cmd/cloner: add regression test for slice nil/empty semantics

We had a misstep with the semantics when applying an optimization that
showed up in the roll into corp. This test ensures that case and related
cases must be retained.

Updates #9410
Updates #9601
Signed-off-by: James Tucker <james@tailscale.com>

cmd/cloner/cloner.go

@@ -128,7 +128,9 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 				if ptr, isPtr := ft.Elem().(*types.Pointer); isPtr {
 					if _, isBasic := ptr.Elem().Underlying().(*types.Basic); isBasic {
 						it.Import("tailscale.com/types/ptr")
+						writef("if src.%s[i] == nil { dst.%s[i] = nil } else {", fname, fname)
 						writef("\tdst.%s[i] = ptr.To(*src.%s[i])", fname, fname)
+						writef("}")
 					} else {
 						writef("\tdst.%s[i] = src.%s[i].Clone()", fname, fname)
 					}

cmd/cloner/cloner_test.go

@@ -0,0 +1,60 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+package main
+
+import (
+	"reflect"
+	"testing"
+
+	"tailscale.com/cmd/cloner/clonerex"
+)
+
+func TestSliceContainer(t *testing.T) {
+	num := 5
+	examples := []struct {
+		name string
+		in   *clonerex.SliceContianer
+	}{
+		{
+			name: "nil",
+			in:   nil,
+		},
+		{
+			name: "zero",
+			in:   &clonerex.SliceContianer{},
+		},
+		{
+			name: "empty",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{},
+			},
+		},
+		{
+			name: "nils",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{nil, nil, nil, nil, nil},
+			},
+		},
+		{
+			name: "one",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{&num},
+			},
+		},
+		{
+			name: "several",
+			in: &clonerex.SliceContianer{
+				Slice: []*int{&num, &num, &num, &num, &num},
+			},
+		},
+	}
+
+	for _, ex := range examples {
+		t.Run(ex.name, func(t *testing.T) {
+			out := ex.in.Clone()
+			if !reflect.DeepEqual(ex.in, out) {
+				t.Errorf("Clone() = %v, want %v", out, ex.in)
+			}
+		})
+	}
+}

cmd/cloner/clonerex/clonerex.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=true -type SliceContianer
+
+package clonerex
+
+type SliceContianer struct {
+	Slice []*int
+}

cmd/cloner/clonerex/clonerex_clone.go

@@ -0,0 +1,54 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by tailscale.com/cmd/cloner; DO NOT EDIT.
+
+package clonerex
+
+import (
+	"tailscale.com/types/ptr"
+)
+
+// Clone makes a deep copy of SliceContianer.
+// The result aliases no memory with the original.
+func (src *SliceContianer) Clone() *SliceContianer {
+	if src == nil {
+		return nil
+	}
+	dst := new(SliceContianer)
+	*dst = *src
+	if src.Slice != nil {
+		dst.Slice = make([]*int, len(src.Slice))
+		for i := range dst.Slice {
+			if src.Slice[i] == nil {
+				dst.Slice[i] = nil
+			} else {
+				dst.Slice[i] = ptr.To(*src.Slice[i])
+			}
+		}
+	}
+	return dst
+}
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _SliceContianerCloneNeedsRegeneration = SliceContianer(struct {
+	Slice []*int
+}{})
+
+// Clone duplicates src into dst and reports whether it succeeded.
+// To succeed, <src, dst> must be of types <*T, *T> or <*T, **T>,
+// where T is one of SliceContianer.
+func Clone(dst, src any) bool {
+	switch src := src.(type) {
+	case *SliceContianer:
+		switch dst := dst.(type) {
+		case *SliceContianer:
+			*dst = *src.Clone()
+			return true
+		case **SliceContianer:
+			*dst = src.Clone()
+			return true
+		}
+	}
+	return false
+}

cmd/viewer/tests/tests_clone.go

@@ -157,9 +157,13 @@ func (src *StructWithSlices) Clone() *StructWithSlices {
 	if src.Ints != nil {
 		dst.Ints = make([]*int, len(src.Ints))
 		for i := range dst.Ints {
+			if src.Ints[i] == nil {
+				dst.Ints[i] = nil
+			} else {
 				dst.Ints[i] = ptr.To(*src.Ints[i])
 			}
 		}
+	}
 	dst.Slice = append(src.Slice[:0:0], src.Slice...)
 	dst.Prefixes = append(src.Prefixes[:0:0], src.Prefixes...)
 	dst.Data = append(src.Data[:0:0], src.Data...)