[BuildStream] Partial local CAS for remote execution - next steps

[Jürg Billeter <j bitron ch>]

Hi everyone,

Initial support for partial local CAS has now been merged (#832, !1225,
!1232, !1278). When using remote execution the actual file blobs of
build-only dependencies are no longer downloaded to the local cache as
part of a build session. Directories and metadata are always downloaded
as they are needed for staging.

This is implemented for both, downloading the build result from the
remote execution CAS server as well as pulling previously built
artifacts from artifact servers.

If artifact pushing is enabled and the artifact server is distinct from
the RE CAS server, the file blobs are still downloaded.

I see two follow-ups as possible next steps with regards to partial
local CAS:

 * Add configuration option to disable fetching file blobs even for the
   specified build targets and their runtime dependencies (when remote
   execution is used). I'm not planning to change the default behavior.

 * Support fetching missing file blobs from RE CAS server (or artifact
   server) on demand for commands that need them such as bst checkout,
   bst shell, or local build. This will likely require discussion about
   CLI options and default behavior and can be considered related to on
   demand pulling of buildtrees.

Any comments or concerns with these plans? Are there users that would
benefit from the above for their current use cases or is this more a
nice to have and not urgent?

Cheers,
Jürg