tvl-depot/nix/lazy-deps/default.nix
Vincent Ambo 0ce396bdab feat(nix/lazy-deps): Add function to generate lazy binary dispatcher
There is a reoccuring problem in readTree-type repositories that use
nix-shell, where evaluation of the full set of dependencies that
should be made available to users takes a noticeable amount of time,
slowing down operations when `direnv` is involved.

In depot, we have so far fixed this by maintaining a manual `//bin`
directory which contains a set of symlinks to a central dispatch
script that can dispatch to various tools in depot lazily.

This script can instead be generated ad-hoc by Nix (pretty fast if we
can make assumptions like `git` and `nix-build` existing on user's
machines already) and added to $PATH.

The function introduced in this commit implements the logic for that.
The structure of the script is based on the existing
`//bin/__dispatch`.

This does not yet switch depot's envrc to use this new method of
installing dependencies lazily.

Change-Id: I92efcd9bb6aa51aa2709ad910a464e9dac97ee89
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5512
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>
2022-05-02 23:34:55 +00:00

75 lines
2.3 KiB
Nix

# Helper function to synthesize a directory of "lazy-built" binaries
# that can be added to $PATH inside of a repository.
#
# Using this, a Nix shell environment in some repository can contain
# several slow-to-build commands without blowing up evaluation and
# build time whenever the shell is loaded.
#
# Note that the generated script is deliberately impure to speed up
# evaluation, and expects both `git` and `nix-build` to exist in the
# user's $PATH. If required, this can be done in the shell
# configuration invoking this function.
{ pkgs, ... }:
let
inherit (builtins) attrNames attrValues mapAttrs;
inherit (pkgs.lib) concatStringsSep;
# Create the case statement for a command invocations, optionally
# overriding the `TARGET_TOOL` variable.
invoke = name: { attr, cmd ? null }: ''
${name})
attr="${attr}"
${if cmd != null then "TARGET_TOOL=\"${cmd}\"\n;;" else ";;"}
'';
# Create command to symlink to the dispatch script for each tool.
link = name: "ln -s $target $out/bin/${name}";
invocations = tools: concatStringsSep "\n" (attrValues (mapAttrs invoke tools));
in
# Attribute set of tools that should be lazily-added to the $PATH.
# The name of each attribute is used as the command name (on $PATH).
# It must contain the keys 'attr' (containing the Nix attribute path
# to the tool's derivation from the top-level), and may optionally
# contain the key 'cmd' to override the name of the binary inside the
# derivation.
tools:
pkgs.writeTextFile {
name = "lazy-dispatch";
executable = true;
destination = "/bin/__dispatch";
text = ''
#!${pkgs.runtimeShell}
set -ue
if ! type git>/dev/null || ! type nix-build>/dev/null; then
echo "The 'git' and 'nix-build' commands must be available." >&2
exit 127
fi
readonly REPO_ROOT=$(git rev-parse --show-toplevel)
TARGET_TOOL=$(basename "$0")
case "''${TARGET_TOOL}" in
${invocations tools}
*)
echo "''${TARGET_TOOL} is currently not installed in this repository." >&2
exit 127
;;
esac
result=$(nix-build --no-out-link --attr "''${attr}" "''${REPO_ROOT}")
PATH="''${result}/bin:$PATH"
exec "''${TARGET_TOOL}" "''${@}"
'';
checkPhase = ''
${pkgs.stdenv.shellDryRun} "$target"
${concatStringsSep "\n" (map link (attrNames tools))}
'';
}