feat: read out flake config if a flake

action.yml

@@ -45,6 +45,15 @@ inputs:
     description: |
       Don't use. For bootstrapping purposes only.
 
+  load_nixConfig:
+    default: true
+    description: |
+      Reads out a flakes 'nixConfig' attribute, if there is one, and loads
+      it into the environment config for old-sytle commands to pick it up.
+
+      If this cause undesired behaviour, you can switch it off by setting
+      this to false.
+
 
 runs:
   using: "composite"
@@ -60,6 +69,15 @@ runs:
         NIX_ON_TMPFS: ${{ inputs.nix_on_tmpfs }}
         GITHUB_ACCESS_TOKEN: ${{ inputs.github_access_token }}
 
+    - name: Preload nixConfig from Flake
+      if: ${{ inputs.load_nixConfig }}
+      run: ${{ github.action_path }}/read-nix-config-from-flake.sh
+      shell: bash
+      env:
+        NIX_VERSION: ${{ inputs.nix_version }}
+        GH_TOKEN: ${{ inputs.github_access_token }} # used by gh cli
+        OWNER_AND_REPO: ${{ github.repository }}
+        SHA: ${{ github.sha }}
 
 branding:
   icon: zap

nix-quick-install.sh

@@ -3,6 +3,8 @@
 set -eu
 set -o pipefail
 
+source "${BASH_SOURCE[0]%/*}/vercomp.sh"
+
 # Figure out system type (TODO don't assume x86_64)
 case "$OSTYPE" in
   darwin*)
@@ -73,6 +75,15 @@ if [[ -n "${GITHUB_ACCESS_TOKEN:-}" ]]; then
     "access-tokens = github.com=$GITHUB_ACCESS_TOKEN"
 fi
 
+# Setup Flakes
+if vergt "$NIX_VERSION" "2.13"; then
+  echo >>"$NIX_CONF_FILE" \
+    "experimental-features = nix-command flakes"
+  echo >>"$NIX_CONF_FILE" \
+    "accept-flake-config = true"
+fi
+
+
 # Populate the nix db
 nix="$(readlink /nix/var/nix-quick-install-action/nix)"
 "$nix/bin/nix-store" --load-db < /nix/var/nix-quick-install-action/registration

nix_config.nix

@@ -0,0 +1,46 @@
+with builtins; let
+  boolToString = b:
+    if b
+    then "true"
+    else "false";
+
+  /*
+  Check whether a value can be coerced to a string.
+  The value must be a string, path, or attribute set.
+
+  String-like values can be used without explicit conversion in
+  string interpolations and in most functions that expect a string.
+  */
+  isStringLike = x:
+    isString x
+    || isPath x
+    || x ? outPath
+    || x ? __toString;
+
+  mapAttrsToList =
+    # A function, given an attribute's name and value, returns a new value.
+    f:
+    # Attribute set to map over.
+    attrs:
+      map (name: f name attrs.${name}) (attrNames attrs);
+
+  mkValueString = v:
+    if v == null
+    then ""
+    else if isInt v
+    then toString v
+    else if isBool v
+    then boolToString v
+    else if isFloat v
+    then toString v
+    else if isList v
+    then concatStringsSep " " v
+    else if isStringLike v
+    then v
+    else "";
+
+  mkKeyValue = k: v: "${k} = ${mkValueString v}";
+
+  mkKeyValuePairs = attrs: concatStringsSep "\n" (mapAttrsToList mkKeyValue attrs);
+in
+  mkKeyValuePairs

read-nix-config-from-flake.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+set -eu
+set -o pipefail
+
+source "${BASH_SOURCE[0]%/*}/vercomp.sh"
+if verlte "$NIX_VERSION" "2.13"; then
+  echo "Do not load flake config on nix version <= 2.13"
+  exit 0
+fi
+
+NIX_CONF_FILE="${XDG_CONFIG_HOME:-$HOME/.config}/nix/nix.conf"
+
+declare nix_conf
+tmp="$(mktemp)"
+
+flake_file=${flake_file:="$tmp"}
+flake_url=${flake_url:="github:$OWNER_AND_REPO/$SHA"}
+
+# only fetch if not (locally) defined (e.g. for testing)
+if [ "$flake_file" = "$tmp" ]; then
+  set -x
+  gh api \
+    -H "Accept: application/vnd.github+json" \
+    -H "X-GitHub-Api-Version: 2022-11-28" \
+    "/repos/$OWNER_AND_REPO/contents/flake.nix?ref=$SHA" |
+    jq -r '.content|gsub("[\n\t]"; "")|@base64d' >"$flake_file"
+  set +x
+fi
+
+nix_conf="$(mktemp -d)/flake-nixConfig.conf"
+NIX_CONFIG=$(nix --experimental-features "nix-command" eval --raw --impure --expr '(import '"$flake_file"').nixConfig or {}' --apply "$(<"${BASH_SOURCE[0]%/*}/nix_config.nix")" | tee "$nix_conf")
+
+NIX_USER_CONF_FILES="$nix_conf:$NIX_CONF_FILE:${NIX_USER_CONF_FILES:-}"
+
+echo "NIX_USER_CONF_FILES=$NIX_USER_CONF_FILES" >> $GITHUB_ENV

vercomp.sh

@@ -0,0 +1,49 @@
+# taken from: https://stackoverflow.com/a/4025065
+vercomp () {
+    if [[ $1 == $2 ]]
+    then
+        return 0
+    fi
+    local IFS=.
+    local i ver1=($1) ver2=($2)
+    # fill empty fields in ver1 with zeros
+    for ((i=${#ver1[@]}; i<${#ver2[@]}; i++))
+    do
+        ver1[i]=0
+    done
+    for ((i=0; i<${#ver1[@]}; i++))
+    do
+        if [[ -z ${ver2[i]} ]]
+        then
+            # fill empty fields in ver2 with zeros
+            ver2[i]=0
+        fi
+        if ((10#${ver1[i]} > 10#${ver2[i]}))
+        then
+            return 1
+        fi
+        if ((10#${ver1[i]} < 10#${ver2[i]}))
+        then
+            return 2
+        fi
+    done
+    return 0
+}
+
+vergt() {
+    vercomp $1 $2
+    case $? in
+        0) return 1;;
+        1) return 0;;
+        2) return 1;;
+    esac
+}
+
+verlte() {
+    vercomp $1 $2
+    case $? in
+        0) return 0;;
+        1) return 1;;
+        2) return 0;;
+    esac
+}