Merge bitcoin#29347: net: enable v2transport by default

0bef104 net: enable v2transport by default (Pieter Wuille)

Pull request description:

  This enables BIP324's v2 transport by default (see bitcoin#27634):
  * Inbound connections will auto-sense whether v1 or v2 is in use.
  * Automatic outbound connections will use v2 if `NODE_P2P_V2` was set in addr gossip, but retry with v1 if met with immediate failure.
  * Manual outbound connections will default to v2, but retry with v1 if met with immediate failure.

  It remains possible to run with `-v2transport=0` to disable all of these, and make all outbound and inbound connections v1. It also remains possible to specify the `v2transport` argument to the `addnode` RPC as `false`, to disable attempting a v2 connection for that particular added node.

ACKs for top commit:
  stratospher:
    ACK 0bef104.
  josibake:
    reACK bitcoin@0bef104
  achow101:
    ACK 0bef104
  naumenkogs:
    ACK 0bef104
  theStack:
    ACK 0bef104
  willcl-ark:
    crACK 0bef104
  BrandonOdiwuor:
    utACK 0bef104
  pablomartin4btc:
    re ACK 0bef104
  kristapsk:
    utACK 0bef104

Tree-SHA512: 3f17a91e318b9304c40c74a7a5b231149f664ae684d13e9739a05be6c05ba9720f3c3c62da6a73ace0ae8ce733f1c8410b211f9fa15694e6a8d28999ab9882d8

src/net.h

@@ -114,7 +114,7 @@ static const bool DEFAULT_FIXEDSEEDS = true;
 static const size_t DEFAULT_MAXRECEIVEBUFFER = 5 * 1000;
 static const size_t DEFAULT_MAXSENDBUFFER    = 1 * 1000;
 
-static constexpr bool DEFAULT_V2_TRANSPORT{false};
+static constexpr bool DEFAULT_V2_TRANSPORT{true};
 
 #if defined USE_KQUEUE
 #define DEFAULT_SOCKETEVENTS "kqueue"

test/functional/test_framework/test_node.py

@@ -128,8 +128,15 @@ def __init__(self, i, datadir, extra_args_from_options, *, chain, rpchost, timew
         # Default behavior from global -v2transport flag is added to args to persist it over restarts.
         # May be overwritten in individual tests, using extra_args.
         self.default_to_v2 = v2transport
-        if self.default_to_v2:
-            self.args.append("-v2transport=1")
+        if self.version_is_at_least(260000):
+            # 26.0 and later support v2transport
+            if v2transport:
+                self.args.append("-v2transport=1")
+            else:
+                self.args.append("-v2transport=0")
+        else:
+            # v2transport requested but not supported for node
+            assert not v2transport
 
         self.cli = TestNodeCLI(bitcoin_cli, self.datadir)
         self.use_cli = use_cli