handling reverse proxy nuisance

index.html

@@ -816,6 +816,21 @@
         // state under run()'s own control.
         let solvingHere = false;
 
+        // Token identifying this tab's in-flight solve. When the tab closes mid
+        // solve we beacon /cancel?token=... so the server stops it and frees the
+        // lock immediately — reliable even behind a reverse proxy, which keeps
+        // the upstream socket open and so hides the disconnect from the server.
+        let activeToken = null;
+
+        function cancelActiveSolve() {
+            if (activeToken && navigator.sendBeacon) {
+                navigator.sendBeacon("/cancel?token=" + encodeURIComponent(activeToken));
+            }
+        }
+        // pagehide fires on tab close / navigation away (not on mere tab switch,
+        // so we don't cancel a solve the user backgrounds and comes back to).
+        window.addEventListener("pagehide", cancelActiveSolve);
+
         async function run() {
             const errBox = document.getElementById("error");
             const out = document.getElementById("output");
@@ -831,11 +846,14 @@
             } catch (e) {errBox.textContent = e.message; pending.remove(); return;}
             solvingHere = true;
             setSolveDisabled(true, "Solving…");
+            const token = (crypto.randomUUID ? crypto.randomUUID()
+                : String(Date.now()) + Math.random());
+            activeToken = token;
             try {
                 const resp = await fetch("/solve", {
                     method: "POST",
                     headers: {"Content-Type": "application/json"},
-                    body: JSON.stringify({problem, max_time_seconds: time}),
+                    body: JSON.stringify({problem, max_time_seconds: time, token}),
                 });
                 if (resp.status === 429) {
                     pending.remove();
@@ -846,7 +864,7 @@
                 if (!resp.ok) {pending.remove(); errBox.textContent = data.error || "Server error"; return;}
                 renderSolution(data, pending);
             } catch (e) {pending.remove(); errBox.textContent = e.message;}
-            finally {solvingHere = false; setSolveDisabled(false);}
+            finally {solvingHere = false; activeToken = null; setSolveDisabled(false);}
         }
 
         // Enable/disable the Solve button with an optional note beside it. `kind`

main.py

@@ -19,6 +19,7 @@ import socket
 import threading
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 from pathlib import Path
+from urllib.parse import parse_qs, urlsplit
 
 from solve import problem_from_dict, solve, solution_to_dict
 
@@ -31,6 +32,23 @@ INDEX_HTML = (Path(__file__).resolve().parent / "index.html").read_text(encoding
 # the busy state by polling /status).
 _solve_lock = threading.Lock()
 
+# Tracks the in-flight solve so /cancel can stop it. When a tab closes, the
+# browser fires navigator.sendBeacon("/cancel?token=...") — a normal small
+# request that survives the close and passes cleanly through reverse proxies,
+# unlike a dropped upstream socket (which the proxy keeps alive). Guarded by
+# _active_lock; only one solve runs at a time, but the lock keeps the token /
+# solver handoff race-free against a concurrent /cancel.
+_active_lock = threading.Lock()
+_active = {"token": None, "solver": None}
+
+
+def _stop_search(solver):
+    # StopSearch() exists in OR-Tools 9.x+; degrade gracefully on older builds
+    # (the solve then just runs to max_time_seconds).
+    stop = getattr(solver, "StopSearch", None)
+    if callable(stop):
+        stop()
+
 
 class Handler(BaseHTTPRequestHandler):
     def _send(self, code, body, content_type="application/json", no_cache=False):
@@ -65,7 +83,11 @@ class Handler(BaseHTTPRequestHandler):
         return False
 
     def do_POST(self):
-        if self.path != "/solve":
+        path = urlsplit(self.path)
+        if path.path == "/cancel":
+            self._handle_cancel(parse_qs(path.query))
+            return
+        if path.path != "/solve":
             self._send(404, json.dumps({"error": "not found"}))
             return
         # Reject immediately if another viewer is already solving.
@@ -73,23 +95,34 @@ class Handler(BaseHTTPRequestHandler):
             self._send(429, json.dumps({"error": "Another solve is already in progress"}))
             return
         released = False
+        token = None
         try:
             length = int(self.headers.get("Content-Length", 0))
             payload = json.loads(self.rfile.read(length) or b"{}")
             problem = problem_from_dict(payload.get("problem", {}))
             max_time = float(payload.get("max_time_seconds", 30.0))
+            token = str(payload.get("token") or "")
+
+            # Mark this token as the in-flight solve so a matching /cancel beacon
+            # (sent when the requesting tab closes) can stop it.
+            with _active_lock:
+                _active["token"] = token
+                _active["solver"] = None
 
             # Run the (blocking) solve in a worker thread so this thread is free
-            # to watch the socket. solver_sink hands us the CpSolver so we can
-            # abort the search if the client disconnects.
+            # to watch the socket. solver_sink hands us the CpSolver so both the
+            # local disconnect watcher and /cancel can abort the search.
             result = {}
-            solver_box = {}
+
+            def register(s):
+                with _active_lock:
+                    if _active["token"] == token:
+                        _active["solver"] = s
 
             def run():
                 try:
                     result["sol"] = solve(
-                        problem, max_time_seconds=max_time,
-                        solver_sink=lambda s: solver_box.setdefault("s", s),
+                        problem, max_time_seconds=max_time, solver_sink=register,
                     )
                 except Exception as exc:
                     result["err"] = exc
@@ -98,12 +131,12 @@ class Handler(BaseHTTPRequestHandler):
             worker.start()
             while worker.is_alive():
                 worker.join(0.25)
+                # Direct connections (no proxy) still get fast release via socket
+                # EOF; behind a proxy the /cancel beacon is what stops the solve.
                 if worker.is_alive() and self._client_gone():
-                    # Tab closed: stop the search and free the lock immediately
-                    # so the next viewer can start. The worker unwinds on its own.
-                    s = solver_box.get("s")
-                    if s is not None:
-                        s.StopSearch()
+                    with _active_lock:
+                        if _active["solver"] is not None:
+                            _stop_search(_active["solver"])
                     _solve_lock.release()
                     released = True
                     return
@@ -114,9 +147,25 @@ class Handler(BaseHTTPRequestHandler):
         except Exception as exc:  # surface errors to the browser
             self._send(400, json.dumps({"error": f"{type(exc).__name__}: {exc}"}))
         finally:
+            with _active_lock:
+                if _active["token"] == token:
+                    _active["token"] = None
+                    _active["solver"] = None
             if not released:
                 _solve_lock.release()
 
+    def _handle_cancel(self, query):
+        # Stop the in-flight solve iff the beacon's token matches it, so a closing
+        # tab can't cancel a *different* viewer's solve. The running /solve handler
+        # then returns normally and releases the lock.
+        token = (query.get("token") or [""])[0]
+        stopped = False
+        with _active_lock:
+            if token and token == _active["token"] and _active["solver"] is not None:
+                _stop_search(_active["solver"])
+                stopped = True
+        self._send(200, json.dumps({"cancelled": stopped}), no_cache=True)
+
     def log_message(self, fmt, *args):  # quieter console
         pass