Merge bitcoin/bitcoin#17160: refactor: net: subnet lookup: use single-result LookupHost()

a989f98d24 refactor: net: subnet lookup: use single-result LookupHost() (Sebastian Falbesoner)

Pull request description:

  plus describe single IP subnet case for more clarity

ACKs for top commit:
  jonatack:
    utACK a989f98d24 the patch rebases cleanly to master, the debug build is green, and it is essentially the same patch as c8991f0251dd2a modulo local variable naming, braced initialization, and a comment
  vasild:
    ACK a989f98d24

Tree-SHA512: 082d3481b1fa5e5f3267b7c4a812954b67b36d1f94c5296fe20110699f053e5042dfa13f728ae20249e9b8d71e930c3b119410125d0faeccdfbdc259223ee3a6

src/netbase.cpp

@@ -682,14 +682,11 @@ bool LookupSubNet(const std::string& strSubnet, CSubNet& ret, DNSLookupFn dns_lo
         return false;
     }
     size_t slash = strSubnet.find_last_of('/');
-    std::vector<CNetAddr> vIP;
+    CNetAddr network;
 
     std::string strAddress = strSubnet.substr(0, slash);
-    // TODO: Use LookupHost(const std::string&, CNetAddr&, bool) instead to just get
+    if (LookupHost(strAddress, network, false, dns_lookup_function))
-    //       one CNetAddr.
-    if (LookupHost(strAddress, vIP, 1, false, dns_lookup_function))
     {
-        CNetAddr network = vIP[0];
         if (slash != strSubnet.npos)
         {
             std::string strNetmask = strSubnet.substr(slash + 1);
@@ -701,14 +698,15 @@ bool LookupSubNet(const std::string& strSubnet, CSubNet& ret, DNSLookupFn dns_lo
             }
             else // If not a valid number, try full netmask syntax
             {
+                CNetAddr netmask;
                 // Never allow lookup for netmask
-                if (LookupHost(strNetmask, vIP, 1, false, dns_lookup_function)) {
+                if (LookupHost(strNetmask, netmask, false, dns_lookup_function)) {
-                    ret = CSubNet(network, vIP[0]);
+                    ret = CSubNet(network, netmask);
                     return ret.IsValid();
                 }
             }
         }
-        else
+        else // Single IP subnet (<ipv4>/32 or <ipv6>/128)
         {
             ret = CSubNet(network);
             return ret.IsValid();