Design review for "Fingerprinting Starlark Functions"

[brandjon]

This is a proposal to systematize how we compute digests for Starlark callback functions that are used in actions (e.g., Args's map_each parameter). Such digests are relevant for shared action detection and for lookups in the local action cache. Currently we're not very rigorous about computing these digests, so this proposal would fix correctness bugs and prevent new ones of the same kind.

The main takeaway is that we would ban non-global functions from being used as callbacks to map_each or similar features. In terms of the discussion in #13730, the preferred alternative is to go with the "clientdata" approach of passing additional fixed values into the Args alongside the map_each callback.

[brandjon]

@alexjski FYI. You're no longer on Bazel team, but I remember working on the map_each digest issue with you in 2020, so pinging in case you're curious.