Wait for handshakes before replacing a server's certificates

update_certs_pem() replaced the certificate, key and client CAs inside
the TLS context while handshakes on other threads were still reading
them, and only the creation of a session was serialized with it. A
server rotating its certificates under load could crash.

Count the handshakes in progress, make an update wait until there are
none, and hold off new handshakes while an update is pending so that a
steady stream of them cannot keep it waiting. The wait is bounded by
the handshake timeout.

test/test.cc

@@ -4657,6 +4657,53 @@ TEST(SSLClientServerTest, UpdateCertsPemWithClientAuth) {
   ASSERT_TRUE(handler_called);
   EXPECT_EQ("ok", res->body);
 }
+
+TEST(SSLClientServerTest, UpdateCertsPemWaitsForHandshakeInProgress) {
+  SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE);
+  ASSERT_TRUE(svr.is_valid());
+  svr.Get("/test", [](const Request &, Response &res) {
+    res.set_content("ok", "text/plain");
+  });
+
+  thread t = thread([&]() { ASSERT_TRUE(svr.listen(HOST, PORT)); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  // A connection that sends no ClientHello keeps its handshake in progress,
+  // once the server has had a moment to start it.
+  auto error = Error::Success;
+  auto sock = detail::create_client_socket(
+      HOST, "", PORT, AF_UNSPEC, false, false, nullptr,
+      /*connection_timeout_sec=*/5, 0,
+      /*read_timeout_sec=*/5, 0,
+      /*write_timeout_sec=*/5, 0, std::string(), error);
+  ASSERT_NE(INVALID_SOCKET, sock);
+  std::this_thread::sleep_for(std::chrono::milliseconds(500));
+
+  std::string cert_pem, key_pem;
+  read_file(SERVER_CERT_FILE, cert_pem);
+  read_file(SERVER_PRIVATE_KEY_FILE, key_pem);
+
+  auto update = std::async(std::launch::async, [&]() {
+    return svr.update_certs_pem(cert_pem.c_str(), key_pem.c_str());
+  });
+  EXPECT_EQ(std::future_status::timeout,
+            update.wait_for(std::chrono::milliseconds(200)));
+
+  detail::close_socket(sock);
+  EXPECT_TRUE(update.get());
+
+  SSLClient cli(HOST, PORT);
+  cli.enable_server_certificate_verification(false);
+  auto res = cli.Get("/test");
+  ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+  EXPECT_EQ(StatusCode::OK_200, res->status);
+}
 #endif
 
 TEST(ErrorHandlerTest, ContentLength) {