nix nar {ls,cat}: Optimize, make nix nar cat work on pipes too#14732
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Ericson2314 merged 3 commits intomasterfrom Dec 8, 2025
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nix nar {ls,cat}: Optimize, make nix nar cat work on pipes too#14732Ericson2314 merged 3 commits intomasterfrom
Ericson2314 merged 3 commits intomasterfrom
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By default windows doesn't allow inheriting handles anyway. These comments are just confusing at this point.
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The whole NarAccessor -> listing -> lazy NarAccessor is very weird. Source can now be seek-ed over when supported, so we can support it pretty easily. Alternatively we could also make it single-pass very easily with a custom FileSystemObjectSink. It will get removed in a follow-up commit anyway.
This was lost after 2.32 while making the accessor lazy. We can restore the support for it pretty easily. Also this is significant optimization for nix nar cat. E.g. with a NAR of a linux repo this speeds up by ~3x: Benchmark 1: nix nar cat /tmp/linux.nar README Time (mean ± σ): 737.2 ms ± 5.6 ms [User: 298.1 ms, System: 435.7 ms] Range (min … max): 728.6 ms … 746.9 ms 10 runs Benchmark 2: build/src/nix/nix nar cat /tmp/linux.nar README Time (mean ± σ): 253.5 ms ± 2.9 ms [User: 56.4 ms, System: 196.3 ms] Range (min … max): 248.1 ms … 258.7 ms 12 runs
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Ericson2314
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Dec 8, 2025
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| GetNarBytes seekableGetNarBytes(Descriptor fd); | ||
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| ref<SourceAccessor> makeLazyNarAccessor(const nlohmann::json & listing, GetNarBytes getNarBytes); |
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BTW this should use the new listing types now that we have them (I missed them before)
Ericson2314
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Dec 8, 2025
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Motivation
This (
nix nar catworking with pipes) was lost after 2.32 while making the accessor lazy. We can restore the support for it pretty easily. Also this is significant optimization for nix nar cat.E.g. with a NAR of a linux repo this speeds up by ~3x:
Context
#14273
cc @myclevorname, you might be interested in this. Do note that we still have to parse the full NAR to validate it fully. Cutting off early wouldn't be correct.
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