pytest-lsp: Forward a server's stderr output

This should be enough in most cases to get pytest to report the
server's log output on failure.

There may be issues if there are multiple servers running
concurrently/within the same session but we will cross that bridge
if/when it becomes an issue.

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)