feat(nix/sparseTree): get a directory with only selected children
Given a path (which points to a directory and a list of paths which are below that path, build a “sparse” version of that directory, so that it only contains the listed paths (and their children): $ nix-build -E 'with import ./. {}; nix.sparseTree ./. [ ./default.nix ./nix/readTree ./nix/buildLisp ./third_party/nixpkgs ./third_party/overlays ]' /nix/store/0ynj0gc613fs6mfp9snqcvdj5gfxbdzg-sparse-depot $ lr -t 'type == d' result/ result/ result/nix result/nix/buildLisp result/nix/buildLisp/example result/nix/readTree result/nix/readTree/tests […] result/third_party result/third_party/nixpkgs result/third_party/overlays result/third_party/overlays/haskell result/third_party/overlays/haskell/patches result/third_party/overlays/patches This is useful if a derivation depends on depot.path (e. g. if it wants to import depot at runtime). Usually this means that on every depot commit (or even worse, every change of .git on a local machine), this derivation has to be rebuild. By using sparseTree you can instead depend on a stripped down version of depot which only contains the bits you actually depend on, avoiding unrelated rebuilds. Change-Id: I127b108c8b177c657fb46786d0a6256516fd2c52 Reviewed-on: https://cl.tvl.fyi/c/depot/+/3503 Tested-by: BuildkiteCI Reviewed-by: tazjin <mail@tazj.in>
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nix/sparseTree/OWNERS
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nix/sparseTree/OWNERS
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inherited: true
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owners:
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- sterni
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nix/sparseTree/default.nix
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nix/sparseTree/default.nix
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# Build a “sparse” version of a given directory, only including contained files
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# and directories if they are listed in a supplied list:
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#
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# # A very minimal depot
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# sparseTree ./depot [
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# ./default.nix
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# ./depot/nix/readTree/default.nix
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# ./third_party/nixpkgs
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# ./third_party/overlays
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# ]
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{ pkgs, lib, ... }:
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# root path to use as a reference point
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root:
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# list of paths below `root` that should be
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# included in the resulting directory
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paths:
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let
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rootLength = builtins.stringLength (toString root);
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# Count slashes in a path.
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#
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# Type: path -> int
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depth = path: lib.pipe path [
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toString
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(builtins.split "/")
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(builtins.filter builtins.isList)
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builtins.length
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];
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# (Parent) directories will be created from deepest to shallowest
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# which should mean no conflicts are caused unless both a child
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# and its parent directory are in the list of paths.
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# TODO(sterni): improve error messages in such cases
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fromDeepest = lib.sort (a: b: depth a < depth b) paths;
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# Create a set which contains the source path to copy / symlink and
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# it's destination, so the path below the destination root including
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# a leading slash. Additionally some sanity checking is done.
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makeSymlink = path:
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let
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strPath = toString path;
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contextPath = "${path}";
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belowRoot = builtins.substring rootLength (-1) strPath;
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prefix = builtins.substring 0 rootLength strPath;
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in assert toString root == prefix; {
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src = contextPath;
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dst = belowRoot;
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};
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symlinks = builtins.map makeSymlink fromDeepest;
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in
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# TODO(sterni): teach readTree to also read symlinked directories,
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# so we ln -sT instead of cp -aT.
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pkgs.runCommandNoCC "sparse-${builtins.baseNameOf root}" {} (
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lib.concatMapStrings ({ src, dst }: ''
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mkdir -p "$(dirname "$out${dst}")"
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cp -aT --reflink=auto "${src}" "$out${dst}"
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'') symlinks
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)
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