fix(host): enforce flush=True to prevent pipeline block buffering

l4d2host/process.py

@@ -46,7 +46,7 @@ def run_command(
         if on_stderr is not None:
             on_stderr(line)
         if passthrough:
-            print(line, file=sys.stderr)
+            print(line, file=sys.stderr, flush=True)
 
     def terminate_process() -> None:
         emit_stderr_message("cancellation requested; terminating subprocess")
@@ -82,7 +82,7 @@ def run_command(
             if callback is not None:
                 callback(line)
             if passthrough:
-                print(line, file=output_stream)
+                print(line, file=output_stream, flush=True)
         stream.close()
 
     stdout_thread = threading.Thread(

l4d2host/tests/test_process.py

@@ -47,3 +47,21 @@ def test_cancelled_command_raises_cancelled_error() -> None:
 def test_run_command_avoids_runtime_unsafe_nested_annotations() -> None:
     source = inspect.getsource(run_command)
     assert "subprocess.Popen[str].stdout" not in source
+
+
+def test_run_command_passthrough_writes_to_sys_streams(monkeypatch: pytest.MonkeyPatch) -> None:
+    import sys
+    from io import StringIO
+
+    mock_stdout = StringIO()
+    mock_stderr = StringIO()
+    monkeypatch.setattr(sys, "stdout", mock_stdout)
+    monkeypatch.setattr(sys, "stderr", mock_stderr)
+
+    run_command(
+        ["python3", "-c", "import sys; print('passed out'); print('passed err', file=sys.stderr)"],
+        passthrough=True,
+    )
+
+    assert mock_stdout.getvalue() == "passed out\n"
+    assert mock_stderr.getvalue() == "passed err\n"

l4d2web/services/host_commands.py

@@ -50,7 +50,7 @@ def run_command(
         if on_stderr is not None:
             on_stderr(line)
         if passthrough:
-            print(line, file=sys.stderr)
+            print(line, file=sys.stderr, flush=True)
 
     def terminate_process() -> None:
         emit_stderr_message("cancellation requested; terminating subprocess")
@@ -86,7 +86,7 @@ def run_command(
             if callback is not None:
                 callback(line)
             if passthrough:
-                print(line, file=output_stream)
+                print(line, file=output_stream, flush=True)
         stream.close()
 
     stdout_thread = threading.Thread(

l4d2web/tests/test_host_commands.py

@@ -1,6 +1,25 @@
+import sys
+from io import StringIO
 import pytest
 
 
+def test_run_command_passthrough_writes_to_sys_streams(monkeypatch: pytest.MonkeyPatch) -> None:
+    from l4d2web.services.host_commands import run_command
+
+    mock_stdout = StringIO()
+    mock_stderr = StringIO()
+    monkeypatch.setattr(sys, "stdout", mock_stdout)
+    monkeypatch.setattr(sys, "stderr", mock_stderr)
+
+    run_command(
+        ["python3", "-c", "import sys; print('passed out'); print('passed err', file=sys.stderr)"],
+        passthrough=True,
+    )
+
+    assert mock_stdout.getvalue() == "passed out\n"
+    assert mock_stderr.getvalue() == "passed err\n"
+
+
 def test_run_command_streams_stdout_and_stderr_callbacks() -> None:
     from l4d2web.services.host_commands import run_command