(core) Update the current time in formulas automatically every hour

Summary: Adds a special user action `UpdateCurrentTime` which invalidates an internal engine dependency node that doesn't belong to any table but is 'used' by the `NOW()` function. Applies the action automatically every hour.

Test Plan: Added a Python test for the user action. Tested the interval periodically applying the action manually: {F43312}

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3389

sandbox/grist/engine.py

@@ -3,7 +3,6 @@
 The data engine ties the code generated from the schema with the document data, and with
 dependency tracking.
 """
-import contextlib
 import itertools
 import re
 import rlcompleter
@@ -1141,6 +1140,23 @@ class Engine(object):
     self.dep_graph.invalidate_deps(table._new_columns_node, depend.ALL_ROWS, self.recompute_map,
                                    include_self=False)
 
+  def update_current_time(self):
+    self.dep_graph.invalidate_deps(self._current_time_node, depend.ALL_ROWS, self.recompute_map,
+                                   include_self=False)
+
+  def use_current_time(self):
+    """
+    Add a dependency on the current time to the current evaluating node,
+    so that calling update_current_time() will invalidate the node and cause its reevaluation.
+    """
+    if not self._current_node:
+      return
+    table_id = self._current_node[0]
+    table = self.tables[table_id]
+    self._use_node(self._current_time_node, table._identity_relation)
+
+  _current_time_node = ("#now", None)
+
   def mark_lookupmap_for_cleanup(self, lookup_map_column):
     """
     Once a LookupMapColumn seems no longer used, it's added here. We'll check after recomputing