86785fd9d1 was broken because it was
storing the full path in the MountedSourceAccessor as the `path` field
in the fetcher cache key (i.e. including the
/nix/store/... prefix). Especially in the case of lazy (virtual) store
paths, this didn't work at all because those paths are different every time.
If a NAR hash is specified, we should probably check
it. Unfortunately, for now this has the side effect of forcing NAR
hash checking of any input that has a NAR hash.
This was broken because MountedSourceAccessor did not return a
fingerprint. Previously fingerprints were global to an accessor, but
with a MountedSourceAccessor the fingerprint can be different for each
mount point.
These got displayed as e.g.
«github:NixOS/nixpkgs/adaa24fbf46737f3f1b5497bf64bae750f82942e?narHash=sha256-qhFMmDkeJX9KJwr5H32f1r7Prs7XbQWtO0h3V0a0rFY%3D»/nix/store/x9wnkly3k1gkq580m90jjn32q9f05q2v-source/pkgs/stdenv/generic/source-stdenv.sh
Now we get
«github:NixOS/nixpkgs/adaa24fbf46737f3f1b5497bf64bae750f82942e?narHash=sha256-qhFMmDkeJX9KJwr5H32f1r7Prs7XbQWtO0h3V0a0rFY%3D»/pkgs/stdenv/generic/source-stdenv.sh
Previously it used the `ThreadPool` default,
i.e. `std:🧵:hardware_concurrency()`. But copying signatures is
not primarily CPU-bound so it makes more sense to use the
`http-connections` setting (since we're typically copying from/to a
binary cache).
(Actually, this adds a position field to *all* values.)
This allows improving the "inefficient double copy" warning by showing
where the source path came from in the source, e.g.
warning: Performing inefficient double copy of path '/home/eelco/Dev/patchelf/' to the store at /home/eelco/Dev/patchelf/flake.nix:30:17. This can typically be avoided by rewriting an attribute like `src = ./.` to `src = builtins.path { path = ./.; name = "source"; }`.
As it turns out using `std::regex` is actually the bottleneck
for root discovery. Just substituting `std::` -> `boost::`
makes root discovery twice as fast (3x if counting only userspace time).
Some rather ad-hoc measurements to motivate the switch:
(On master)
```
nix build github:nixos/nix/1e822bd4149a8bce1da81ee2ad9404986b07914c#nix-cli --out-link result-1e822bd4149a8bce1da81ee2ad9404986b07914c
taskset -c 2,3 hyperfine "result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run --max 0"
Benchmark 1: result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run --max 0
Time (mean ± σ): 481.6 ms ± 3.9 ms [User: 336.2 ms, System: 142.0 ms]
Range (min … max): 474.6 ms … 487.7 ms 10 runs
```
(After this patch)
```
taskset -c 2,3 hyperfine "result/bin/nix store gc --dry-run --max 0"
Benchmark 1: result/bin/nix store gc --dry-run --max 0
Time (mean ± σ): 254.7 ms ± 9.7 ms [User: 111.1 ms, System: 141.3 ms]
Range (min … max): 246.5 ms … 281.3 ms 10 runs
```
`boost::regex` is a drop-in replacement for `std::regex`, but much faster.
Doing a simple before/after comparison doesn't surface any change in behavior:
```
result/bin/nix store gc --dry-run -vvvvv --max 0 |& grep "got additional" | wc -l
result-1e822bd4149a8bce1da81ee2ad9404986b07914c/bin/nix store gc --dry-run -vvvvv --max 0 |& grep "got additional" | wc -l
```
(cherry picked from commit 3a1301cd6d)
