test: reject malformed coverage metrics

scripts/check_coverage_thresholds.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import argparse
 import json
+import math
 from pathlib import Path
 from typing import Any
 
@@ -23,7 +24,11 @@ def validate_coverage(
     )
     for label, key, minimum in metrics:
         value = totals.get(key)
-        if not isinstance(value, (int, float)):
+        if (
+            isinstance(value, bool)
+            or not isinstance(value, (int, float))
+            or not math.isfinite(value)
+        ):
             failures.append(f"coverage report is missing numeric {key}")
         elif value < minimum:
             failures.append(

tests/test_coverage_thresholds.py

@@ -43,3 +43,16 @@ def test_validate_coverage_rejects_malformed_totals():
     assert MODULE.validate_coverage(
         {}, min_line=49.0, min_branch=36.0,
     ) == ["coverage report is missing the totals object"]
+
+
+def test_validate_coverage_rejects_non_numeric_metrics():
+    report = _report()
+    report["totals"]["percent_statements_covered"] = True
+    report["totals"]["percent_branches_covered"] = float("nan")
+
+    assert MODULE.validate_coverage(
+        report, min_line=49.0, min_branch=36.0,
+    ) == [
+        "coverage report is missing numeric percent_statements_covered",
+        "coverage report is missing numeric percent_branches_covered",
+    ]