fetchTree performs fetch even when a narHash is provided

[yajo]

Describe the bug

According to the docs, when you add narHash to a fetchTree call, the result won't be fetched if it's already available:

nix/src/libexpr/primops/fetchTree.cc

Line 218 in 864fc85

If `narHash` is set, the source is first looked up is the Nix store and [substituters](@docroot@/command-ref/conf-file.md#conf-substituters), and only fetched if not available.

Well, that's not happening.

Steps To Reproduce

The 1st time you fetch, it downloads ✅:

➤ nix eval --no-eval-cache --debug --expr 'builtins.fetchTree {type="github"; owner="OCA"; repo="account-analytic"; rev="bc8dbde0477634d64915d027093756defcf0ebc3"; narHash="sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc=";}'
did not find cache entry for '{"rev":"bc8dbde0477634d64915d027093756defcf0ebc3","type":"git-tarball"}'
did not find cache entry for '{"name":"source","type":"tarball","url":"https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz"}'
did not find cache entry for '{"name":"source","type":"file","url":"https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz"}'
downloading 'https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz'...
starting download of https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz
finished download of 'https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz'; curl status = 0, HTTP status = 200, body = 1089274 bytes
performing daemon worker op: 39
acquiring write lock on '/nix/var/nix/temproots/575565'
performing daemon worker op: 7
performing daemon worker op: 26
{ lastModified = 1697447248; lastModifiedDate = "20231016090728"; narHash = "sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc="; outPath = "/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source"; rev = "bc8dbde0477634d64915d027093756defcf0ebc3"; shortRev = "bc8dbde"; }
download thread shutting down

The next time you download, it says "using cache entry". That's suspicious 🕵️‍♂️. It shouldn't need any cache because there's a narHash parameter passed:

➤ nix eval --no-eval-cache --debug --expr 'builtins.fetchTree {type="github"; owner="OCA"; repo="account-analytic"; rev="bc8dbde0477634d64915d027093756defcf0ebc3"; narHash="sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc=";}'
performing daemon worker op: 11
acquiring write lock on '/nix/var/nix/temproots/575669'
performing daemon worker op: 1
using cache entry '{"rev":"bc8dbde0477634d64915d027093756defcf0ebc3","type":"git-tarball"}' -> '{"lastModified":1697447248,"rev":"bc8dbde0477634d64915d027093756defcf0ebc3"}', '/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source'
performing daemon worker op: 26
{ lastModified = 1697447248; lastModifiedDate = "20231016090728"; narHash = "sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc="; outPath = "/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source"; rev = "bc8dbde0477634d64915d027093756defcf0ebc3"; shortRev = "bc8dbde"; }

Let's remove cache and retry. We can see how fetches again. ❌ It shouldn't!:

➤ rm -rf ~/.cache/nix

➤ nix eval --no-eval-cache --debug --expr 'builtins.fetchTree {type="github"; owner="OCA"; repo="account-analytic"; rev="bc8dbde0477634d64915d027093756defcf0ebc3"; narHash="sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc=";}'
did not find cache entry for '{"rev":"bc8dbde0477634d64915d027093756defcf0ebc3","type":"git-tarball"}'
did not find cache entry for '{"name":"source","type":"tarball","url":"https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz"}'
did not find cache entry for '{"name":"source","type":"file","url":"https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz"}'
downloading 'https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz'...
starting download of https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz
finished download of 'https://github.com/OCA/account-analytic/archive/bc8dbde0477634d64915d027093756defcf0ebc3.tar.gz'; curl status = 0, HTTP status = 200, body = 1089274 bytes
performing daemon worker op: 39
acquiring write lock on '/nix/var/nix/temproots/575717'
performing daemon worker op: 7
performing daemon worker op: 26
{ lastModified = 1697447248; lastModifiedDate = "20231016090728"; narHash = "sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc="; outPath = "/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source"; rev = "bc8dbde0477634d64915d027093756defcf0ebc3"; shortRev = "bc8dbde"; }
download thread shutting down

Expected behavior

When calling with a NAR hash, it shouldn't need to even check the cache. Nix should just check the store and skip the fetching/caching completely.

nix-env --version output
nix-env (Nix) 2.18.1

Additional context

This might be duplicate of #9077 or #9570, but I'm not sure so I open separately.

Priorities

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[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-03-04-nix-team-meeting-minute-130/40830/1

[edolstra]

This appears to have been fixed in Nix 2.19:

$ nix shell nix/2.19.0

$ rm -rf ~/.cache/nix/

$ $ nix eval --no-eval-cache --debug --expr 'builtins.fetchTree {type="github"; owner="OCA"; repo="account-analytic"; rev="bc8dbde0477634d64915d027093756defcf0ebc3"; narHash="sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc=";}'
evaluating file '/derivation-internal.nix'
performing daemon worker op: 10
using substituted/cached input 'github:OCA/account-analytic/bc8dbde0477634d64915d027093756defcf0ebc3' in '/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source'
{ narHash = "sha256-sD75xS86JRr7q1be+U8FMI7ijDj4tgexlyNGVEa8fpc="; outPath = "/nix/store/q3fw9j28hdlyal51xgqlrac9vcb59p2f-source"; rev = "bc8dbde0477634d64915d027093756defcf0ebc3"; shortRev = "bc8dbde"; }