test(quic): SendSync returns for a send the peer answered at once

ShouldReturnFromSendSyncQUIC: 500 rounds of four fresh connections, each
sending one message on its first stream to a server that echoes it with
a FIN and drops the stream; every SendSync must return and every echo
come back. The race it guards needs the sending thread descheduled for
a round trip, so it is a stress test, not a deterministic one: against
the previous SendSync it failed 6 runs of 7, with one or two of the 2000
sends throwing each time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ShouldReturnFromSendSyncQUIC/main.cpp

@@ -0,0 +1,109 @@
+//SPDX-License-Identifier: LGPL-3.0-only
+//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// QUICStream::SendSync returns once msquic has taken its buffer, and throws
+// only when that send failed. It used to wait on one flag per stream and,
+// once woken, throw QUICClosedException whenever the stream had finished
+// shutting down meanwhile, though its own send had completed. A peer that
+// answers at once and finishes the stream (here: echo, FIN, and drop the
+// stream, whose Stop() stops our sending side too) can complete the stream
+// within a round trip, which on loopback is shorter than a busy machine can
+// take to wake the sending thread: the sender threw for a send the peer had
+// already answered.
+//
+// The race needs the sending thread to be descheduled at the wrong moment,
+// so no test hits it every time. The shape that showed it: the first stream
+// of a fresh connection, four connections at once, one exchange each; a
+// throwaway run of that shape threw once in 3000 exchanges, the echo of the
+// failed send arriving intact. Here 2000 such exchanges must all return
+// from SendSync, and all be echoed.
+
+#include <cstdio>
+
+import Crafter.Network;
+import Crafter.Thread;
+import std;
+using namespace Crafter;
+
+namespace {
+
+int failures = 0;
+
+void Check(bool ok, std::string_view what) {
+    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
+    if (!ok) ++failures;
+}
+
+} // namespace
+
+int main() {
+    ThreadPool::Start();
+    std::thread([] {
+        std::this_thread::sleep_for(std::chrono::seconds(50));
+        std::println("FAIL  timed out");
+        std::fflush(stdout);
+        std::_Exit(1);
+    }).detach();
+
+    const std::string alpn = "crafter/test-send-sync";
+    QUICServerCredentials serverCreds;
+    serverCreds.selfSigned = true;
+    QUICClientCredentials clientCreds;
+    clientCreds.insecureNoServerValidation = true;
+
+    constexpr int rounds = 500;
+    constexpr int together = 4;
+    std::atomic<int> threw = 0;
+    std::atomic<int> echoed = 0;
+    std::atomic<int> other = 0;
+    try {
+        // Answer each stream with what arrived and a FIN, then drop it: its
+        // destructor's Stop() aborts our receive side, which also ends the
+        // client's sending side.
+        ListenerQUIC listener("127.0.0.1", 0, alpn, serverCreds, [](ClientQUIC* peer) {
+            peer->OnStream([](QUICStream stream) {
+                try {
+                    std::vector<char> got = stream.RecieveSync();
+                    stream.SendSync(got.data(), static_cast<std::uint32_t>(got.size()), true);
+                } catch (...) {}
+            });
+        });
+        listener.ListenAsyncAsync();
+
+        for (int round = 0; round < rounds; ++round) {
+            std::vector<std::thread> clients;
+            for (int i = 0; i < together; ++i) {
+                clients.emplace_back([&] {
+                    try {
+                        ClientQUIC client("127.0.0.1", listener.Port(), alpn, clientCreds);
+                        QUICStream stream = client.OpenStream();
+                        try {
+                            stream.SendSync("ping", 4, false);
+                        } catch (const QUICClosedException&) {
+                            ++threw;
+                        }
+                        std::vector<char> back = stream.RecieveUntilCloseSync();
+                        if (std::string(back.begin(), back.end()) == "ping") ++echoed;
+                    } catch (const std::exception& e) {
+                        if (++other == 1) std::println("      {}", e.what());
+                    }
+                });
+            }
+            for (std::thread& client : clients) client.join();
+        }
+    } catch (const std::exception& e) {
+        Check(false, std::format("threw: {}", e.what()));
+    }
+    constexpr int exchanges = rounds * together;
+    Check(threw.load() == 0,
+          std::format("SendSync returned for every send msquic took ({} of {} threw QUICClosedException)",
+                      threw.load(), exchanges));
+    Check(echoed.load() == exchanges && other.load() == 0,
+          std::format("and every echo came back ({} of {}, {} other errors)", echoed.load(), exchanges, other.load()));
+
+    std::println("{}", failures == 0 ? "all checks passed" : "check(s) failed");
+    // The accepted peers are left to the process: skip msquic's static
+    // teardown, as the other QUIC tests do. _Exit doesn't flush.
+    std::fflush(stdout);
+    std::_Exit(failures == 0 ? 0 : 1);
+}