91d8b86b39
This adds a new Website/Docs for Snix, using Thulite / Doks, which is mostly hugo and a bit of npm. Change-Id: Iea10d4068fa783ec0ddd6bcaba5c8d92b1a1168f
45 lines
1.7 KiB
Markdown
45 lines
1.7 KiB
Markdown
---
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title: "Reference Scanning"
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summary: ""
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date: 2025-03-14T14:14:35+01:00
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lastmod: 2025-03-14T14:14:35+01:00
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draft: false
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weight: 45
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toc: true
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---
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At the end of a build, Nix does scan a store path for references to other store
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paths (*out of the set of all store paths present during the build*).
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It does do this by (only) looking for a list of nixbase32-encoded hashes in
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filenames (?), symlink targets and blob contents.
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As outlined in the [Builder Protocol]({{< relref "protocol" >}}) page, we
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don't want to introduce Nix specifics to the builder protocol, but if we simply
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do refscanning on the coordinator side, that side would need to download the
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produced inputs and scan them locally.
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This is undesireable, as the builder already has all produced
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outputs locally, and it'd make more sense for it do do it.
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Instead, we want to describe reference scanning in a generic, non-Nix-specific
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fashion.
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## Proposal
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One way to do this is to add an additional field `refscan_needles` to the
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`BuildRequest` message.
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If this is an non-empty list, all `outputs` are scanned for these.
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The `Build` response message would then be extended with an `outputs_needles`
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field, containing the same number of elements as the existing `outputs` field,
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describing which `refscan_needles` are found for each output.
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If there's needles found, they will be a list of indexes into the
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`refscan_needles` field specified in the `BuildRequest` field.
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For Nix, `refscan_needles` would be populated with the nixbase32 hash parts of
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every input store path and output store path. The latter is necessary to scan
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for references between multi-output derivations.
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This is sufficient to construct the referred store paths in each build output on
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the build client.
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