Merge bitcoin/bitcoin#25355: I2P: add support for transient addresses for outbound connections

59aa54f7312f3441692c89feed86b8756d9d6b7a i2p: log "SAM session" instead of "session" (Vasil Dimov)
d7ec30b648721133b5a5ac3f52275f779c54310f doc: add release notes about the I2P transient addresses (Vasil Dimov)
47c0d02f126c73755288c3084402098567964329 doc: document I2P transient addresses usage in doc/i2p.md (Vasil Dimov)
3914e472f5685c29aa3d1c6dc5af9a758313d6c1 test: add a test that -i2pacceptincoming=0 creates a transient session (Vasil Dimov)
ae1e97ce863609e06be44a2632fb9d1fbb8e5698 net: use transient I2P session for outbound if -i2pacceptincoming=0 (Vasil Dimov)
a1580a04f5d7c9ecb30ee0d3bfdae519843a67ac net: store an optional I2P session in CNode (Vasil Dimov)
2b781ad66e34000037f589c71366c203255ed058 i2p: add support for creating transient sessions (Vasil Dimov)

Pull request description:

  Add support for generating a transient, one-time I2P address for ourselves when making I2P outbound connection and discard it once the connection is closed.

  Background
  ---
  In I2P connections, the host that receives the connection knows the I2P address of the connection initiator. This is unlike the Tor network where the recipient does not know who is connecting to them, not even the initiator's Tor address.

  Persistent vs transient I2P addresses
  ---
  Even if an I2P node is not accepting incoming connections, they are known to other nodes by their outgoing I2P address. This creates an opportunity to white-list given nodes or treat them differently based on their I2P address. However, this also creates an opportunity to fingerprint or analyze a given node because it always uses the same I2P address when it connects to other nodes. If this is undesirable, then a node operator can use the newly introduced `-i2ptransientout` to generate a transient (disposable), one-time I2P address for each new outgoing connection. That address is never going to be reused again, not even if reconnecting to the same peer later.

ACKs for top commit:
  mzumsande:
    ACK 59aa54f7312f3441692c89feed86b8756d9d6b7a (verified via range-diff that just a typo / `unique_ptr` initialisation were fixed)
  achow101:
    re-ACK 59aa54f7312f3441692c89feed86b8756d9d6b7a
  jonatack:
    utACK 59aa54f7312f3441692c89feed86b8756d9d6b7a reviewed range diff, rebased to master, debug build + relevant tests + review at each commit

Tree-SHA512: 2be9b9dd7502b2d44a75e095aaece61700766bff9af0a2846c29ca4e152b0a92bdfa30f61e8e32b6edb1225f74f1a78d19b7bf069f00b8f8173e69705414a93e

1 files changed, 24 insertions, 5 deletions

diff --git a/src/net.h b/src/net.h
index 2036e9078c..7156a2259f 100644
--- a/src/net.h
+++ b/src/net.h
@@ -513,10 +513,16 @@ public:
      * criterium in CConnman::AttemptToEvictConnection. */
     std::atomic<std::chrono::microseconds> m_min_ping_time{std::chrono::microseconds::max()};
 
-    CNode(NodeId id, std::shared_ptr<Sock> sock, const CAddress& addrIn,
-          uint64_t nKeyedNetGroupIn, uint64_t nLocalHostNonceIn,
-          const CAddress& addrBindIn, const std::string& addrNameIn,
-          ConnectionType conn_type_in, bool inbound_onion);
+    CNode(NodeId id,
+          std::shared_ptr<Sock> sock,
+          const CAddress& addrIn,
+          uint64_t nKeyedNetGroupIn,
+          uint64_t nLocalHostNonceIn,
+          const CAddress& addrBindIn,
+          const std::string& addrNameIn,
+          ConnectionType conn_type_in,
+          bool inbound_onion,
+          std::unique_ptr<i2p::sam::Session>&& i2p_sam_session = nullptr);
     CNode(const CNode&) = delete;
     CNode& operator=(const CNode&) = delete;
 
@@ -596,6 +602,18 @@ private:
 
     mapMsgTypeSize mapSendBytesPerMsgType GUARDED_BY(cs_vSend);
     mapMsgTypeSize mapRecvBytesPerMsgType GUARDED_BY(cs_vRecv);
+
+    /**
+     * If an I2P session is created per connection (for outbound transient I2P
+     * connections) then it is stored here so that it can be destroyed when the
+     * socket is closed. I2P sessions involve a data/transport socket (in `m_sock`)
+     * and a control socket (in `m_i2p_sam_session`). For transient sessions, once
+     * the data socket is closed, the control socket is not going to be used anymore
+     * and is just taking up resources. So better close it as soon as `m_sock` is
+     * closed.
+     * Otherwise this unique_ptr is empty.
+     */
+    std::unique_ptr<i2p::sam::Session> m_i2p_sam_session GUARDED_BY(m_sock_mutex);
 };
 
 /**
@@ -1072,7 +1090,8 @@ private:
 
     /**
      * I2P SAM session.
-     * Used to accept incoming and make outgoing I2P connections.
+     * Used to accept incoming and make outgoing I2P connections from a persistent
+     * address.
      */
     std::unique_ptr<i2p::sam::Session> m_i2p_sam_session;