ci: build newly added variants in the PR matrix (#11549)

* ci: build newly added variants in the PR matrix

A new board declares board_level = release, so it gets no CI build until
after merge. Build the first env of each platformio.ini added by a PR,
regardless of board_level.

Only added files qualify; adding an env to an existing config does not.

* ci: also detect added variants in merge_group runs

merge_group uses the same --level pr subset as pull_request, so a newly
added variant was skipped there. Derive the diff base from
github.event.merge_group.base_sha for those runs.

* ci: fail the matrix step when the variant diff errors

Process substitution hides the exit status, so a failed diff silently
yielded an empty list and dropped the new board from the matrix. Capture
into a variable so 'set -e' aborts the step instead.

.github/workflows/main_matrix.yml

@@ -55,6 +55,9 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v7
+        with:
+          # Needed to diff against the base branch for newly added variants.
+          fetch-depth: 0
       - uses: actions/setup-python@v6
         with:
           python-version: 3.x
@@ -62,11 +65,33 @@ jobs:
       - run: pip install -U platformio
       - name: Generate matrix
         id: jsonStep
+        env:
+          BASE_REF: ${{ github.base_ref }}
+          MERGE_GROUP_BASE_SHA: ${{ github.event.merge_group.base_sha }}
         run: |
+          # A new board is 'release' and gets no CI until after merge, so force-build the
+          # first env of each ADDED variant config. A new env in an existing one does not count.
+          DIFF_BASE=""
+          if [[ "$GITHUB_EVENT_NAME" == "pull_request" ]]; then
+            git fetch --no-tags --depth=1 origin "$BASE_REF"
+            DIFF_BASE=$(git merge-base FETCH_HEAD HEAD)
+          elif [[ "$GITHUB_EVENT_NAME" == "merge_group" ]]; then
+            DIFF_BASE="$MERGE_GROUP_BASE_SHA"
+          fi
+          ADDED_ARGS=()
+          if [[ -n "$DIFF_BASE" ]]; then
+            # Assign rather than pipe: a failing diff must abort the step under 'set -e',
+            # not silently yield an empty list and drop the new board from the matrix.
+            ADDED_CONFIGS=$(git diff --name-only --diff-filter=A \
+              "$DIFF_BASE" HEAD -- 'variants/**/platformio.ini')
+            while IFS= read -r cfg; do
+              [[ -n "$cfg" ]] && ADDED_ARGS+=(--added-config "$cfg")
+            done <<<"$ADDED_CONFIGS"
+          fi
           # PRs and (for now) merge_group builds use the narrowed --level pr board
           # subset. Full-matrix builds run on push / schedule / workflow_dispatch.
           if [[ "$GITHUB_EVENT_NAME" == "pull_request" || "$GITHUB_EVENT_NAME" == "merge_group" ]]; then
-            TARGETS=$(./bin/generate_ci_matrix.py all --level pr)
+            TARGETS=$(./bin/generate_ci_matrix.py all --level pr "${ADDED_ARGS[@]}")
           else
             TARGETS=$(./bin/generate_ci_matrix.py all)
           fi

bin/generate_ci_matrix.py

@@ -23,10 +23,29 @@ parser.add_argument(
   default=[],
   help="Board level to build for (omit for the 'pr' + 'release' matrix)",
 )
+parser.add_argument(
+  "--added-config",
+  action="append",
+  default=[],
+  metavar="PATH",
+  help="platformio.ini added by this PR; its first env is built regardless of board_level",
+)
 args = parser.parse_args()
 
 outlist = []
 
+# A brand-new board is normally 'release', so it would get no CI until after merge.
+# Build the first env of each newly added config so it is compiled at least once.
+forced_envs = set()
+for added_path in args.added_config:
+  try:
+    with open(added_path, encoding="utf-8") as added_file:
+      first_env = re.search(r"^[ \t]*\[env:([^\]]+)\]", added_file.read(), re.MULTILINE)
+  except OSError:
+    continue
+  if first_env:
+    forced_envs.add(first_env.group(1).strip())
+
 cfg = ProjectConfig.get_instance()
 pio_envs = cfg.envs()
 
@@ -69,6 +88,9 @@ for env in all_envs:
     # Always include board_level = 'pr'
     if env["board_level"] == "pr":
       outlist.append(env["ci"])
+    # Include the first env of a platformio.ini added by this PR
+    elif env["ci"]["board"] in forced_envs:
+      outlist.append(env["ci"])
     # Include board_level = 'extra' when requested
     elif "extra" in args.level and env["board_level"] == "extra":
       outlist.append(env["ci"])