Merge pull request #260 from pathsim/fix/native-stdout-protocol

Fix native stdout protocol

Author: milanofthe

pathview/app.py

@@ -63,11 +63,22 @@ def send_message(self, msg: dict) -> None:
         self.process.stdin.flush()
 
     def read_line(self) -> dict | None:
-        """Read one JSON line from the subprocess stdout."""
-        line = self.process.stdout.readline()
-        if not line:
-            return None
-        return json.loads(line.strip())
+        """Read one JSON line from the subprocess stdout.
+
+        Skips blank or non-JSON lines that may leak from native C/C++
+        libraries writing directly to the OS-level stdout fd.
+        """
+        while True:
+            line = self.process.stdout.readline()
+            if not line:
+                return None
+            stripped = line.strip()
+            if not stripped:
+                continue
+            try:
+                return json.loads(stripped)
+            except json.JSONDecodeError:
+                continue
 
     def read_line_timeout(self, timeout: float = EXEC_TIMEOUT) -> dict | None:
         """Read one JSON line with a timeout. Returns None on EOF or timeout.

pathview/cli.py

@@ -46,6 +46,8 @@ def open_browser_when_ready():
         if args.debug:
             app.run(host=args.host, port=args.port, debug=True, threaded=True)
         else:
+            import logging
+            logging.getLogger("waitress.queue").setLevel(logging.ERROR)
             from waitress import serve
             serve(app, host=args.host, port=args.port, threads=4)
     except KeyboardInterrupt:

pathview/worker.py

@@ -15,6 +15,7 @@
 """
 
 import sys
+import os
 import json
 import subprocess
 import threading
@@ -26,7 +27,15 @@
 _stdout_lock = threading.Lock()
 
 # Keep a reference to the real stdout pipe — protocol messages go here.
-_real_stdout = sys.stdout
+# We dup() fd 1 so that _real_stdout survives the fd-level redirect below.
+_real_stdout_fd = os.dup(1)
+_real_stdout = os.fdopen(_real_stdout_fd, "w")
+
+# Redirect OS-level fd 1 to devnull so that C/C++ libraries (e.g. jsbsim)
+# writing directly to stdout via printf/cout cannot corrupt the JSON protocol.
+_devnull_fd = os.open(os.devnull, os.O_WRONLY)
+os.dup2(_devnull_fd, 1)
+os.close(_devnull_fd)
 
 # Worker state
 _namespace = {}