wgengine/tsdns: truncate Map.PrettyDiffFrom string at 1KB

Hello's were painful.

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

wgengine/tsdns/map.go

@@ -92,6 +92,9 @@ func (m *Map) PrettyDiffFrom(old *Map) string {
 	}
 
 	buf := new(strings.Builder)
+	space := func() bool {
+		return buf.Len() < (1 << 10)
+	}
 
 	for len(oldNames) > 0 && len(newNames) > 0 {
 		var name string
@@ -109,6 +112,9 @@ func (m *Map) PrettyDiffFrom(old *Map) string {
 			oldNames = oldNames[1:]
 			newNames = newNames[1:]
 		}
+		if !space() {
+			continue
+		}
 
 		ipOld, inOld := oldNameToIP[name]
 		ipNew, inNew := newNameToIP[name]
@@ -128,6 +134,9 @@ func (m *Map) PrettyDiffFrom(old *Map) string {
 	}
 
 	for _, name := range oldNames {
+		if !space() {
+			break
+		}
 		if _, ok := newNameToIP[name]; !ok {
 			buf.WriteByte('-')
 			printSingleNameIP(buf, name, oldNameToIP[name])
@@ -135,11 +144,17 @@ func (m *Map) PrettyDiffFrom(old *Map) string {
 	}
 
 	for _, name := range newNames {
+		if !space() {
+			break
+		}
 		if _, ok := oldNameToIP[name]; !ok {
 			buf.WriteByte('+')
 			printSingleNameIP(buf, name, newNameToIP[name])
 		}
 	}
+	if !space() {
+		buf.WriteString("... [truncated]\n")
+	}
 
 	return buf.String()
 }

wgengine/tsdns/map_test.go

@@ -5,6 +5,8 @@
 package tsdns
 
 import (
+	"fmt"
+	"strings"
 	"testing"
 
 	"inet.af/netaddr"
@@ -135,4 +137,20 @@ func TestPrettyDiffFrom(t *testing.T) {
 			}
 		})
 	}
+
+	t.Run("truncated", func(t *testing.T) {
+		small := NewMap(nil, nil)
+		m := map[string]netaddr.IP{}
+		for i := 0; i < 5000; i++ {
+			m[fmt.Sprintf("host%d.ipn.dev.", i)] = netaddr.IPv4(100, 64, 1, 1)
+		}
+		veryBig := NewMap(m, nil)
+		diff := veryBig.PrettyDiffFrom(small)
+		if len(diff) > 3<<10 {
+			t.Errorf("pretty diff too large: %d bytes", len(diff))
+		}
+		if !strings.Contains(diff, "truncated") {
+			t.Errorf("big diff not truncated")
+		}
+	})
 }