Drop the SNI of a failed handshake on the Mbed TLS server

The Mbed TLS server keeps the SNI from the ClientHello in a
thread-local until the handshake succeeds, and its SNI callback only
runs for a ClientHello that carries one. A handshake that failed left
its SNI behind, so the next connection served by the same thread
without SNI reported the earlier client's name in req.sni(). Start
each handshake with the thread-local cleared.

test/test.cc

@@ -12977,6 +12977,44 @@ TEST(SNI_AutoDetectionTest, SNI_Logic) {
     }
   }
 }
+
+// The SNI of a failed handshake does not reach the next connection that the
+// same worker thread serves
+TEST(SNI_AutoDetectionTest, FailedHandshakeLeavesNoSNI) {
+  SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE);
+  ASSERT_TRUE(svr.is_valid());
+  svr.new_task_queue = [] { return new ThreadPool(1); };
+  svr.Get("/sni", [](const Request &req, Response &res) {
+    res.set_content(req.sni(), "text/plain");
+  });
+
+  auto port = svr.bind_to_any_port("127.0.0.1");
+  ASSERT_LT(0, port);
+  thread t = thread([&] { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+  svr.wait_until_ready();
+
+  {
+    // Sends SNI, then rejects the server certificate, which the CA did not sign
+    SSLClient cli("localhost", port);
+    cli.set_hostname_addr_map({{"localhost", "127.0.0.1"}});
+    cli.set_ca_cert_path(CLIENT_CA_CERT_FILE);
+    auto res = cli.Get("/sni");
+    ASSERT_FALSE(res);
+    EXPECT_EQ(Error::SSLServerVerification, res.error());
+  }
+
+  // An IP host sends no SNI (RFC 6066)
+  SSLClient cli("127.0.0.1", port);
+  cli.enable_server_certificate_verification(false);
+  auto res = cli.Get("/sni");
+  ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+  EXPECT_EQ("", res->body);
+}
 #endif
 
 TEST(ClientProblemDetectionTest, ContentProvider) {