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.github.ci: hand the lazy store's fetch list from the eval job over
In the benchmark run (PR 68) the neoprism job spent 29 s between
fetching lazy-store and mounting: the dry-run for the paths to
substitute (13 s in the shared job for the same machine) and their
narinfo closure (the rest, 2,878 paths), both after the 21 s hestia
prefetch of the drvs, which the dry-run needs.
The eval job has the drvs and dry-runs the machines anyway for the
shared derivations; it now keeps each machine's "will be fetched" list
and uploads it with the shared ones. The machine job collects the
closure of that list in the background while hestia prefetches the
drvs, then mounts. Computing the closure in the eval job too would only
move its time onto the eval job, before every machine job starts.
For a single machine the eval job now dry-runs it (about 13 s), against
about 29 s less in the machine job; with more machines the dry-runs ran
already, and in parallel.
author
Hermes
committer
lassulus
date
(Oct 17, 2026, 11:40 AM CEST)
commit
c3fad322
c3fad32222eaae74f1c50bc068476867b7b8b679
parent
7d3f6b7c
7d3f6b7c65596d3381e231c75bfdfcf55330c78d
+61
-37
+26
-16
.github/lazy-store.sh
+26
-16
.github/lazy-store.sh
···
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#!/usr/bin/env bash
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# Mounts a lazy /nix/store for building INSTALLABLE... (the machine jobs of
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# Mounts a lazy /nix/store for building a machine (the machine jobs of
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# ci.yml): every store path the build would substitute from cache.nixos.org
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# or cache.lassul.us is registered as valid from its narinfo and served by
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# lazy-store (tools/lazy-store), which downloads a path's NAR only when a
···
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# is built or substituted (also from hestia) as usual; writes go to the
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# real store dir below the mount.
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#
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# `closure FETCH` collects the closure of the paths in FETCH, those the
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# build would substitute (from the eval job's dry-run, .github/ci-shared.sh),
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# from the caches' narinfos; it needs no drvs and can run while they are
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# fetched. `mount` mounts it on /nix/store and registers the paths, after
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# `closure` and once the drvs are in the store.
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#
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# Needs sudo, /dev/fuse and LAZY_STORE, the lazy-store package's out path
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# (substitutable). If anything fails before the paths are registered, the
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# mount is undone and the build substitutes eagerly: this script always
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# exits 0. The FUSE log ends up in $RUNNER_TEMP/lazy-store/fuse.log.
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# Usage: lazy-store.sh INSTALLABLE...
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# Usage: lazy-store.sh closure FETCH
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# lazy-store.sh mount
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set -euo pipefail
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L=$RUNNER_TEMP/lazy-store
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mkdir -p "$L"
···
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fallback() {
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set +e
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echo "::error::lazy store: $1; the build substitutes eagerly instead"
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rm -f "$L/ready"
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if [ -s "$L/fuse.log" ]; then
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tail -n 50 "$L/fuse.log"
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fi
···
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}
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trap 'fallback "line $LINENO failed"' ERR
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nix-store --realise "$LAZY_STORE" >/dev/null || fallback "cannot get $LAZY_STORE"
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lazy=$LAZY_STORE/bin/lazy-store
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nixbin=$(dirname "$(readlink -f "$(command -v nix-store)")")
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nix build --dry-run "$@" 2>"$L/plan" || fallback "nix build --dry-run failed: $(cat "$L/plan")"
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# the path lines below the "will be fetched" header, up to the next header
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awk '/^[^ ]/ { f = /will be fetched/; next } f && $1 ~ /^\/nix\/store\// { print $1 }' \
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"$L/plan" >"$L/fetch"
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if [ ! -s "$L/fetch" ]; then
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echo "lazy store: nothing to substitute"
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if [ "$1" = closure ]; then
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if [ ! -s "$2" ]; then
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echo "lazy store: nothing to substitute"
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exit 0
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fi
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nix-store --realise "$LAZY_STORE" >/dev/null || fallback "cannot get $LAZY_STORE"
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"$lazy" closure "$L" <"$2" >/dev/null || fallback "closure failed"
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jq -r '"lazy store: \(.paths) of the \(.roots) paths to substitute and their closure (\(.file_size / 1048576 | floor) MiB to download eagerly); \(.missing | length) not in the caches, \(.incomplete) referring to those"' \
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"$L/closure.json"
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if [ "$(jq length "$L/paths.json")" != 0 ]; then
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touch "$L/ready"
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fi
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exit 0
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fi
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"$lazy" closure "$L" <"$L/fetch" >/dev/null || fallback "closure failed"
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jq -r '"lazy store: \(.paths) of the \(.roots) paths to substitute and their closure (\(.file_size / 1048576 | floor) MiB to download eagerly); \(.missing | length) not in the caches, \(.incomplete) referring to those"' \
