net/sockstat: fix per-interface statistics not always being available

withSockStats may be called before setLinkMonitor, in which case we
don't have a populated knownInterfaces map. Since we pre-populate the
per-interface counters at creation time, we would end up with an
empty map. To mitigate this, we do an on-demand request for the list of
interfaces.

This would most often happen with the logtail instrumentation, since we
initialize it very early on.

Updates tailscale/corp#9230

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/sockstats/sockstats_tsgo.go

@@ -53,9 +53,22 @@ func withSockStats(ctx context.Context, label Label) context.Context {
 			rxBytesByInterface: make(map[int]*atomic.Uint64),
 			txBytesByInterface: make(map[int]*atomic.Uint64),
 		}
-		for iface := range sockStats.knownInterfaces {
-			counters.rxBytesByInterface[iface] = &atomic.Uint64{}
-			counters.txBytesByInterface[iface] = &atomic.Uint64{}
+
+		// We might be called before setLinkMonitor has been called (and we've
+		// had a chance to populate knownInterfaces). In that case, we'll have
+		// to get the list of interfaces ourselves.
+		if len(sockStats.knownInterfaces) == 0 {
+			if ifaces, err := interfaces.GetList(); err == nil {
+				for _, iface := range ifaces {
+					counters.rxBytesByInterface[iface.Index] = &atomic.Uint64{}
+					counters.txBytesByInterface[iface.Index] = &atomic.Uint64{}
+				}
+			}
+		} else {
+			for iface := range sockStats.knownInterfaces {
+				counters.rxBytesByInterface[iface] = &atomic.Uint64{}
+				counters.txBytesByInterface[iface] = &atomic.Uint64{}
+			}
 		}
 		sockStats.countersByLabel[label] = counters
 	}