fix: fail a truth diff that compared nothing

scripts/diff_truth.py

@@ -179,26 +179,23 @@ def main() -> None:
 
     reports: list[dict] = []
     formatter = _format_markdown if args.format == "markdown" else _format_terminal
+    compared = 0
 
     for platform in platforms:
         truth = _load_truth(args.truth_dir, platform)
         if truth is None:
-            if not args.json_output:
-                print(
-                    f"skip {platform}: no truth YAML in {args.truth_dir}/",
-                    file=sys.stderr,
-                )
+            print(f"skip {platform}: no truth YAML in {args.truth_dir}/", file=sys.stderr)
             continue
 
         try:
             scraped = load_platform_config(platform, args.platforms_dir)
         except FileNotFoundError:
-            if not args.json_output:
-                print(f"skip {platform}: no scraped config", file=sys.stderr)
+            print(f"skip {platform}: no scraped config", file=sys.stderr)
             continue
 
         report = diff_platform_truth(truth, scraped)
         report["platform"] = platform
+        compared += 1
 
         if args.json_output:
             reports.append(report)
@@ -210,6 +207,10 @@ def main() -> None:
         json.dump(reports, sys.stdout, indent=2)
         print()
 
+    if compared < len(platforms) and (args.platform or not compared):
+        print(f"compared {compared} of {len(platforms)} platform(s)", file=sys.stderr)
+        sys.exit(1)
+
 
 if __name__ == "__main__":
     main()

tests/test_truth_diff.py

@@ -56,6 +56,25 @@ class ATargetedModelIsItsOwnArtifact(unittest.TestCase):
             )
 
 
+class ADiffThatComparedNothingFails(unittest.TestCase):
+    """diff_truth --all skipped every platform without a model and exited 0."""
+
+    def test_an_empty_truth_directory_is_an_error(self):
+        import subprocess
+        import tempfile
+
+        repo = pathlib.Path(__file__).resolve().parent.parent
+        with tempfile.TemporaryDirectory(dir=str(repo / "tmp")) as directory:
+            for selection in (["--all"], ["--platform", "retroarch"]):
+                proc = subprocess.run(
+                    [sys.executable, "scripts/diff_truth.py", *selection,
+                     "--truth-dir", directory, "--json"],
+                    capture_output=True, text=True, cwd=str(repo), timeout=120,
+                )
+                self.assertEqual(proc.returncode, 1, selection)
+                self.assertIn("skip", proc.stderr)
+
+
 class RenameMatching(unittest.TestCase):
     def test_a_shared_sha1_pairs_the_two_names(self):
         truth = [_entry("bios_CD_U.bin", sha1="a" * 40)]