Re-run tests serially if parallel testing failed

Use the same approach that we’ve taken in SourceKit-LSP to capture more actionable output.

Author: ahoppen

Utilities/build-script-helper.py

@@ -150,8 +150,15 @@ def run_tests(swift_exec: str, args: argparse.Namespace) -> None:
     print('Cleaning ' + tests)
     shutil.rmtree(tests, ignore_errors=True)
 
-    cmd = [swift_exec, 'test', '--parallel', '--test-product', 'IndexStoreDBPackageTests'] + swiftpm_args
-    check_call(cmd, additional_env=additional_env, verbose=args.verbose)
+    cmd = [swift_exec, 'test', '--test-product', 'IndexStoreDBPackageTests'] + swiftpm_args
+    try:
+        check_call(cmd + ['--parallel'], additional_env=additional_env, verbose=args.verbose)
+    except:
+        print('--- Running tests in parallel failed. Re-running tests serially to capture more actionable output.')
+        sys.stdout.flush()
+        check_call(cmd, additional_env=additional_env, verbose=args.verbose)
+        # Return with non-zero exit code even if serial test execution succeeds.
+        raise SystemExit(1)
 
 
 def handle_invocation(swift_exec: str, args: argparse.Namespace) -> None: