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Git hashing for Git fetching #9025

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@Ericson2314 Ericson2314 commented Sep 21, 2023

Motivation

The git fetcher is now more tree-hash-oriented, and we will want to integrate this with git fetching eventually. This PR exposes treeHash inputs and outputs in a few ways for this purpose.

Eventually, we should add something like builtins.derivation's outputHashMode to builtins.fetchTree, in order to specify we should use git hashing, and then this and the store-layer git hashing should meet together, ensuring we have the same tree hash end-to-end.

Context

Part of RFC 133

Tracking issue #8919

depends on #8918

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@github-actions github-actions bot added new-cli Relating to the "nix" command with-tests Issues related to testing. PRs with tests have some priority store Issues and pull requests concerning the Nix store fetching Networking with the outside (non-Nix) world, input locking labels Sep 21, 2023
@Ericson2314 Ericson2314 changed the title Git hashing for fetching Git hashing for Git fetching Sep 21, 2023
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Cursory review

src/libexpr/primops/fetchTree.cc Outdated Show resolved Hide resolved
src/libfetchers/git.cc Outdated Show resolved Hide resolved
src/libfetchers/git.cc Outdated Show resolved Hide resolved
src/libstore/store-api.cc Outdated Show resolved Hide resolved
src/libutil/fs-sink.hh Outdated Show resolved Hide resolved
@github-actions github-actions bot removed the with-tests Issues related to testing. PRs with tests have some priority label Oct 6, 2023
@github-actions github-actions bot added the with-tests Issues related to testing. PRs with tests have some priority label Oct 6, 2023
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dpulls bot commented Feb 28, 2024

🎉 All dependencies have been resolved !

@Ericson2314 Ericson2314 added this to the git-hashing stabilisation milestone Mar 22, 2024
@Ericson2314 Ericson2314 force-pushed the git-hashing-for-fetching branch 3 times, most recently from f0d1e1a to 33e945b Compare March 25, 2024 16:31
@Ericson2314 Ericson2314 marked this pull request as ready for review March 25, 2024 16:31
Ericson2314 and others added 2 commits March 27, 2024 10:38
The git fetcher is now more tree-hash-oriented, and we will want to
integrate this with git fetching eventually. This PR exposes `treeHash`
inputs and outputs in a few ways for this purpose.

Eventually, we should add something like `builtins.derivation`'s
`outputHashMode` to `builtins.fetchTree`, in order to specify we should
use git hashing, and then this and the store-layer git hashing should
meet together, ensuring we have the same tree hash end-to-end.

Part of RFC 133

Co-Authored-By: Matthew Bauer <[email protected]>
Co-Authored-By: Carlo Nucera <[email protected]>
Co-authored-by: Robert Hensing <[email protected]>
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roberth commented Mar 27, 2024

and then this and the store-layer git hashing should meet together

They already meet, and they involve a transformation that turns git trees without submodules and filtering into git trees where those things are resolved.
These are in general different trees, even if they happen to line up exactly in the simple cases.

Could you clarify what "meeting" entails and how we benefit from that?

if (auto narHash = input.getNarHash()) {
attrs.alloc("narHash").mkString(narHash->to_string(HashFormat::SRI, true));
} else if (auto treeHash = input.getTreeHash()) {
attrs.alloc("treeHash").mkString(treeHash->to_string(HashFormat::SRI, true));
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We have two kinds of tree hashes:

  • tree hashes from git commits, which do not correspond to fetched trees in general
  • tree hashes from file system objects returned by fetchTree

Which one is this, and what is the purpose of exposing it to the expression language?

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  • The meaning of the tree hash must be unambiguous.
  • We need a concrete benefit to users before exposing arbitrary data to the expression language.

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