ci: cache gnu32 nix store

Author: Sjors

.github/workflows/ci.yml

@@ -131,6 +131,10 @@ jobs:
     runs-on: ubuntu-latest
 
     env:
+      NIXPKGS_CHANNEL: nixos-25.05
+      NIX_EXTRA_CONFIG: |
+        keep-env-derivations = true
+        keep-outputs = true
       NIX_EXTRA_CONFIG_ACT: |
         sandbox = false
         filter-syscalls = false
@@ -145,14 +149,35 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
+      - name: Determine CI configuration
+        id: config
+        env:
+          CI_CONFIG: ci/configs/${{ matrix.config }}.bash
+        run: ci/scripts/config.sh
+
       - name: Install Nix
         uses: cachix/install-nix-action@v31       # 2025-05-27, from https://github.com/cachix/install-nix-action/tags
         with:
-          nix_path: nixpkgs=channel:nixos-25.05   # latest release
+          nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/${{ steps.config.outputs.nixpkgs_rev }}.tar.gz
           # Act executes inside an unprivileged container (Docker or Podman),
           # so KVM support isn't available.
           enable_kvm: "${{ github.actor != 'nektos/act' }}"
-          extra_nix_config: ${{ github.actor == 'nektos/act' && env.NIX_EXTRA_CONFIG_ACT || '' }}
+          extra_nix_config: |
+            ${{ env.NIX_EXTRA_CONFIG }}
+            ${{ github.actor == 'nektos/act' && env.NIX_EXTRA_CONFIG_ACT || '' }}
+
+      # Cache the heaviest Nix job to stay within GitHub's cache budget while
+      # still avoiding repeated gnu32 cross-toolchain downloads and builds.
+      - name: Cache Nix store
+        if: steps.config.outputs.cache_nix_store == 'true'
+        uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ matrix.config }}-${{ steps.config.outputs.nixpkgs_rev }}-${{ hashFiles('shell.nix', 'ci/patches/*.patch', format('ci/configs/{0}.bash', matrix.config)) }}
+          restore-prefixes-first-match: |
+            nix-${{ runner.os }}-${{ matrix.config }}-${{ steps.config.outputs.nixpkgs_rev }}-
+            nix-${{ runner.os }}-${{ matrix.config }}-
+            nix-${{ runner.os }}-
+          gc-max-store-size-linux: 10G
 
       - name: Run CI script
         env:

ci/configs/gnu32.bash

@@ -1,5 +1,6 @@
 CI_DESC="CI job cross-compiling to 32-bit"
 CI_DIR=build-gnu32
+CI_CACHE_NIX_STORE=true
 NIX_ARGS=(
   --arg minimal true
   --arg crossPkgs 'import <nixpkgs> { crossSystem = { config = "i686-unknown-linux-gnu"; }; }'

ci/scripts/config.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Source CI configuration and output variables needed by the workflow.
+
+set -o errexit -o nounset -o pipefail -o xtrace
+
+readonly SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+
+source "${SCRIPT_DIR}/ci_helpers.sh"
+
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+
+# Resolve the nixpkgs channel to a specific revision for use in cache keys.
+if [[ -n "${NIXPKGS_CHANNEL:-}" ]]; then
+  rev="$(curl --fail --location --silent --show-error "https://channels.nixos.org/${NIXPKGS_CHANNEL}/git-revision")"
+  test -n "${rev}"
+  write_output_var nixpkgs_rev "${rev}"
+fi
+
+write_output_var cache_nix_store "${CI_CACHE_NIX_STORE:-false}"

ci/scripts/run.sh

@@ -11,3 +11,13 @@ set -o errexit -o nounset -o pipefail -o xtrace
 [ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
 
 nix develop --ignore-environment --keep CI_CONFIG --keep CI_CLEAN "${NIX_ARGS[@]+"${NIX_ARGS[@]}"}" -f shell.nix --command ci/scripts/ci.sh
+
+# Create a GC root for the shell closure so the cache-nix-action save step
+# does not garbage-collect it.
+if [[ "${CI_CACHE_NIX_STORE:-}" == "true" ]]; then
+  mkdir -p .nix-gc-roots
+  nix-build shell.nix \
+    -o .nix-gc-roots/shell \
+    "${NIX_ARGS[@]+"${NIX_ARGS[@]}"}"
+  nix-store --query --requisites .nix-gc-roots/shell >/dev/null
+fi