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fetchToStore(): Avoid duplicate copying if the input is already a store path #10511

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@edolstra edolstra commented Apr 15, 2024

Motivation

This is needed for the path:// input scheme (until it returns a FSInputAccessor to the original path, but that's blocked by #10089) and the Mercurial input scheme.

Note: it would be cleaner to have fetchToStore() as an overridable method on InputAccessor (as was originally the case), but the latter got moved to libutil so we can't do that anymore.

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@github-actions github-actions bot added the fetching Networking with the outside (non-Nix) world, input locking label Apr 15, 2024
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Do we need isStorePath? I don't think so

// an FSInputAccessor pointing to a store path.
if (path.accessor->isStorePath
&& path.path.isRoot()
&& method == FileIngestionMethod::Recursive
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What goes wrong if we skip this? I am trying to special-case NAR hashing less in Nix currently.

Suggested change
&& method == FileIngestionMethod::Recursive

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Meant that to not be approval

@Ericson2314 Ericson2314 self-assigned this Apr 15, 2024
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Do we need isStorePath? I don't think so

We do need that to ensure it's only used for store paths where we know their ingestion method.

As mentioned above, the architecturally cleaner way is to make fetchToStore() a method in InputAccessor (which is pretty much why that class exists in the first place), but it would require moving InputAccessor back to libfetcher. Which would be a much bigger change.

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I think I rather solve this in a different way:

  1. SourceAccessor has optional content address (on the FSO level, so (FileIngestionMethod, Hash) pair)
  2. fetchToStore can use that to construct a store path and see if it is in the store
  3. only add to store if it isn't.

I am still hoping InputAccessor can go away entirely.

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If you could take a stab at #10467 (review) @edolstra, I would be happy to take on this one per my plan above.

…re path

This is needed for the path:// input scheme (until it returns a
FSInputAccessor to the original path, but that's blocked by NixOS#10089)
and the Mercurial input scheme.
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