(core) External requests

Summary:
Adds a Python function `REQUEST` which makes an HTTP GET request. Behind the scenes it:

- Raises a special exception to stop trying to evaluate the current cell and just keep the existing value.
- Notes the request arguments which will be returned by `apply_user_actions`.
- Makes the actual request in NodeJS, which sends back the raw response data in a new action `RespondToRequests` which reevaluates the cell(s) that made the request.
- Wraps the response data in a class which mimics the `Response` class of the `requests` library.

In certain cases, this asynchronous flow doesn't work and the sandbox will instead synchronously call an exported JS method:

- When reevaluating a single cell to get a formula error, the request is made synchronously.
- When a formula makes multiple requests, the earlier responses are retrieved synchronously from files which store responses as long as needed to complete evaluating formulas. See https://grist.slack.com/archives/CL1LQ8AT0/p1653399747810139

Test Plan: Added Python and nbrowser tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz, dsagal

Differential Revision: https://phab.getgrist.com/D3429

sandbox/grist/useractions.py

@@ -9,6 +9,7 @@ import six
 from six.moves import xrange
 
 import acl
+import depend
 import gencode
 from acl_formula import parse_acl_formula_json
 import actions
@@ -331,6 +332,25 @@ class UserActions(object):
     """
     self._engine.update_current_time()
 
+  @useraction
+  def RespondToRequests(self, responses, cached_keys):
+    """
+    Reevaluate formulas which called the REQUEST function using the now available responses.
+    """
+    engine = self._engine
+
+    # The actual raw responses which will be returned to the REQUEST function
+    engine._request_responses = responses
+    # Keys for older requests which are stored in files and can be retrieved synchronously
+    engine._cached_request_keys = set(cached_keys)
+
+    # Invalidate the exact cells which made the exact requests which are being responded to here.
+    for response in six.itervalues(responses):
+      for table_id, table_deps in six.iteritems(response.pop("deps")):
+        for col_id, row_ids in six.iteritems(table_deps):
+          node = depend.Node(table_id, col_id)
+          engine.dep_graph.invalidate_deps(node, row_ids, engine.recompute_map)
+
   #----------------------------------------
   # User actions on records.
   #----------------------------------------