ci: Add a way to run libm tests that would otherwise be skipped

As part of this, commonize the way to get a `PrInfo` object from
environment variables.

Author: tgross35

ci/ci-util.py

@@ -59,6 +59,8 @@
 # accidentally spending huge amounts of CI time.
 ALLOW_MANY_EXTENSIVE_DIRECTIVE = "ci: allow-many-extensive"
 MANY_EXTENSIVE_THRESHOLD = 20
+# Require tests for `libm`
+TEST_LIBM_DIRECTIVE = "ci: test-libm"
 
 # Don't run exhaustive tests if these files change, even if they contaiin a function
 # definition.
@@ -89,9 +91,25 @@ class PrInfo:
     created_at: str
     number: int
 
+    @classmethod
+    def from_env(cls, gh_ref: str | None = None) -> Self | None:
+        """Create a PR object from the PR_NUMBER environment if set, using a ref
+        as a fallback. Returns `None` is there is no PR found.
+        """
+        pr_env = os.environ.get("PR_NUMBER")
+        if pr_env is not None and len(pr_env) > 0:
+            return cls(*PrInfo.load(pr_env))
+
+        if gh_ref is None:
+            return None
+
+        # Split `refs/pull/1234/merge`
+        pr_number = int(gh_ref.split("/")[2])
+        return cls(*PrInfo.load(pr_number))
+
     @classmethod
     def load(cls, pr_number: int | str) -> Self:
-        """For a given PR number, query the body and commit list"""
+        """For a given PR number, query the body and commit list."""
         pr_info = sp.check_output(
             [
                 "gh",
@@ -207,22 +225,31 @@ def may_skip_libm_ci(self) -> bool:
         """If this is a PR and no libm files were changed, allow skipping libm
         jobs."""
 
-        if self.is_pr():
-            return all(not re.match(TRIGGER_LIBM_PR_CI, str(f)) for f in self.changed)
+        # Always run on merge CI
+        if not self.is_pr():
+            return False
+
+        pr = PrInfo.from_env()
+        if pr is None:
+            eprint("Is a PR but couldn't load PrInfo")
+            exit(1)
+
+        # Allow opting in to libm tests
+        if pr.contains_directive(TEST_LIBM_DIRECTIVE):
+            return True
 
-        return False
+        return all(not re.match(TRIGGER_LIBM_PR_CI, str(f)) for f in self.changed)
 
     def emit_workflow_output(self):
         """Create a JSON object a list items for each type's changed files, if any
         did change, and the routines that were affected by the change.
         """
 
-        pr_number = os.environ.get("PR_NUMBER")
         skip_tests = False
         error_on_many_tests = False
 
-        if pr_number is not None and len(pr_number) > 0:
-            pr = PrInfo.load(pr_number)
+        pr = PrInfo.from_env()
+        if pr is not None:
             skip_tests = pr.contains_directive(SKIP_EXTENSIVE_DIRECTIVE)
             error_on_many_tests = not pr.contains_directive(
                 ALLOW_MANY_EXTENSIVE_DIRECTIVE