Merge bitcoin/bitcoin#29984: net: Replace ifname check with IFF_LOOPBACK in Discover

a68fed1 net: Fix misleading comment for Discover (laanwj)
7766dd2 net: Replace ifname check with IFF_LOOPBACK in Discover (laanwj)

Pull request description:

  Checking the interface name is kind of brittle. In the age of network namespaces and containers, there is no reason a loopback interface can't be called differently.

  Check for the `IFF_LOOPBACK` flag to detect loopback interface instead.

  Also remove a misleading comment in Discover's doc comment.

ACKs for top commit:
  sipa:
    utACK a68fed1
  willcl-ark:
    utACK a68fed1
  theuni:
    utACK a68fed1. Satoshi-era brittleness :)

Tree-SHA512: e2d7fc541f40f6a6af08286e7bcb0873ff55debdcd8b38b03f274897b673a6fb51d84d6c7241a02a9567ddf2645f50231d91bb1f55307ba7c6e68196c29b0edf

Author: fanquake

src/net.cpp

@@ -3113,8 +3113,7 @@ void Discover()
         {
             if (ifa->ifa_addr == nullptr) continue;
             if ((ifa->ifa_flags & IFF_UP) == 0) continue;
-            if (strcmp(ifa->ifa_name, "lo") == 0) continue;
-            if (strcmp(ifa->ifa_name, "lo0") == 0) continue;
+            if ((ifa->ifa_flags & IFF_LOOPBACK) != 0) continue;
             if (ifa->ifa_addr->sa_family == AF_INET)
             {
                 struct sockaddr_in* s4 = (struct sockaddr_in*)(ifa->ifa_addr);

src/net.h

@@ -137,8 +137,7 @@ struct CSerializedNetMsg {
 /**
  * Look up IP addresses from all interfaces on the machine and add them to the
  * list of local addresses to self-advertise.
- * The loopback interface is skipped and only the first address from each
- * interface is used.
+ * The loopback interface is skipped.
  */
 void Discover();