ci: only build required plugins in pull requests

Author: shdwmtr

.github/workflows/pr.yml

@@ -32,11 +32,44 @@ jobs:
           path: scripts/build/
           retention-days: 1
 
+      - name: Determine changed plugins
+        id: changed-plugins
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          set -euo pipefail
+
+          if [[ "${{ github.event_name }}" != "pull_request" ]]; then
+            echo "Not a pull_request event; building all plugins."
+            echo "apply-filter=false" >> "$GITHUB_OUTPUT"
+            echo "changed-plugins=" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          changed=$(gh api "repos/$REPO/pulls/$PR_NUMBER/files" --paginate \
+            --jq '.[] | select(.filename | startswith("plugins/")) | .filename' \
+            | awk -F/ '{print $1"/"$2}' | sort -u)
+
+          echo "Changed plugin paths:"
+          echo "${changed:-<none>}"
+
+          echo "apply-filter=true" >> "$GITHUB_OUTPUT"
+          {
+            echo "changed-plugins<<EOF"
+            echo "$changed"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
+
       - name: Discover submodules
         id: discover-submodules
+        env:
+          APPLY_FILTER: ${{ steps.changed-plugins.outputs.apply-filter }}
+          CHANGED_PLUGINS: ${{ steps.changed-plugins.outputs.changed-plugins }}
         run: |
           sudo bash ./scripts/_gen_metadata.sh
-          sudo bash ./scripts/_submodules.sh > submodules.json
+          sudo -E bash ./scripts/_submodules.sh > submodules.json
           cat submodules.json
 
           echo "submodule-matrix=$(cat submodules.json)" >> $GITHUB_OUTPUT

scripts/_submodules.sh

@@ -4,7 +4,11 @@ submodules=()
 while read -r line; do
     sha=$(echo "$line" | awk '{print $1}')
     name=$(echo "$line" | awk '{print $2}')
-    
+
+    if [[ "${APPLY_FILTER:-false}" == "true" ]] && ! grep -qxF "$name" <<< "${CHANGED_PLUGINS:-}"; then
+        continue
+    fi
+
     url=$(git config --get submodule."$name".url)
     if [[ -n "$url" ]]; then
         owner=$(echo "$url" | sed -E 's|https://github.com/([^/]+)/.*|\1|')