Continue FreeCAD Web UI parity repairs

tools/build_web_frontend.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Build the configured operator frontend from its fixed working directory.
+
+No npm lifecycle hooks, standalone typechecks, tests or native builds. The
+caller groups related frontend edits before invoking this delivery command.
+"""
+import os
+import fcntl
+from pathlib import Path
+import shutil
+import subprocess
+import sys
+import time
+
+
+def main():
+    web = Path(__file__).resolve().parents[1] / "web"
+    vite = web / "node_modules/vite/bin/vite.js"
+    node = shutil.which("node")
+    if not node or not vite.is_file():
+        print(f"Frontend dependency unavailable: node={node}, Vite={vite}", file=sys.stderr)
+        return 2
+    env = {**os.environ, "FREECAD_WEB_FAST_LINK": "ON"}
+    # The foreground session and the sole scheduler share web/dist. Keep the
+    # lock outside dist: Vite clears that directory before writing a bundle.
+    with (web / ".freecad-web-frontend-build.lock").open("a") as lock:
+        started = time.monotonic()
+        waiting = False
+        while True:
+            try:
+                fcntl.flock(lock.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                break
+            except BlockingIOError:
+                if not waiting:
+                    print("Waiting for the existing frontend delivery to release web/dist", flush=True)
+                    waiting = True
+                if time.monotonic() - started >= 1800:
+                    print("Frontend delivery lock timed out; retain edits and resume after the active build", file=sys.stderr)
+                    return 124
+                time.sleep(0.25)
+        # Vite retains the descriptor if the outer runner exits mid-delivery.
+        return subprocess.run([node, str(vite), "build"], cwd=web, env=env,
+                              pass_fds=(lock.fileno(),), check=False).returncode
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())