Route py_func/lua_func subprocess stdio into the parent log

stdout/stderr of the subprocesses were going to DEVNULL — early-startup
errors (lua require failures, exceptions before stdio_redir kicks in)
were lost.

New helper proc_drain.drain_to_log spawns a daemon thread per pipe that
print()s each line through stdio_redir, so it reaches the log + live
output. Used by py_process and lua_process with [py_func]/[lua_func]
prefixes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/testium/interpreter/utils/lua_process.py

@@ -8,6 +8,7 @@ from runtime.jrpc import JsonRpcClient
 from interpreter.utils.paths import subproc_path
 from runtime.tum_except import ETUMRuntimeError
 from interpreter.utils import bins
+from interpreter.utils.proc_drain import drain_to_log
 
 
 class LuaProcessBase:
@@ -93,10 +94,14 @@ class LuaProcessBase:
         self._process = subprocess.Popen(
             params, env=env, cwd=func_proc_path,
             stdin=subprocess.DEVNULL,
-            stdout=subprocess.DEVNULL,
-            stderr=subprocess.DEVNULL,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
             restore_signals=False,
         )
+        # Route subprocess stdout/stderr (lua require failures, syntax
+        # errors, anything written to fd 1/2 before the in-script
+        # remote_print is set up) into the parent's log.
+        drain_to_log(self._process, prefix="[lua_func] ")
 
         self._rpc = JsonRpcClient(
             "localhost", self._port, req_handler=self._req_handler

src/testium/interpreter/utils/proc_drain.py

@@ -0,0 +1,48 @@
+"""Drain a subprocess stdout/stderr into testium's print pipeline.
+
+Captured lines go through the parent's stdio_redir, so they reach the
+test log AND the live output (terminal in batch mode, GUI text panel
+in -r mode). This is essential for diagnosing early-startup errors
+of py_func / lua_func subprocesses (missing modules, unhandled
+exceptions before the in-process redirection kicks in, lua
+``require`` failures, anything written to fd 1/2 directly).
+"""
+import threading
+
+
+def _drain_pipe(pipe, prefix):
+    try:
+        for raw in iter(pipe.readline, b""):
+            line = raw.decode("utf-8", errors="replace").rstrip("\r\n")
+            if not line:
+                continue
+            if prefix:
+                print(f"{prefix}{line}")
+            else:
+                print(line)
+    finally:
+        try:
+            pipe.close()
+        except Exception:
+            pass
+
+
+def drain_to_log(process, prefix=""):
+    """Spawn daemon threads that read ``process.stdout`` and
+    ``process.stderr`` line by line and print each line through the
+    parent's stdout (so it reaches the log + live output).
+
+    Each thread exits cleanly when the subprocess closes the
+    corresponding pipe (i.e. when it exits). Daemon flag ensures they
+    do not block testium exit.
+    """
+    threads = []
+    for pipe in (process.stdout, process.stderr):
+        if pipe is None:
+            continue
+        t = threading.Thread(
+            target=_drain_pipe, args=(pipe, prefix), daemon=True,
+        )
+        t.start()
+        threads.append(t)
+    return threads

src/testium/interpreter/utils/py_process.py

@@ -7,6 +7,7 @@ import api.testium as tm
 from runtime.tum_except import ETUMRuntimeError
 from interpreter.utils.paths import testium_path, subproc_path
 from interpreter.utils import bins
+from interpreter.utils.proc_drain import drain_to_log
 
 
 class PyProcessBase:
@@ -77,10 +78,15 @@ class PyProcessBase:
         self._process = subprocess.Popen(
             params, env=env, cwd=func_proc_path,
             stdin=subprocess.DEVNULL,
-            stdout=subprocess.DEVNULL,
-            stderr=subprocess.DEVNULL,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
             restore_signals=False,
         )
+        # Route subprocess stdout/stderr (early-startup errors,
+        # unhandled exceptions, anything written to fd 1/2 before the
+        # in-process JSON-RPC stdio_redir kicks in) into the parent's
+        # log.
+        drain_to_log(self._process, prefix="[py_func] ")
 
         self._rpc = JsonRpcClient(
             "localhost", self._port, req_handler=self._req_handler