discover: allow result to be uploaded to Nix cache

discover/action.yml

@@ -2,6 +2,10 @@ name: "Standard Discovery"
 description: "Find out what to build & run in a Standard project."
 
 inputs:
+  cache:
+    description: "Nix cache to upload evaluation results to."
+    required: false
+    default: auto
   github_pat:
     description: "GitHub token for accessing private repos."
     required: false
@@ -52,6 +56,7 @@ runs:
       id: eval
       env:
         FLAKE: "github:${{ github.repository }}/${{ github.sha }}"
+        CACHE: ${{ inputs.cache }}
       run: |
         ${{ github.action_path }}/eval.sh
       shell: bash

discover/eval.sh

@@ -2,15 +2,16 @@
 
 set -e
 
-declare JSON
+declare JSON result
 
 function eval() {
   echo "::group::Nix Evaluation"
 
   local system delim
 
   system="$(nix eval --raw --impure --expr 'builtins.currentSystem')"
-  JSON="$(nix eval "$FLAKE#__std.ci'.$system" --json | jq -c '
+  result=$(nix eval "$FLAKE#__std.ci'.$system" --json)
+  JSON="$(jq -c '
       group_by(.block)
       | map({
         key: .[0].block,
@@ -23,7 +24,7 @@ function eval() {
           | from_entries
         )
       })
-      | from_entries'
+      | from_entries' <<< "$result"
   )"
 
   nix_conf=("$(nix eval --raw "$FLAKE#__std.nixConfig")")
@@ -42,4 +43,23 @@ function eval() {
   echo "::endgroup::"
 }
 
+function cache() {
+  echo "::group::Cache Evaluation"
+
+  local drvs
+
+  drvs=$(jq -r '.[]|select(.targetDrv != null)|.targetDrv' <<< "$result")
+
+  if [[ -n $drvs ]]; then
+    #shellcheck disable=SC2086
+    nix copy --derivation --to "$CACHE" $drvs
+  fi
+
+  echo "::endgroup::"
+}
+
 eval
+
+if [[ $CACHE != 'auto' ]]; then
+  cache
+fi