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"$L/closure.json"
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if [ ! -e "$L/ready" ]; then
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exit 0
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fi
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n=$(jq length "$L/paths.json")
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if [ "$n" = 0 ]; then
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exit 0
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fi
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nixbin=$(dirname "$(readlink -f "$(command -v nix-store)")")
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sudo modprobe fuse || true
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# The upper layer is the real store dir, reached through a private bind
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.github/workflows/ci.yml
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.github/workflows/ci.yml
···
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run: |
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echo "not cached: $(jq -r '[.include[].name] | join(" ")' <<<"$MATRIX")" |
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tee -a "$GITHUB_STEP_SUMMARY"
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# the derivations the shared job builds, none for a single machine
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# the derivations the shared job builds and per machine the paths its
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# lazy store registers
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- id: plan
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if: steps.matrix.outputs.any-jobs == 'true'
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env:
···
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matrix=$RUNNER_TEMP/matrix.json
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plans=$RUNNER_TEMP/plans
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printf '%s' "$MATRIX" > "$matrix"
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mkdir -p "$plans"
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: > "$plans/shared.drvs"
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if [ "$(jq '.include | length' "$matrix")" -ge 2 ]; then
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.github/ci-shared.sh "$matrix" "$plans"
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{
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for name in $(jq -r '.include[].name' "$matrix"); do
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echo "- $name: $(wc -l < "$plans/$name.drvs") derivations to build"
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done
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echo "- shared (needed by >= 2 machines): $(wc -l < "$plans/shared.drvs")"
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sed -E 's|^/nix/store/[^-]+-(.*)\.drv$| \1|' "$plans/shared.drvs"
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} | tee -a "$GITHUB_STEP_SUMMARY"
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fi
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.github/ci-shared.sh "$matrix" "$plans"
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{
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for name in $(jq -r '.include[].name' "$matrix"); do
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echo "- $name: $(wc -l < "$plans/$name.drvs") derivations to build"
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done
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echo "- shared (needed by >= 2 machines): $(wc -l < "$plans/shared.drvs")"
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sed -E 's|^/nix/store/[^-]+-(.*)\.drv$| \1|' "$plans/shared.drvs"
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} | tee -a "$GITHUB_STEP_SUMMARY"
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if [ -s "$plans/shared.drvs" ]; then
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echo "shared=true" >> "$GITHUB_OUTPUT"
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fi
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- if: steps.plan.outputs.shared == 'true'
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- if: steps.matrix.outputs.any-jobs == 'true'
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uses: actions/upload-artifact@v4
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with:
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name: plans
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path: ${{ runner.temp }}/plans/shared.drvs
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path: |
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${{ runner.temp }}/plans/shared.drvs
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${{ runner.temp }}/plans/*.fetch
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shared:
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needs: eval
···
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run: |
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nix-store --realise "$NIKS3"
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echo "$NIKS3/bin" >> "$GITHUB_PATH"
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- uses: actions/download-artifact@v4
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with:
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name: plans
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path: ${{ runner.temp }}/plans
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- name: build ${{ matrix.name }}
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env:
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NAME: ${{ matrix.name }}
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INSTALLABLES: ${{ matrix.installables }}
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LAZY_STORE: ${{ needs.eval.outputs.lazy-store }}
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run: |
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read -ra installables <<<"$INSTALLABLES"
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# the closure needs only the caches, so it runs while the drvs are
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# fetched
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.github/lazy-store.sh closure "$RUNNER_TEMP/plans/$NAME.fetch" &
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closure=$!
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"$HESTIA_BIN" prefetch "${installables[@]}"
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.github/lazy-store.sh "${installables[@]}"
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wait "$closure"
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.github/lazy-store.sh mount
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.github/ci-build.sh "${installables[@]}"
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- name: lazy store log
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if: always()