Commits on Dec 8, 2020

1. Store metadata about drv outputs realisations …

   For each known realisation, store:
   - its output
   - its output path
   
   This comes with a set of needed changes:
   
   - New `realisations` module declaring the types needed for describing
     these mappings
   - New `Store::registerDrvOutput` method registering all the needed informations
     about a derivation output (also replaces `LocalStore::linkDeriverToPath`)
   - new `Store::queryDrvOutputInfo` method to retrieve the informations for a
     derivations
   
   This introcudes some redundancy on the remote-store side between
   `wopQueryDerivationOutputMap` and `wopQueryDrvOutputInfo`.
   However we might need to keep both (regardless of backwards compat)
   because we sometimes need to get some infos for all the outputs of a
   derivation (where `wopQueryDerivationOutputMap` is handy), but all the
   stores can't implement it − because listing all the outputs of a
   derivation isn't really possible for binary caches where the server
   doesn't allow to list a directory.

   

   thufschmitt committed Dec 8, 2020
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2. Rework the db schema for derivation outputs …

   Add a new table for tracking the derivation output mappings.
   
   We used to hijack the `DerivationOutputs` table for that, but (despite its
   name), it isn't a really good fit:
   
   - Its entries depend on the drv being a valid path, making it play badly with
     garbage collection and preventing us to copy a drv output without copying
     the whole drv closure too;
   - It dosen't guaranty that the output path exists;
   
   By using a different table, we can experiment with a different schema better
   suited for tracking the output mappings of CA derivations.
   (incidentally, this also fixes #4138)

   

   thufschmitt committed Dec 8, 2020
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