URL cache resolution broken with newer Nix versions

[infinisil]

Description

nix-zsh-completions substitutes channel: and https:// entries in NIX_PATH with cached versions thereof. To do that it uses this function:

https://github.com/spwhitt/nix-zsh-completions/blob/468d8cf752a62b877eba1a196fbbebb4ce4ebb6f/_nix-common-options#L338-L356

However this function doesn't work anymore, because it relies on ~/.cache/nix/tarballs, which isn't getting populated anymore since Nix version 2.4. Nix now uses an sqlite database instead to store information on cached tarballs, this was changed in this commit: NixOS/nix@462421d

The effect of this is that (with an empty ~/.cache/nix/tarballs), any channel: and https:// entry on NIX_PATH gets replaced with an empty string, breaking evaluation if you depend on those. There is also no error message indicating a failed evaluation.

Reproduction steps

Assuming a zsh with nix-zsh-completions installed

1. Create a default.nix file containing

   import <nixpkgs> {}

2. Set NIX_PATH to contain a channel: or https:// entry:

   $ export NIX_PATH=nixpkgs=channel:nixpkgs-unstable
   

3. Make sure the entry is fetched and cached:

   $ nix-instantiate -A hello
   warning: you did not specify '--add-root'; the result might be removed by the garbage collector
   /nix/store/2nlfpvl89vc9jq2vli9mni3xfylcfqpm-hello-2.12.drv
   

4. Try to tab complete:

   $ nix-instantiate -A <TAB>
   

5. Notice that tab completion doesn't work, without printing any error message

Completion trace

+_nix_attr_paths:24> result=          +_nix_attr_paths:24> _nix_eval_stdin
 +_nix_eval_stdin:1> setopt local_options pipefail
 +_nix_eval_stdin:3> local i override=''
 +_nix_eval_stdin:4> i=1
 +_nix_eval_stdin:4> i < 3
 +_nix_eval_stdin:5> case nix-instantiate (-I | --include)
 +_nix_eval_stdin:4> i++
 +_nix_eval_stdin:4> i < 3
 +_nix_eval_stdin:5> case -A (-I | --include)
 +_nix_eval_stdin:4> i++
 +_nix_eval_stdin:4> i < 3
 +_nix_eval_stdin:11> override+='nixpkgs=channel:nixpkgs-unstable' 
 +_nix_eval_stdin:14> [[ 'nixpkgs=channel:nixpkgs-unstable' == *(=|:)channel:* ]]
 +_nix_eval_stdin:15> local channel=nixpkgs-unstable
 +_nix_eval_stdin:16> channel=channel:nixpkgs-unstable 
 +_nix_eval_stdin:17> local url=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz
 +_nix_eval_stdin:19> override='nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz' 
 +_nix_eval_stdin:14> [[ 'nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz' == *(=|:)channel:* ]]
 +_nix_eval_stdin:23> [[ 'nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz' == *https://* ]]
 +_nix_eval_stdin:25> local url=nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz
 +_nix_eval_stdin:27> url=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz 
 +_nix_eval_stdin:28> _nix_resolve_url https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz
  +_nix_resolve_url:1> local url=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz
  +_nix_resolve_url:2> nix-instantiate --version
  +_nix_resolve_url:2> local version='nix-instantiate (Nix) 2.9.0'
  +_nix_resolve_url:3> local input
  +_nix_resolve_url:4> [[ 2.9.0 == 1.11.* ]]
  +_nix_resolve_url:9> input='nixexprs.tar.xz\0https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz' 
  +_nix_resolve_url:11> local sha
  +_nix_resolve_url:12> sha=            +_nix_resolve_url:12> printf 'nixexprs.tar.xz\0https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz'
  +_nix_resolve_url:12> sha=            +_nix_resolve_url:12> nix-hash --flat --base32 --type sha256 /proc/self/fd/27
  +_nix_resolve_url:12> sha=12zgms82w6c4ykwa6bfji5ykg68c5z306lim2qa6ch71zd04p10m 
  +_nix_resolve_url:13> local cache=/home/infinisil/.cache/nix/tarballs
  +_nix_resolve_url:14> local link=/home/infinisil/.cache/nix/tarballs/12zgms82w6c4ykwa6bfji5ykg68c5z306lim2qa6ch71zd04p10m-file
  +_nix_resolve_url:15> [[ -e /home/infinisil/.cache/nix/tarballs/12zgms82w6c4ykwa6bfji5ykg68c5z306lim2qa6ch71zd04p10m-file ]]
 +_nix_eval_stdin:28> local cache=''
 +_nix_eval_stdin:30> override='nixpkgs=' 
 +_nix_eval_stdin:23> [[ 'nixpkgs=' == *https://* ]]
 +_nix_eval_stdin:33> NIX_PATH='nixpkgs=' nix-instantiate --eval -
 +_nix_eval_stdin:33> tr '[]"' ' '
 +_nix_eval_stdin:34> return 1
+_nix_attr_paths:24> result=( ) 
+_nix_attr_paths:48> [[ 1 > 0 ]]
+_nix_attr_paths:49> _message $'Eval failed, can\'t complete (an URL might not be cached):\n'