/*
Copyright 2021 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package handler

import (
	"sync"
)

// HandlerCache represents a lazy cache for generating a byte array
// It is used to lazily marshal OpenAPI v2/v3 and lazily generate the ETag
type HandlerCache struct {
	BuildCache func() ([]byte, error)
	once       sync.Once
	bytes      []byte
	err        error
}

// Get either returns the cached value or calls BuildCache() once before caching and returning
// its results. If BuildCache returns an error, the last valid value for the cache (from prior
// calls to New()) is used instead if possible.
func (c *HandlerCache) Get() ([]byte, error) {
	c.once.Do(func() {
		bytes, err := c.BuildCache()
		// if there is an error updating the cache, there can be situations where
		// c.bytes contains a valid value (carried over from the previous update)
		// but c.err is also not nil; the cache user is expected to check for this
		c.err = err
		if c.err == nil {
			// don't override previous spec if we had an error
			c.bytes = bytes
		}
	})
	return c.bytes, c.err
}

// New creates a new HandlerCache for situations where a cache refresh is needed.
// This function is not thread-safe and should not be called at the same time as Get().
func (c *HandlerCache) New(cacheBuilder func() ([]byte, error)) HandlerCache {
	return HandlerCache{
		bytes:      c.bytes,
		BuildCache: cacheBuilder,
	}
}