nodemap: gate the feature behind a new requirement
Now that the feature is working smoothly, a question was still open, should we
gate the feature behind a new requirement or just treat it as a cache to be
warmed by those who can and ignored by other.
The advantage of using the cache approach is a transparent upgrade/downgrade
story, making the feature easier to move to. However having out of date cache
can come with a significant performance hit for process who expect an up to
date cache but found none. In this case the file needs to be stored under
`.hg/cache`.
The "requirement" approach guarantee that the persistent nodemap is up to date.
However, it comes with a less flexible activation story since an explicite
upgrade is required. In this case the file can be stored in `.hg/store`.
This wiki page is relevant to this questions:
https://www.mercurial-scm.org/wiki/ComputedIndexPlan
So which one should we take? Another element came into plan, the persistent
nodemap use the `add` method of the transaction, it is used to keep track of a
file content before a transaction in case we need to rollback it back. It turns
out that the `transaction.add` API does not support file stored anywhere than
`.hg/store`. Making it support file stored elsewhere is possible, require a
change in on disk transaction format. Updating on disk file requires…
introducing a new requirements.
As a result, we pick the second option "gating the persistent nodemap behind a
new requirements".
Differential Revision: https://phab.mercurial-scm.org/D8417
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----