Add a test for the output of a binary signaling an error

tests/BUILD

@@ -138,3 +138,28 @@ py_test(
         "@io_abseil_py//absl/testing:absltest",
     ],
 )
+
+elisp_binary(
+    name = "signal",
+    src = "signal.el",
+)
+
+py_test(
+    name = "signal_test",
+    size = "medium",
+    timeout = "short",
+    srcs = ["signal_test.py"],
+    args = [
+        # See https://github.com/bazelbuild/bazel/issues/12313 why we need to
+        # add additional quoting.
+        shell.quote("--binary=$(rlocationpath :signal)"),
+    ],
+    data = [":signal"],
+    python_version = "PY3",
+    srcs_version = "PY3",
+    deps = [
+        "//elisp:runfiles",
+        "@io_abseil_py//absl/flags",
+        "@io_abseil_py//absl/testing:absltest",
+    ],
+)

tests/signal.el

@@ -0,0 +1,25 @@
+;;; signal.el --- signal an error -*- lexical-binding: t; -*-
+
+;; Copyright 2023, 2024 Google LLC
+;;
+;; Licensed under the Apache License, Version 2.0 (the "License");
+;; you may not use this file except in compliance with the License.
+;; You may obtain a copy of the License at
+;;
+;;     https://www.apache.org/licenses/LICENSE-2.0
+;;
+;; Unless required by applicable law or agreed to in writing, software
+;; distributed under the License is distributed on an "AS IS" BASIS,
+;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+;; See the License for the specific language governing permissions and
+;; limitations under the License.
+
+;;; Commentary:
+
+;; This is a trivial binary that only signals an error.
+
+;;; Code:
+
+(error "Foo")
+
+;;; signal.el ends here

tests/signal_test.py

@@ -0,0 +1,76 @@
+# Copyright 2020, 2021, 2022, 2023, 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for an Emacs Lisp binary that signals an error."""
+
+import pathlib
+import platform
+import subprocess
+
+from absl import flags
+from absl.testing import absltest
+
+from elisp import runfiles
+
+FLAGS = flags.FLAGS
+
+flags.DEFINE_string(
+    'binary', None, 'runfile location of the Emacs Lisp binary to test',
+    required=True)
+
+class EmptyTest(absltest.TestCase):
+    """Unit tests for an empty Emacs Lisp binary."""
+
+    def test_run(self) -> None:
+        """Tests that the binary exits as expected.
+
+        These tests test some implementation details to catch some unwanted
+        behavior like launcher warnings on standard error.
+        """
+        run_files = runfiles.Runfiles()
+        binary = run_files.resolve(pathlib.PurePosixPath(FLAGS.binary))
+        process = subprocess.run([str(binary)], env=run_files.environment(),
+                                 stdin=subprocess.DEVNULL, capture_output=True,
+                                 check=False, encoding='utf-8')
+        # Emacs exits with a code of −1 in case of a signal, which gets
+        # converted to an exit status with all bits set: 8 bits on Unix systems,
+        # 32 bits on Windows.
+        self.assertEqual(process.returncode,
+                         0xFFFFFFFF if platform.system() == 'Windows' else 0xFF)
+        if process.stdout:
+            # Emacs 29: error message on standard error, backtrace on standard
+            # output.
+            self.assertEqual(process.stderr, 'Foo\n')
+            self.assertStartsWith(process.stdout, '\nError: error ("Foo")\n')
+            self.assertEndsWith(process.stdout, '\n  normal-top-level()\n')
+            self.assertContainsSubsequence(process.stdout.splitlines(),
+                                           ['Error: error ("Foo")',
+                                            '  error("Foo")',
+                                            '  normal-top-level()'])
+        else:
+            # Emacs 28: error message followed with backtrace on standard error,
+            # empty standard output.
+            self.assertStartsWith(
+                process.stderr,
+                'Debugger entered--Lisp error: (error "Foo")\n')
+            self.assertEndsWith(process.stderr, '\n  normal-top-level()\n\n')
+            self.assertContainsSubsequence(
+                process.stderr.splitlines(),
+                ['Debugger entered--Lisp error: (error "Foo")',
+                 '  error("Foo")',
+                 '  normal-top-level()'])
+
+
+if __name__ == '__main__':
+    absltest.main()