util/mak: move tailssh's mapSet into a new package for reuse elsewhere

Change-Id: Idfe95db82275fd2be6ca88f245830731a0d5aecf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/mak/mak.go

@@ -0,0 +1,53 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mak helps make maps. It contains generic helpers to make/assign
+// things, notably to maps, but also slices.
+package mak
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// Set populates an entry in a map, making the map if necessary.
+//
+// That is, it assigns (*m)[k] = v, making *m if it was nil.
+func Set[K comparable, V any, T ~map[K]V](m *T, k K, v V) {
+	if *m == nil {
+		*m = make(map[K]V)
+	}
+	(*m)[k] = v
+}
+
+// NonNil takes a pointer to a Go data structure
+// (currently only a slice or a map) and makes sure it's non-nil for
+// JSON serialization. (In particular, JavaScript clients usually want
+// the field to be defined after they decode the JSON.)
+// MakeNonNil takes a pointer to a Go data structure
+// (currently only a slice or a map) and makes sure it's non-nil for
+// JSON serialization. (In particular, JavaScript clients usually want
+// the field to be defined after they decode the JSON.)
+func NonNil(ptr interface{}) {
+	if ptr == nil {
+		panic("nil interface")
+	}
+	rv := reflect.ValueOf(ptr)
+	if rv.Kind() != reflect.Ptr {
+		panic(fmt.Sprintf("kind %v, not Ptr", rv.Kind()))
+	}
+	if rv.Pointer() == 0 {
+		panic("nil pointer")
+	}
+	rv = rv.Elem()
+	if rv.Pointer() != 0 {
+		return
+	}
+	switch rv.Type().Kind() {
+	case reflect.Slice:
+		rv.Set(reflect.MakeSlice(rv.Type(), 0, 0))
+	case reflect.Map:
+		rv.Set(reflect.MakeMap(rv.Type()))
+	}
+}

util/mak/mak_test.go

@@ -0,0 +1,71 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mak contains code to help make things.
+package mak
+
+import (
+	"reflect"
+	"testing"
+)
+
+type M map[string]int
+
+func TestSet(t *testing.T) {
+	t.Run("unnamed", func(t *testing.T) {
+		var m map[string]int
+		Set(&m, "foo", 42)
+		Set(&m, "bar", 1)
+		Set(&m, "bar", 2)
+		want := map[string]int{
+			"foo": 42,
+			"bar": 2,
+		}
+		if got := m; !reflect.DeepEqual(got, want) {
+			t.Errorf("got %v; want %v", got, want)
+		}
+	})
+	t.Run("named", func(t *testing.T) {
+		var m M
+		Set(&m, "foo", 1)
+		Set(&m, "bar", 1)
+		Set(&m, "bar", 2)
+		want := M{
+			"foo": 1,
+			"bar": 2,
+		}
+		if got := m; !reflect.DeepEqual(got, want) {
+			t.Errorf("got %v; want %v", got, want)
+		}
+	})
+}
+
+func TestNonNil(t *testing.T) {
+	var s []string
+	NonNil(&s)
+	if len(s) != 0 {
+		t.Errorf("slice len = %d; want 0", len(s))
+	}
+	if s == nil {
+		t.Error("slice still nil")
+	}
+
+	s = append(s, "foo")
+	NonNil(&s)
+	if len(s) != 1 {
+		t.Errorf("len = %d; want 1", len(s))
+	}
+	if s[0] != "foo" {
+		t.Errorf("value = %q; want foo", s)
+	}
+
+	var m map[string]string
+	NonNil(&m)
+	if len(m) != 0 {
+		t.Errorf("map len = %d; want 0", len(s))
+	}
+	if m == nil {
+		t.Error("map still nil")
+	}
+}