Problem

select_multiversion_binaries runs once per build variant and resolves every version listed in
evergreen/multiversion_selection.sh – currently last-lts, last-continuous, 7.0, 8.0 and
8.0.16 – into multiversion-downloads.json. Every multiversion task on that variant then hands
that file to db-contrib-tool setup-repro-env, which downloads all of them. Passing the tool a
.json path makes it a single VERSIONS_FILE request that it expands wholesale; there is no way
to ask for a subset.

So a jstestfuzz task testing only against last-LTS also downloads 7.0, 8.0 and 8.0.16. Only the
multiversion and multiversion_auth suites need those, via major_version_upgrade.js and
skip_level_upgrade.js, which walk every supported release. Nothing is cached between tasks
either, since multiversion_setup.sh removes the install and link dirs first.

Change

Trim multiversion-downloads.json to what the task needs before db-contrib-tool sees it, in
buildscripts/multiversion_downloads_filter.py. Two cases, no inference:

• the task generator passed the versions this task needs -> keep those entries, drop the rest
• it passed nothing -> leave the file alone and download everything, as today

The versions come from multiversion_setup_versions, added to the do multiversion setup
function call by mongo-task-generator (see linked ticket). Until a generator release that sets it
is pinned in evergreen/prelude_mongo_task_generator.sh, every task takes the second branch and
behaviour is unchanged.

Result

Measured on replica_sets_jscore_passthrough_last_lts_new_new_old: one binary instead of four,
and the multiversion setup step drops from ~38s to ~17s. Suites that test against several
versions (no_multiversion_generate_tasks) are unaffected and still download everything.

Testing

Evergreen patches covering both paths on enterprise-rhel-8-64-bit-multiversion and
enterprise-rhel-8-64-bit-large-txns-format: fan-out tasks trim to one binary, opt-out tasks
download all four, tests pass in both. 20 unit tests, run by buildscripts_test.