immediately, rather than waiting for a timeout or
+   a keypress.
+
 To upgrade the database:
 
  - run "runtill syncdb" to create the new stocktype metadata table
@@ -29,6 +33,24 @@ ALTER TABLE users
 ALTER TABLE stocklines
         ADD COLUMN note character varying DEFAULT ''::character varying NOT NULL;
 
+CREATE OR REPLACE FUNCTION notify_user_change() RETURNS trigger
+    LANGUAGE plpgsql
+    AS $$
+DECLARE
+BEGIN
+  IF NEW.transid IS DISTINCT FROM OLD.transid
+    OR NEW.register IS DISTINCT FROM OLD.register THEN
+    PERFORM pg_notify('user_register', CAST(NEW.id AS text));
+  END IF;
+  RETURN NEW;
+END;
+$$;
+
+CREATE TRIGGER user_changed
+	AFTER UPDATE ON users
+	FOR EACH ROW
+	EXECUTE PROCEDURE public.notify_user_change();
+
 COMMIT;
 ```