Adding Dandelion++ support to public networks:

- New flag in NOTIFY_NEW_TRANSACTION to indicate stem mode
  - Stem loops detected in tx_pool.cpp
  - Embargo timeout for a blackhole attack during stem phase

src/crypto/duration.h

@@ -0,0 +1,70 @@
+// Copyright (c) 2020, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <chrono>
+#include "crypto/crypto.h"
+
+namespace crypto
+{
+  //! Generate poisson distributed values in discrete `D` time units.
+  template<typename D>
+  struct random_poisson_duration
+  {
+    using result_type = D;                 //!< std::chrono::duration time unit precision
+    using rep = typename result_type::rep; //!< Type used to represent duration value
+
+    //! \param average for generated durations
+    explicit random_poisson_duration(result_type average)
+      : dist(average.count() < 0 ? 0 : average.count())
+    {}
+
+    //! Generate a crypto-secure random duration
+    result_type operator()()
+    {
+      crypto::random_device rand{};
+      return result_type{dist(rand)};
+    }
+
+  private:
+    std::poisson_distribution<rep> dist;
+  };
+
+    /* A custom duration is used for subsecond precision because of the
+       variance. If 5000 milliseconds is given, 95% of the values fall between
+       4859ms-5141ms in 1ms increments (not enough time variance). Providing 1/4
+       seconds would yield 95% of the values between 3s-7.25s in 1/4s
+       increments. */
+
+  //! Generate random durations with 1 second precision
+  using random_poisson_seconds = random_poisson_duration<std::chrono::seconds>;
+  //! Generate random duration with 1/4 second precision
+  using random_poisson_subseconds =
+    random_poisson_duration<std::chrono::duration<std::chrono::milliseconds::rep, std::ratio<1, 4>>>;
+}