Use a separate thread to read stderr when needed

dangerzone/isolation_provider/base.py

@@ -5,6 +5,8 @@
 import signal
 import subprocess
 import sys
+import threading
+from queue import Queue
 from abc import ABC, abstractmethod
 from typing import IO, Callable, Iterator, Optional
 
@@ -92,6 +94,7 @@ def __init__(self, debug: bool = False) -> None:
             self.proc_stderr = subprocess.PIPE
         else:
             self.proc_stderr = subprocess.DEVNULL
+        self.stderr_queue = Queue()
 
     def should_capture_stderr(self) -> bool:
         return self.debug or getattr(sys, "dangerzone_dev", False)
@@ -363,3 +366,23 @@ def doc_to_pixels_proc(
                     f"{debug_log}"  # no need for an extra newline here
                     f"{DOC_TO_PIXELS_LOG_END}"
                 )
+
+    def _stream_stderr(self, stderr, queue: Queue) -> None:
+        """Read stderr in a separate thread to avoid blocking"""
+        try:
+            for line in stderr:
+                queue.put(line)
+                if self.debug:
+                    log.debug(line.decode().strip())
+        except (ValueError, IOError) as e:
+            log.debug(f"Stderr stream closed: {e}")
+
+    def start_stderr_thread(self, process: subprocess.Popen) -> None:
+        """Start a thread to read stderr from the process"""
+        if process.stderr:
+            stderr_thread = threading.Thread(
+                target=self._stream_stderr,
+                args=(process.stderr, self.stderr_queue),
+                daemon=True
+            )
+            stderr_thread.start()

dangerzone/isolation_provider/container.py

@@ -189,7 +189,22 @@ def exec_container(
             + command
         )
         args = [container_runtime] + args
-        return self.exec(args)
+        args_str = " ".join(shlex.quote(s) for s in args)
+        log.info("> " + args_str)
+
+        process = subprocess.Popen(
+            args,
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,  # Always pipe stderr
+            startupinfo=startupinfo,
+            start_new_session=True,
+        )
+
+        # Start stderr reader thread
+        self.start_stderr_thread(process)
+
+        return process
 
     def kill_container(self, name: str) -> None:
         """Terminate a spawned container.

dangerzone/logic.py

@@ -66,11 +66,25 @@ def convert_documents(
     ) -> None:
         def convert_doc(document: Document) -> None:
             try:
+                # Clear any existing stderr output
+                while not self.isolation_provider.stderr_queue.empty():
+                    self.isolation_provider.stderr_queue.get_nowait()
+
                 self.isolation_provider.convert(
                     document,
                     ocr_lang,
                     stdout_callback,
                 )
+
+                # Process any stderr output that was captured
+                while not self.isolation_provider.stderr_queue.empty():
+                    try:
+                        line = self.isolation_provider.stderr_queue.get_nowait()
+                        if stdout_callback:
+                            stdout_callback(True, line.decode().strip(), -1)
+                    except Exception as e:
+                        log.error(f"Error processing stderr: {e}")
+
             except Exception:
                 log.exception(
                     f"Unexpected error occurred while converting '{document}'"