Don't lock the register if a transaction closes within 3 seconds

If a Square transaction was in progress when the user locked the
register, it is likely to complete very quickly the next time the user
taps in. If this happens within 3 seconds of the user tapping in, don't
lock automatically — the user probably wants to start a new transaction.

See #288 on github.

quicktill/register.py

@@ -29,6 +29,7 @@
 
 from . import tillconfig
 import math
+import time
 from . import td, ui, keyboard, printer
 import quicktill.stocktype
 from . import linekeys
@@ -778,6 +779,7 @@ def __init__(self, user, hotkeys, autolock=None, timeout=300):
         # trans and user are needed for "pagename" which is called in
         # basicpage.__init__()
         # transid is now a transaction ID, not a models.Transaction object
+        self._start_time = time.time()
         self.transid = None
         self.user = user
         log.info("Page created for %s", self.user.fullname)
@@ -1017,7 +1019,9 @@ def close_if_balanced(self):
             self.balance = zero
             td.s.flush()
             self._clear_marks()
-            if self._autolock:
+            # Check how long it has been since the register was
+            # created — if it's under a few seconds, do not lock
+            if self._autolock and time.time() - self._start_time >= 3.0:
                 self.locked = True
 
     def linekey(self, kb):