Merge bitcoin/bitcoin#22211: net: relay I2P addresses even if not reachable (by us)

7593b06bd1 test: ensure I2P addresses are relayed (Vasil Dimov)
e7468139a1 test: make CAddress in functional tests comparable (Vasil Dimov)
33e211d2a4 test: implement ser/unser of I2P addresses in functional tests (Vasil Dimov)
86742811ce test: use NODE_* constants instead of magic numbers (Vasil Dimov)
ba45f02708 net: relay I2P addresses even if not reachable (by us) (Vasil Dimov)

Pull request description:

  Nodes that can reach the I2P network (have set `-i2psam=`) will relay
  I2P addresses even without this patch. However, nodes that can't reach
  the I2P network will not. This was done as a precaution in
  https://github.com/bitcoin/bitcoin/pull/20119 before anybody could
  connect to I2P because then, for sure, it would have been useless.

  Now, however, we have I2P support and a bunch of I2P nodes, so get all
  nodes on the network to relay I2P addresses to help with propagation,
  similarly to what we do with Tor addresses.

ACKs for top commit:
  jonatack:
    ACK 7593b06bd1
  naumenkogs:
    ACK 7593b06bd1.
  laanwj:
    Code review ACK 7593b06bd1
  kristapsk:
    ACK 7593b06bd1. Code looks correct, tested that functional test suite passes and also that `test/functional/p2p_addrv2_replay.py` fails if I undo changes in `IsRelayable()`.

Tree-SHA512: c9feec4a9546cc06bc2fec6d74f999a3c0abd3d15b7c421c21fcf2d610eb94611489e33d61bdcd5a4f42041a6d84aa892f7ae293b0d4f755309a8560b113b735

test/functional/test_framework/messages.py

@@ -18,6 +18,7 @@ ser_*, deser_*: functions that handle serialization/deserialization.
 Classes use __slots__ to ensure extraneous attributes aren't accidentally added
 by tests, compromising their intended effect.
 """
+from base64 import b32decode, b32encode
 from codecs import encode
 import copy
 import hashlib
@@ -213,15 +214,20 @@ class CAddress:
 
     # see https://github.com/bitcoin/bips/blob/master/bip-0155.mediawiki
     NET_IPV4 = 1
+    NET_I2P = 5
 
     ADDRV2_NET_NAME = {
-        NET_IPV4: "IPv4"
+        NET_IPV4: "IPv4",
+        NET_I2P: "I2P"
     }
 
     ADDRV2_ADDRESS_LENGTH = {
-        NET_IPV4: 4
+        NET_IPV4: 4,
+        NET_I2P: 32
     }
 
+    I2P_PAD = "===="
+
     def __init__(self):
         self.time = 0
         self.nServices = 1
@@ -229,6 +235,9 @@ class CAddress:
         self.ip = "0.0.0.0"
         self.port = 0
 
+    def __eq__(self, other):
+        return self.net == other.net and self.ip == other.ip and self.nServices == other.nServices and self.port == other.port and self.time == other.time
+
     def deserialize(self, f, *, with_time=True):
         """Deserialize from addrv1 format (pre-BIP155)"""
         if with_time:
@@ -261,24 +270,33 @@ class CAddress:
         self.nServices = deser_compact_size(f)
 
         self.net = struct.unpack("B", f.read(1))[0]
-        assert self.net == self.NET_IPV4
+        assert self.net in (self.NET_IPV4, self.NET_I2P)
 
         address_length = deser_compact_size(f)
         assert address_length == self.ADDRV2_ADDRESS_LENGTH[self.net]
 
-        self.ip = socket.inet_ntoa(f.read(4))
+        addr_bytes = f.read(address_length)
+        if self.net == self.NET_IPV4:
+            self.ip = socket.inet_ntoa(addr_bytes)
+        else:
+            self.ip = b32encode(addr_bytes)[0:-len(self.I2P_PAD)].decode("ascii").lower() + ".b32.i2p"
 
         self.port = struct.unpack(">H", f.read(2))[0]
 
     def serialize_v2(self):
         """Serialize in addrv2 format (BIP155)"""
-        assert self.net == self.NET_IPV4
+        assert self.net in (self.NET_IPV4, self.NET_I2P)
         r = b""
         r += struct.pack("<I", self.time)
         r += ser_compact_size(self.nServices)
         r += struct.pack("B", self.net)
         r += ser_compact_size(self.ADDRV2_ADDRESS_LENGTH[self.net])
-        r += socket.inet_aton(self.ip)
+        if self.net == self.NET_IPV4:
+            r += socket.inet_aton(self.ip)
+        else:
+            sfx = ".b32.i2p"
+            assert self.ip.endswith(sfx)
+            r += b32decode(self.ip[0:-len(sfx)] + self.I2P_PAD, True)
         r += struct.pack(">H", self.port)
         return r