Restore compatibility with old CSubNet serialization

Author: sipa

src/netaddress.h

@@ -468,7 +468,21 @@ class CSubNet
         friend bool operator!=(const CSubNet& a, const CSubNet& b) { return !(a == b); }
         friend bool operator<(const CSubNet& a, const CSubNet& b);
 
-        SERIALIZE_METHODS(CSubNet, obj) { READWRITE(obj.network, obj.netmask, obj.valid); }
+        SERIALIZE_METHODS(CSubNet, obj)
+        {
+            READWRITE(obj.network);
+            if (obj.network.IsIPv4()) {
+                // Before commit 102867c587f5f7954232fb8ed8e85cda78bb4d32, CSubNet used the last 4 bytes of netmask
+                // to store the relevant bytes for an IPv4 mask. For compatiblity reasons, keep doing so in
+                // serialized form.
+                unsigned char dummy[12] = {0};
+                READWRITE(dummy);
+                READWRITE(MakeSpan(obj.netmask).first(4));
+            } else {
+                READWRITE(obj.netmask);
+            }
+            READWRITE(obj.valid);
+        }
 };
 
 /** A combination of a network address (CNetAddr) and a (TCP) port */