jsondb: small package to load/save JSON DBs.

Signed-off-by: David Anderson <danderson@tailscale.com>

jsondb/db.go

@@ -0,0 +1,58 @@
+// 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 jsondb provides a trivial "database": a Go object saved to
+// disk as JSON.
+package jsondb
+
+import (
+	"encoding/json"
+	"errors"
+	"io/fs"
+	"os"
+
+	"tailscale.com/atomicfile"
+)
+
+// DB is a database backed by a JSON file.
+type DB[T any] struct {
+	// Data is the contents of the database.
+	Data *T
+
+	path string
+}
+
+// Open opens the database at path, creating it with a zero value if
+// necessary.
+func Open[T any](path string) (*DB[T], error) {
+	bs, err := os.ReadFile(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return &DB[T]{
+			Data: new(T),
+			path: path,
+		}, nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	var val T
+	if err := json.Unmarshal(bs, &val); err != nil {
+		return nil, err
+	}
+
+	return &DB[T]{
+		Data: &val,
+		path: path,
+	}, nil
+}
+
+// Save writes db.Data back to disk.
+func (db *DB[T]) Save() error {
+	bs, err := json.Marshal(db.Data)
+	if err != nil {
+		return err
+	}
+
+	return atomicfile.WriteFile(db.path, bs, 0600)
+}