(core) Put user code in linecache so that source lines show in tracebacks

Summary: See title. This should improve the user experience, but more importantly it's something I've wanted several times when developing (including just now) so I've been meaning to do this.

Test Plan: Expanded existing unit tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2910

sandbox/grist/gencode.py

@@ -15,6 +15,7 @@ The schema for grist data is:
     "formula": <opt_string>,
   }
 """
+import linecache
 import re
 import imp
 from collections import OrderedDict
@@ -194,6 +195,17 @@ def _is_special_table(table_id):
 
 def exec_module_text(module_text):
   # pylint: disable=exec-used
-  mod = imp.new_module("usercode")
-  exec(module_text, mod.__dict__)
+  filename = "usercode"
+  mod = imp.new_module(filename)
+
+  # Ensure that source lines show in tracebacks
+  linecache.cache[filename] = (
+    len(module_text),
+    None,
+    [line + '\n' for line in module_text.splitlines()],
+    filename,
+  )
+
+  code_obj = compile(module_text, filename, "exec")
+  exec(code_obj, mod.__dict__)
   return mod