We saw issues with /srv/zulip_npm_cache being cleaned incorrectly by
this tool in production (more correctly, we noticed broken symlinks to
those directories, even from the current deployment). Print-debugging
showed that indeed older deployments were being ignored, because the
logic for `get_caches_in_use` was totally broken (this was sorta
masked because we also keep the last week's deployments).
The specific bug here turned out to be that we weren't passing the
`production` argument to generate_sha1sum_node_modules, but the
broader problem is that this logic isn't robust to changes in the
hashing algorithm.
Fix this by replacing the broken logic for trying to compute the
correct hash for that deployment with just checking the symlink inside
the deployment to let it self-report.
We can't easily do this same change for clean-venv-cache, because we
use multiple virtualenvs there. But a similar change could be useful
for the emoji cache as well.
Fixes#8116.
Apparently, this was checking the wrong path in Travis CI, and thus
never actually running (meaning we'd accumulate every `node_modules`
directory ever in the Travis caches, which in turn resulted in very
slow builds).
This commit re-arranges the arguments of `purge_unused_caches()`
function in order to remain consistent with other similar functions
in the library like `may_be_perform_caching()`.
Based on the `dry_run` flag, this function either purges the list
of directories passed to them or prints a listing of the directories
it would have purged/kept_back, had the `dry_run` flag been false.
