Do not allow exceptions to propagate from backend (#40)

Author: stephenfin

src/pyproject_api/_backend.py

@@ -97,8 +97,6 @@ def run(argv):
                 result["exc_msg"] = str(exception)
                 if not isinstance(exception, MissingCommand):  # for missing command do not print stack
                     traceback.print_exc()
-                if not isinstance(exception, Exception):  # allow SystemExit/KeyboardInterrupt to go through
-                    raise
             finally:
                 try:
                     with open(result_file, "w") as file_handler:

tests/test_backend.py

@@ -0,0 +1,126 @@
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+import pytest
+import pytest_mock
+
+from pyproject_api._backend import BackendProxy, run
+
+
+def test_invalid_module(capsys: pytest.CaptureFixture[str]) -> None:
+    with pytest.raises(ImportError):
+        run([str(False), "an.invalid.module"])
+
+    captured = capsys.readouterr()
+    assert "failed to start backend" in captured.err
+
+
+def test_invalid_request(mocker: pytest_mock.MockerFixture, capsys: pytest.CaptureFixture[str]) -> None:
+    """Validate behavior when an invalid request is issued."""
+    command = "invalid json"
+
+    backend_proxy = mocker.MagicMock(spec=BackendProxy)
+    backend_proxy.return_value = "dummy_result"
+    backend_proxy.__str__.return_value = "FakeBackendProxy"
+    mocker.patch("pyproject_api._backend.BackendProxy", return_value=backend_proxy)
+    mocker.patch("pyproject_api._backend.read_line", return_value=bytearray(command, "utf-8"))
+
+    ret = run([str(False), "a.dummy.module"])
+
+    assert ret == 0
+    captured = capsys.readouterr()
+    assert "started backend " in captured.out
+    assert "Backend: incorrect request to backend: " in captured.err
+
+
+def test_exception(mocker: pytest_mock.MockerFixture, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
+    """Ensure an exception in the backend is not bubbled up."""
+    result = str(tmp_path / "result")
+    command = json.dumps({"cmd": "dummy_command", "kwargs": {"foo": "bar"}, "result": result})
+
+    backend_proxy = mocker.MagicMock(spec=BackendProxy)
+    backend_proxy.side_effect = SystemExit(1)
+    backend_proxy.__str__.return_value = "FakeBackendProxy"
+    mocker.patch("pyproject_api._backend.BackendProxy", return_value=backend_proxy)
+    mocker.patch("pyproject_api._backend.read_line", return_value=bytearray(command, "utf-8"))
+
+    ret = run([str(False), "a.dummy.module"])
+
+    # We still return 0 and write a result file. The exception should *not* bubble up
+    assert ret == 0
+    captured = capsys.readouterr()
+    assert "started backend FakeBackendProxy" in captured.out
+    assert "Backend: run command dummy_command with args {'foo': 'bar'}" in captured.out
+    assert "Backend: Wrote response " in captured.out
+    assert "SystemExit: 1" in captured.err
+
+
+def test_valid_request(mocker: pytest_mock.MockerFixture, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
+    """Validate the "success" path."""
+    result = str(tmp_path / "result")
+    command = json.dumps({"cmd": "dummy_command", "kwargs": {"foo": "bar"}, "result": result})
+
+    backend_proxy = mocker.MagicMock(spec=BackendProxy)
+    backend_proxy.return_value = "dummy-result"
+    backend_proxy.__str__.return_value = "FakeBackendProxy"
+    mocker.patch("pyproject_api._backend.BackendProxy", return_value=backend_proxy)
+    mocker.patch("pyproject_api._backend.read_line", return_value=bytearray(command, "utf-8"))
+
+    ret = run([str(False), "a.dummy.module"])
+
+    assert ret == 0
+    captured = capsys.readouterr()
+    assert "started backend FakeBackendProxy" in captured.out
+    assert "Backend: run command dummy_command with args {'foo': 'bar'}" in captured.out
+    assert "Backend: Wrote response " in captured.out
+    assert "" == captured.err
+
+
+def test_reuse_process(mocker: pytest_mock.MockerFixture, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
+    """Validate behavior when reusing the backend proxy process.
+
+    There are a couple of things we'd like to check here:
+
+    - Ensure we can actually reuse the process.
+    - Ensure an exception in a call to the backend does not affect subsequent calls.
+    - Ensure we can exit safely by calling the '_exit' command.
+    """
+    results = [
+        str(tmp_path / "result_a"),
+        str(tmp_path / "result_b"),
+        str(tmp_path / "result_c"),
+        str(tmp_path / "result_d"),
+    ]
+    commands = [
+        json.dumps({"cmd": "dummy_command_a", "kwargs": {"foo": "bar"}, "result": results[0]}),
+        json.dumps({"cmd": "dummy_command_b", "kwargs": {"baz": "qux"}, "result": results[1]}),
+        json.dumps({"cmd": "dummy_command_c", "kwargs": {"win": "wow"}, "result": results[2]}),
+        json.dumps({"cmd": "_exit", "kwargs": {}, "result": results[3]}),
+    ]
+
+    def fake_backend(name: str, *args: Any, **kwargs: Any) -> Any:  # noqa: U100
+        if name == "dummy_command_b":
+            raise SystemExit(2)
+
+        return "dummy-result"
+
+    backend_proxy = mocker.MagicMock(spec=BackendProxy)
+    backend_proxy.side_effect = fake_backend
+    backend_proxy.__str__.return_value = "FakeBackendProxy"
+    mocker.patch("pyproject_api._backend.BackendProxy", return_value=backend_proxy)
+    mocker.patch("pyproject_api._backend.read_line", side_effect=[bytearray(x, "utf-8") for x in commands])
+
+    ret = run([str(True), "a.dummy.module"])
+
+    # We still return 0 and write a result file. The exception should *not* bubble up and all commands should execute.
+    # It is the responsibility of the caller to handle errors.
+    assert ret == 0
+    captured = capsys.readouterr()
+    assert "started backend FakeBackendProxy" in captured.out
+    assert "Backend: run command dummy_command_a with args {'foo': 'bar'}" in captured.out
+    assert "Backend: run command dummy_command_b with args {'baz': 'qux'}" in captured.out
+    assert "Backend: run command dummy_command_c with args {'win': 'wow'}" in captured.out
+    assert "SystemExit: 2" in captured.err

tests/test_fronted.py tests/test_frontend.pytests/test_fronted.py renamed to tests/test_frontend.py

@@ -1,5 +1,6 @@
 autoclass
 autodoc
+capsys
 cfg
 delenv
 exe
@@ -17,6 +18,7 @@ py311
 py38
 pygments
 pyproject
+readouterr
 sdist
 setenv
 tmpdir