commit 4f458e7f03a67755e691904467ad65e4c91179f9
author Vadim Shtayura <vadimsh@chromium.org> Mon Feb 04 20:27:19 2019
committer Commit Bot <commit-bot@chromium.org> Mon Feb 04 20:27:19 2019
tree a4f45034765b8ad7f5f4a53a8570b6626ebd64df
parent 209795ace740f404761ac7dcaba3863849abcbe4

[lucicfg] Implement lucicfg.var().

Vars can be used to implicitly propagate information down 'exec' call stack,
but not up or "sideways".

Useful for declaring module-level default values for fields instead of plumbing
them through all calls explicitly.

Subject to following rules:

1. Only modules that are invoked via 'exec' can set or get variables. Libraries
   (modules loaded via 'load') can only declare them. This is because libraries
   are loaded lazily (so essentially at unpredictable moments) and their module
   dict is cached and reused. The state inside library's module dict must not
   depend on whatever state variables had when the library was loaded.
2. Each 'exec' call opens a new scope for variables. Variables set within a
   scope are "forgotten" when exiting the scope. Variable value lookup start
   with the innermost scope, and goes up to the top scope. Thus variable values
   propagate down 'exec' call stack, but not up or "sideways".
3. Variables are set-only. If a variable was set in the current scope or any
   of parent scopes, assigning a new value to it is an error.

R=tandrii@chromium.org
BUG=833946

Change-Id: Idaf44866125c7cf8ef63247587b5b67eabf0782f
Reviewed-on: https://chromium-review.googlesource.com/c/1451101
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>