Selected package revision does not respect index-state

[hamishmack]

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plan-to-nix does not include the revisions, instead just choosing the latest one (via revisions.default). haskell-nix.hackage-package also does not respect index-state when choosing the revision of the package itself to use #903 (it currently chooses the oldest one, but we may change that inline with plan-to-nix).

This can cause two problems:

• Unwanted rebuilds when the hackage.nix pin is updated (or if we point to a new haskell.nix commit with updated hackage.nix). For instance when the unix package recently got a new revision it caused a lot of unexpected rebuilds.
• Broken builds if a constraint is added that makes a plan that was valid for an older index-state invalid.

Solution might be to capture the timestamp for each revision here and then have hackage-to-nix write out revTimestamp along with the revNum and sha256.

Then haskell.nix could be updated to filter out revisions where revTimestamp > index-state.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[andreabedini]

The way we deal with revisions has been completey reworked (plan-nix now includes the textual revision).