pytest-lsp: Forward a server's stderr output

lib/pytest-lsp/changes/143.enhancement.md

@@ -0,0 +1 @@
+When a test fails `pytest-lsp` will now show the server's `stderr` output (if any)

lib/pytest-lsp/pytest_lsp/client.py

@@ -82,22 +82,33 @@ def __init__(self, *args, configuration: Optional[Dict[str, Any]] = None, **kwar
         self._last_log_index = 0
         """Used to keep track of which log messages correspond with which test case."""
 
+        self._stderr_forwarder: Optional[asyncio.Task] = None
+        """A task that forwards the server's stderr to the test process."""
+
+    async def start_io(self, cmd: str, *args, **kwargs):
+        await super().start_io(cmd, *args, **kwargs)
+
+        # Forward the server's stderr to this process' stderr
+        if self._server and self._server.stderr:
+            self._stderr_forwarder = asyncio.create_task(forward_stderr(self._server))
+
+    async def stop(self):
+        if self._stderr_forwarder:
+            self._stderr_forwarder.cancel()
+
+        return await super().stop()
+
     async def server_exit(self, server: asyncio.subprocess.Process):
         """Called when the server process exits."""
         logger.debug("Server process exited with code: %s", server.returncode)
 
         if self._stop_event.is_set():
             return
 
-        stderr = ""
-        if server.stderr is not None:
-            stderr_bytes = await server.stderr.read()
-            stderr = stderr_bytes.decode("utf8")
-
         loop = asyncio.get_running_loop()
         loop.call_soon(
             cancel_all_tasks,
-            f"Server process exited with return code: {server.returncode}\n{stderr}",
+            f"Server process exited with return code: {server.returncode}",
         )
 
     def report_server_error(
@@ -259,6 +270,15 @@ async def wait_for_notification(self, method: str):
         return await self.protocol.wait_for_notification_async(method)
 
 
+async def forward_stderr(server: asyncio.subprocess.Process):
+    if server.stderr is None:
+        return
+
+    # EOF is signalled with an empty bytestring
+    while (line := await server.stderr.readline()) != b"":
+        sys.stderr.buffer.write(line)
+
+
 def cancel_all_tasks(message: str):
     """Called to cancel all awaited tasks."""
 

lib/pytest-lsp/tests/examples/server-stderr/server.py

@@ -0,0 +1,21 @@
+import sys
+
+from lsprotocol import types
+from pygls.server import LanguageServer
+
+server = LanguageServer("server-stderr", "v1")
+
+
+@server.feature(types.TEXT_DOCUMENT_COMPLETION)
+def completion(params: types.CompletionParams):
+    items = []
+
+    for i in range(10):
+        print(f"Suggesting item {i}", file=sys.stderr, flush=True)
+        items.append(types.CompletionItem(label=f"item-{i}"))
+
+    return items
+
+
+if __name__ == "__main__":
+    server.start_io()

lib/pytest-lsp/tests/examples/server-stderr/t_server.py

@@ -0,0 +1,46 @@
+import sys
+
+from lsprotocol.types import ClientCapabilities
+from lsprotocol.types import CompletionList
+from lsprotocol.types import CompletionParams
+from lsprotocol.types import InitializeParams
+from lsprotocol.types import Position
+from lsprotocol.types import TextDocumentIdentifier
+
+import pytest_lsp
+from pytest_lsp import ClientServerConfig
+from pytest_lsp import LanguageClient
+
+
+@pytest_lsp.fixture(
+    config=ClientServerConfig(server_command=[sys.executable, "server.py"]),
+)
+async def client(lsp_client: LanguageClient):
+    # Setup
+    params = InitializeParams(capabilities=ClientCapabilities())
+    await lsp_client.initialize_session(params)
+
+    yield
+
+    # Teardown
+    await lsp_client.shutdown_session()
+
+
+async def test_completions(client: LanguageClient):
+    results = await client.text_document_completion_async(
+        params=CompletionParams(
+            position=Position(line=1, character=0),
+            text_document=TextDocumentIdentifier(uri="file:///path/to/file.txt"),
+        )
+    )
+
+    assert results is not None
+
+    if isinstance(results, CompletionList):
+        items = results.items
+    else:
+        items = results
+
+    labels = [item.label for item in items]
+    assert labels == [f"item-{i}" for i in range(10)]
+    assert False  # Force the test case to fail.

lib/pytest-lsp/tests/test_examples.py

@@ -122,6 +122,27 @@ def test_generic_rpc(pytester: pytest.Pytester):
     results.stdout.fnmatch_lines(" *LOG: b=2")
 
 
+def test_server_stderr_fail(pytester: pytest.Pytester):
+    """Ensure that the server's stderr stream is presented on failure."""
+
+    setup_test(pytester, "server-stderr")
+
+    results = pytester.runpytest()
+    results.assert_outcomes(failed=1)
+
+    results.stdout.fnmatch_lines("-* Captured stderr call -*")
+    results.stdout.fnmatch_lines("Suggesting item 0")
+    results.stdout.fnmatch_lines("Suggesting item 1")
+    results.stdout.fnmatch_lines("Suggesting item 2")
+    results.stdout.fnmatch_lines("Suggesting item 3")
+    results.stdout.fnmatch_lines("Suggesting item 4")
+    results.stdout.fnmatch_lines("Suggesting item 5")
+    results.stdout.fnmatch_lines("Suggesting item 6")
+    results.stdout.fnmatch_lines("Suggesting item 7")
+    results.stdout.fnmatch_lines("Suggesting item 8")
+    results.stdout.fnmatch_lines("Suggesting item 9")
+
+
 def test_window_log_message_fail(pytester: pytest.Pytester):
     """Ensure that the initial getting started example fails as expected."""
 

lib/pytest-lsp/tests/test_plugin.py

@@ -195,7 +195,7 @@ async def test_capabilities(client):
     else:
         message = [
             "E*asyncio.exceptions.CancelledError: Server process exited with return code: 1",  # noqa: E501
-            "E*ZeroDivisionError: division by zero",
+            "ZeroDivisionError: division by zero",
         ]
 
     results.stdout.fnmatch_lines(message)