fix: read the PR number from the event payload, not only from GITHUB_REF

src/config.py

@@ -176,6 +176,40 @@ def _env_required(name: str) -> str:
     return val
 
 
+def _pr_number_from_event() -> int | None:
+    """Read the PR number from the event payload GitHub writes for the run.
+
+    Every pull_request payload carries `.pull_request.number`, for every
+    activity type, which GITHUB_REF does not.
+
+    Returns None rather than raising for anything unexpected — no payload path,
+    an unreadable or malformed file, a payload for some other event, a number
+    that is not one. The caller still has GITHUB_REF to fall back on, and a
+    raise here would turn a recoverable case into a failed run.
+
+    Returns:
+        The PR number, or None if the payload does not yield one.
+    """
+    path = os.environ.get("GITHUB_EVENT_PATH")
+    if not path:
+        return None
+    try:
+        with open(path, encoding="utf-8") as handle:
+            payload = json.load(handle)
+    except (OSError, ValueError):
+        return None
+    if not isinstance(payload, dict):
+        return None
+    pull_request = payload.get("pull_request")
+    if not isinstance(pull_request, dict):
+        return None
+    number = pull_request.get("number")
+    # bool is an int subclass, and `true` is valid JSON in that position.
+    if isinstance(number, bool) or not isinstance(number, int):
+        return None
+    return number
+
+
 @dataclass(frozen=True)
 class Config:
     """Parsed and validated configuration from GitHub Actions inputs.
@@ -305,10 +339,15 @@ def parse_config() -> Config:
     repo = _env_required("GITHUB_REPOSITORY")
     event_name = os.environ.get("GITHUB_EVENT_NAME", "unknown")
 
-    # Extract PR number from GITHUB_REF (refs/pull/<number>/merge)
-    pr_number = None
+    # The event payload first, GITHUB_REF only as a fallback. Every
+    # pull_request payload carries .pull_request.number, whereas GITHUB_REF is
+    # refs/pull/<number>/merge only while GitHub has a merge ref to point the
+    # run at. When it points elsewhere the run used to abort with "Could not
+    # determine PR number" having measured nothing, which reads to the author
+    # as a coverage verdict on code the action never looked at.
+    pr_number = _pr_number_from_event()
     github_ref = os.environ.get("GITHUB_REF", "")
-    match = re.search(r"refs/pull/(\d+)/", github_ref)
+    match = None if pr_number is not None else re.search(r"refs/pull/(\d+)/", github_ref)
     if match:
         pr_number = int(match.group(1))
 

src/main.py

@@ -197,7 +197,11 @@ def main() -> None:
         return
 
     if config.pr_number is None:
-        print("::error::Could not determine PR number from GITHUB_REF.")
+        print(
+            "::error::Could not determine the PR number from either the event payload "
+            "at GITHUB_EVENT_PATH or GITHUB_REF. Nothing was measured, so this is not a "
+            "test-adequacy verdict."
+        )
         sys.exit(1)
 
     try:

tests/test_config.py

@@ -1,6 +1,8 @@
 # pyright: reportUnknownParameterType=false, reportMissingParameterType=false, reportUnknownMemberType=false
 """Tests for configuration parsing."""
 
+import json
+
 import pytest
 
 from src.config import parse_config
@@ -217,9 +219,82 @@ def test_pr_number_from_env(self, monkeypatch):
         monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
         monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
         monkeypatch.setenv("GITHUB_REF", "refs/pull/42/merge")
+        # This suite runs inside Actions too, where GITHUB_EVENT_PATH points at a
+        # real payload naming a real PR. Without this the assertion below would
+        # measure that PR instead of the ref under test.
+        monkeypatch.delenv("GITHUB_EVENT_PATH", raising=False)
         cfg = parse_config()
         assert cfg.pr_number == 42
 
+    def test_pr_number_from_event_payload_when_the_ref_has_none(self, monkeypatch, tmp_path):
+        # The case the action used to abort on: a pull_request event whose ref is
+        # not refs/pull/<n>/merge. Observed on the `edited` activity type, where
+        # the run failed with "Could not determine PR number" without having
+        # measured anything.
+        payload = tmp_path / "event.json"
+        payload.write_text(json.dumps({"pull_request": {"number": 180}}), encoding="utf-8")
+        monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
+        monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
+        monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
+        monkeypatch.setenv("GITHUB_REF", "refs/heads/main")
+        monkeypatch.setenv("GITHUB_EVENT_PATH", str(payload))
+        assert parse_config().pr_number == 180
+
+    def test_the_event_payload_wins_over_the_ref(self, monkeypatch, tmp_path):
+        # The payload describes the event being processed; the ref is a pointer
+        # that may lag it. When they disagree, the payload is the one to trust.
+        payload = tmp_path / "event.json"
+        payload.write_text(json.dumps({"pull_request": {"number": 180}}), encoding="utf-8")
+        monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
+        monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
+        monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
+        monkeypatch.setenv("GITHUB_REF", "refs/pull/42/merge")
+        monkeypatch.setenv("GITHUB_EVENT_PATH", str(payload))
+        assert parse_config().pr_number == 180
+
+    @pytest.mark.parametrize(
+        "payload_text",
+        [
+            pytest.param('{"pull_request": {"number": ', id="truncated json"),
+            pytest.param("not json at all", id="not json"),
+            pytest.param('{"push": {"ref": "refs/heads/main"}}', id="another event"),
+            pytest.param('{"pull_request": null}', id="null pull_request"),
+            pytest.param('{"pull_request": {"number": "180"}}', id="number as a string"),
+            pytest.param('{"pull_request": {"number": true}}', id="number as a boolean"),
+            pytest.param('{"pull_request": {}}', id="no number"),
+            pytest.param("[]", id="payload is a list"),
+        ],
+    )
+    def test_an_unusable_event_payload_falls_back_to_the_ref(
+        self, monkeypatch, tmp_path, payload_text
+    ):
+        # None of these may raise. Each still has a usable ref, and a raise here
+        # would turn a recoverable run into a failed one.
+        payload = tmp_path / "event.json"
+        payload.write_text(payload_text, encoding="utf-8")
+        monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
+        monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
+        monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
+        monkeypatch.setenv("GITHUB_REF", "refs/pull/42/merge")
+        monkeypatch.setenv("GITHUB_EVENT_PATH", str(payload))
+        assert parse_config().pr_number == 42
+
+    def test_a_missing_event_file_falls_back_to_the_ref(self, monkeypatch, tmp_path):
+        monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
+        monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
+        monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
+        monkeypatch.setenv("GITHUB_REF", "refs/pull/42/merge")
+        monkeypatch.setenv("GITHUB_EVENT_PATH", str(tmp_path / "absent.json"))
+        assert parse_config().pr_number == 42
+
+    def test_no_pr_number_when_neither_source_has_one(self, monkeypatch):
+        monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
+        monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")
+        monkeypatch.setenv("GITHUB_EVENT_NAME", "push")
+        monkeypatch.setenv("GITHUB_REF", "refs/heads/main")
+        monkeypatch.delenv("GITHUB_EVENT_PATH", raising=False)
+        assert parse_config().pr_number is None
+
     def test_coverage_threshold_range_validation(self, monkeypatch):
         monkeypatch.setenv("GITHUB_TOKEN", "ghp_fake")
         monkeypatch.setenv("GITHUB_REPOSITORY", "owner/repo")