cmd/cloner: preserve nil-valued entries when cloning map (#19749)

The codegen path for map-of-slice-of-pointer fields, skipped
nil-valued entries. That dropped the key from the map.

This broke how dns.Config.Routes uses nil values sentinels.

Fixes #19730
Fixes #19732
Fixes #19746
Fixes #19744

Change-Id: Ic6400227f4ab21b3ca0e8c0eeecf9b83d145a9ab

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

cmd/cloner/cloner.go

@@ -176,6 +176,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 				if codegen.ContainsPointers(sliceType.Elem()) {
 					writef("\tfor k, sv := range src.%s {", fname)
 					writef("\t\tif sv == nil {")
+					writef("\t\t\tdst.%s[k] = nil", fname)
 					writef("\t\t\tcontinue")
 					writef("\t\t}")
 					writef("\t\tdst.%s[k] = make([]%s, len(sv))", fname, n)

cmd/cloner/cloner_test.go

@@ -7,6 +7,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"tailscale.com/cmd/cloner/clonerex"
 )
 
@@ -208,6 +209,20 @@ func TestMapSlicePointerContainer(t *testing.T) {
 	}
 }
 
+func TestMapSlicePointerContainerNilValue(t *testing.T) {
+	num := 7
+	orig := &clonerex.MapSlicePointerContainer{
+		Routes: map[string][]*clonerex.SliceContainer{
+			"nil-value": nil,
+			"non-nil":   {{Slice: []*int{&num}}},
+		},
+	}
+	cloned := orig.Clone()
+	if diff := cmp.Diff(orig.Routes, cloned.Routes); diff != "" {
+		t.Errorf("Clone() Routes mismatch (-orig +cloned):\n%s", diff)
+	}
+}
+
 func TestDeeplyNestedMap(t *testing.T) {
 	num := 123
 	orig := &clonerex.DeeplyNestedMap{

cmd/cloner/clonerex/clonerex_clone.go

@@ -188,6 +188,7 @@ func (src *MapSlicePointerContainer) Clone() *MapSlicePointerContainer {
 		dst.Routes = map[string][]*SliceContainer{}
 		for k, sv := range src.Routes {
 			if sv == nil {
+				dst.Routes[k] = nil
 				continue
 			}
 			dst.Routes[k] = make([]*SliceContainer, len(sv))