Source roots result in /nix/store/<hash>-<hash>-source

[edolstra]

Describe the bug

In a flake with a derivation attribute like

src = ./.;

the tree gets copied into the store as /nix/store/<hash1>-<hash2>-source. This is because (pre lazy-trees) the basename of the tree is /nix/store/<hash2>-source.

This double-copying is obviously undesirable, but we can't fix it without a breaking change (i.e. without changing derivation paths).

On lazy-trees, we can at least avoid writing /nix/store/<hash2>-source to disk, but we do need to hash the input to compute <hash2>. (See c5ae41d.)

Steps To Reproduce

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Expected behavior

A clear and concise description of what you expected to happen.

nix-env --version output

Additional context

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• Add pure-storepath-builtin experimental feature #12141 (comment)

Priorities

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[KiaraGrouwstra]

we could already improve on the user experience here by warning the user to avoid such paths.

[roberth]

This is a pre-flakes behavior that would be a breaking change.

we could already improve on the user experience here by warning the user to avoid such paths.

I believe this would trigger quite often, and without being all that actionable for most users.

We'd better address the root cause, which is putting flakes in the store in the first place, making this problem prevalent instead of niche.

We could do so with either

• Lazy fetchTree outPath path values #10252
  • Lazy paths #11367
• maybe a different change based on Lazy trees #6530 ideas

[KiaraGrouwstra]

@roberth:

without being all that actionable for most users

alternatives seem (edited):

• general: builtins.path { path = ./.; name = "my-dir-name"; }
• flakes: self.outPath / "${self}" or even just an implicitly cast src = self

I believe this would trigger quite often, and without being all that actionable for most users.

if a proper fix seems not within reach, an intermediate option could be warnings on an opt-in basis, such that more adventurous users could act as the canary and report such occurrences at their upstream projects.