"spack install" will not update the binary index if given a concrete
spec, which causes it to fall back to direct fetches when a simple
index update would have helped. For S3 buckets in particular, this
significantly and needlessly slows down the install process.
This commit alters the logic so that the binary index is updated
whenever a by-hash lookup fails. The lookup is attempted again with
the updated index before falling back to direct fetches. To avoid
updating too frequently (potentially once for each spec being
installed), BinaryCacheIndex.update now includes a "cooldown"
option, and when this option is enabled it will not update more
than once in a cooldown window (set in config.yaml).
Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>
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