Merge #20993: test: store subversion (user agent) as string in msg_version

de85af5cce test: store subversion (user agent) as string in msg_version (Sebastian Falbesoner)

Pull request description:

  It seems more natural to treat the "subversion" field (=user agent string, see [BIP 14](https://github.com/bitcoin/bips/blob/master/bip-0014.mediawiki#Proposal)) of a node as pure string rather than a bytestring within the test framework. This is also suggested with the naming prefix in `msg_version.strSubVer`: one probably wouldn't expect a field starting with "str" to be a bytestring that needs further decoding to be useful. This PR moves the encoding/decoding parts to the serialization/deserialization routines so that the user doesn't have to bother with that anymore.

  Note that currently, in the master branch the `msg_version.strSubVer` is never read (only in `msg_version.__repr__`); However, one issue that is solved by this PR came up while testing #19509 (not merged yet): A decoding script for binary message capture files takes use of the functional test framework convert it into JSON format. Bytestrings will be convered to hexstrings, while pure strings will (surprise surprise) end up without modification in the file.

  So without this patch, we get:

  ```
  $ jq . out.json | grep -m5 strSubVer
        "strSubVer": "2f5361746f7368693a32312e39392e302f"
        "strSubVer": "2f5361746f7368693a302e32302e312f"
        "strSubVer": "2f5361746f7368693a32312e39392e302f"
        "strSubVer": "2f5361746f7368693a302e32302e312f"
        "strSubVer": "2f5361746f7368693a32312e39392e302f"
  ```

  After this patch:

  ```
  $ jq . out2.json | grep -m5 strSubVer
        "strSubVer": "/Satoshi:21.99.0/"
        "strSubVer": "/Satoshi:0.20.1/"
        "strSubVer": "/Satoshi:21.99.0/"
        "strSubVer": "/Satoshi:0.20.1/"
        "strSubVer": "/Satoshi:21.99.0/"
  ```

ACKs for top commit:
  jnewbery:
    utACK de85af5cce

Tree-SHA512: ff23642705c858e8387a625537dfec82e6b8a15da6d99b8d12152560e52d243ba17431b602b26f60996d897e00e3f37dcf8dc8a303ffb1d544df29a5937080f9

test/functional/test_framework/messages.py

@@ -33,7 +33,7 @@ from test_framework.util import hex_str_to_bytes, assert_equal
 
 MIN_VERSION_SUPPORTED = 60001
 MY_VERSION = 70016  # past wtxid relay
-MY_SUBVERSION = b"/python-p2p-tester:0.0.3/"
+MY_SUBVERSION = "/python-p2p-tester:0.0.3/"
 MY_RELAY = 1 # from version 70001 onwards, fRelay should be appended to version messages (BIP37)
 
 MAX_LOCATOR_SZ = 101
@@ -1048,7 +1048,7 @@ class msg_version:
         self.addrFrom = CAddress()
         self.addrFrom.deserialize(f, with_time=False)
         self.nNonce = struct.unpack("<Q", f.read(8))[0]
-        self.strSubVer = deser_string(f)
+        self.strSubVer = deser_string(f).decode('utf-8')
 
         self.nStartingHeight = struct.unpack("<i", f.read(4))[0]
 
@@ -1069,7 +1069,7 @@ class msg_version:
         r += self.addrTo.serialize(with_time=False)
         r += self.addrFrom.serialize(with_time=False)
         r += struct.pack("<Q", self.nNonce)
-        r += ser_string(self.strSubVer)
+        r += ser_string(self.strSubVer.encode('utf-8'))
         r += struct.pack("<i", self.nStartingHeight)
         r += struct.pack("<b", self.nRelay)
         return r

test/functional/test_framework/test_node.py

@@ -572,7 +572,7 @@ class TestNode():
 
     def num_test_p2p_connections(self):
         """Return number of test framework p2p connections to the node."""
-        return len([peer for peer in self.getpeerinfo() if peer['subver'] == MY_SUBVERSION.decode("utf-8")])
+        return len([peer for peer in self.getpeerinfo() if peer['subver'] == MY_SUBVERSION])
 
     def disconnect_p2ps(self):
         """Close all p2p connections to the node."""