wgengine/wgcfg/nmcfg: coalesce, limit some debug logs

Updates #14881

Change-Id: I708d29244fe901ab037203a5d7c2cae3c77e4c78
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgcfg/nmcfg/nmcfg.go

@@ -81,8 +81,8 @@ func WGCfg(nm *netmap.NetworkMap, logf logger.Logf, flags netmap.WGConfigFlags,
 
 	// Logging buffers
 	skippedUnselected := new(bytes.Buffer)
-	skippedIPs := new(bytes.Buffer)
 	skippedSubnets := new(bytes.Buffer)
+	skippedExpired := new(bytes.Buffer)
 
 	for _, peer := range nm.Peers {
 		if peer.DiscoKey().IsZero() && peer.HomeDERP() == 0 && !peer.IsWireGuardOnly() {
@@ -95,7 +95,16 @@ func WGCfg(nm *netmap.NetworkMap, logf logger.Logf, flags netmap.WGConfigFlags,
 		// anyway, since control intentionally breaks node keys for
 		// expired peers so that we can't discover endpoints via DERP.
 		if peer.Expired() {
-			logf("[v1] wgcfg: skipped expired peer %s", peer.Key().ShortString())
+			if skippedExpired.Len() >= 1<<10 {
+				if !bytes.HasSuffix(skippedExpired.Bytes(), []byte("...")) {
+					skippedExpired.WriteString("...")
+				}
+			} else {
+				if skippedExpired.Len() > 0 {
+					skippedExpired.WriteString(", ")
+				}
+				fmt.Fprintf(skippedExpired, "%s/%v", peer.StableID(), peer.Key().ShortString())
+			}
 			continue
 		}
 
@@ -137,12 +146,11 @@ func WGCfg(nm *netmap.NetworkMap, logf logger.Logf, flags netmap.WGConfigFlags,
 	if skippedUnselected.Len() > 0 {
 		logf("[v1] wgcfg: skipped unselected default routes from: %s", skippedUnselected.Bytes())
 	}
-	if skippedIPs.Len() > 0 {
-		logf("[v1] wgcfg: skipped node IPs: %s", skippedIPs)
-	}
 	if skippedSubnets.Len() > 0 {
 		logf("[v1] wgcfg: did not accept subnet routes: %s", skippedSubnets)
 	}
-
+	if skippedExpired.Len() > 0 {
+		logf("[v1] wgcfg: skipped expired peer: %s", skippedExpired)
+	}
 	return cfg, nil
 }