Merge bitcoin/bitcoin#22079: zmq: Add support to listen on IPv6 addresses

e6998838e5 doc: Add IPv6 address to zmq example (nthumann)
8abe5703a9 test: Add IPv6 test to zmq (nthumann)
ded449b726 zmq: Enable IPv6 on listening socket (nthumann)

Pull request description:

  This PR adds support for listening on IPv6 addresses with bitcoinds ZMQ interface, just like the RPC server.
  Currently, it is not possible to specify an IPv6 address, as the `ZMQ_IPV6` [socket option](http://api.zeromq.org/master:zmq-setsockopt#toc27) is not set and therefore the ZMQ initialization fails, if one does so. The absence of this option has also been noted [here](https://github.com/bitcoin/bitcoin/issues/15198#issuecomment-617378512).
  With this PR one can e.g. set `-zmqpubhashblock=tcp://[::1]:28333` to listen on the IPv6 loopback address.

ACKs for top commit:
  laanwj:
    Code review ACK e6998838e5
  theStack:
    Tested ACK e6998838e5 🌱

Tree-SHA512: 43c3043d8d5c79794d475926259c1be975b694db4fcc1f7750a9a28e242f0fa1b531735a63ea5777498003aa5834f6243f39742d0f3941f2f37593d0c7890700

test/functional/interface_zmq.py

@@ -24,6 +24,7 @@ from test_framework.util import (
     assert_equal,
     assert_raises_rpc_error,
 )
+from test_framework.netutil import test_ipv6_local
 from io import BytesIO
 from time import sleep
 
@@ -119,6 +120,7 @@ class ZMQTest (BitcoinTestFramework):
             self.test_mempool_sync()
             self.test_reorg()
             self.test_multiple_interfaces()
+            self.test_ipv6()
         finally:
             # Destroy the ZMQ context.
             self.log.debug("Destroying ZMQ context")
@@ -126,10 +128,12 @@ class ZMQTest (BitcoinTestFramework):
 
     # Restart node with the specified zmq notifications enabled, subscribe to
     # all of them and return the corresponding ZMQSubscriber objects.
-    def setup_zmq_test(self, services, *, recv_timeout=60, sync_blocks=True):
+    def setup_zmq_test(self, services, *, recv_timeout=60, sync_blocks=True, ipv6=False):
         subscribers = []
         for topic, address in services:
             socket = self.ctx.socket(zmq.SUB)
+            if ipv6:
+                socket.setsockopt(zmq.IPV6, 1)
             subscribers.append(ZMQSubscriber(socket, topic.encode()))
 
         self.restart_node(0, [f"-zmqpub{topic}={address}" for topic, address in services] +
@@ -568,5 +572,22 @@ class ZMQTest (BitcoinTestFramework):
         assert_equal(self.nodes[0].getbestblockhash(), subscribers[0].receive().hex())
         assert_equal(self.nodes[0].getbestblockhash(), subscribers[1].receive().hex())
 
+    def test_ipv6(self):
+        if not test_ipv6_local():
+            self.log.info("Skipping IPv6 test, because IPv6 is not supported.")
+            return
+        self.log.info("Testing IPv6")
+        # Set up subscriber using IPv6 loopback address
+        subscribers = self.setup_zmq_test([
+            ("hashblock", "tcp://[::1]:28332")
+        ], ipv6=True)
+
+        # Generate 1 block in nodes[0]
+        self.nodes[0].generatetoaddress(1, ADDRESS_BCRT1_UNSPENDABLE)
+
+        # Should receive the same block hash
+        assert_equal(self.nodes[0].getbestblockhash(), subscribers[0].receive().hex())
+
+
 if __name__ == '__main__':
     ZMQTest().main()