From 005d2d845889eb955f09aec6e74f123130a09e1f Mon Sep 17 00:00:00 2001
From: mwiegand <martin.wiegand@seibert.group>
Date: Wed, 6 May 2026 20:30:00 +0200
Subject: [PATCH] fix(host): enforce flush=True to prevent pipeline block
 buffering

---
 l4d2host/process.py                 |  4 ++--
 l4d2host/tests/test_process.py      | 18 ++++++++++++++++++
 l4d2web/services/host_commands.py   |  4 ++--
 l4d2web/tests/test_host_commands.py | 19 +++++++++++++++++++
 4 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/l4d2host/process.py b/l4d2host/process.py
index e7853b9..4992216 100644
--- a/l4d2host/process.py
+++ b/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(
diff --git a/l4d2host/tests/test_process.py b/l4d2host/tests/test_process.py
index 99c896f..6ec12ea 100644
--- a/l4d2host/tests/test_process.py
+++ b/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"
diff --git a/l4d2web/services/host_commands.py b/l4d2web/services/host_commands.py
index 340fd2c..a925b9e 100644
--- a/l4d2web/services/host_commands.py
+++ b/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(
diff --git a/l4d2web/tests/test_host_commands.py b/l4d2web/tests/test_host_commands.py
index 023792d..f9a6512 100644
--- a/l4d2web/tests/test_host_commands.py
+++ b/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