// Copyright 2019 Yunion
//
// 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 runtime

import (
	"sync"
	"time"
)

// Cache stores the PodStatus for the pods. It represents *all* the visible
// pods/containers in the container runtime. All cache entries are at least as
// new or newer than the global timestamp (set by UpdateTime()), while
// individual entries may be slightly newer than the global timestamp. If a pod
// has no states known by the runtime, Cache returns an empty PodStatus object
// with ID populated.
//
// Cache provides two methods to retrieve the PodStatus: the non-blocking Get()
// and the blocking GetNewerThan() method. The component responsible for
// populating the cache is expected to call Delete() to explicitly free the
// cache entries.
type Cache interface {
	Get(string) (*PodStatus, error)
	Set(string, *PodStatus, error, time.Time)
	// GetNewerThan is a blocking call that only returns the status
	// when it is newer than the given time.
	GetNewerThan(string, time.Time) (*PodStatus, error)
	Delete(string)
	UpdateTime(time.Time)
}

type data struct {
	// Status of the pod.
	status *PodStatus
	// Error got when trying to inspect the pod.
	err error
	// Time when the data was last modified.
	modified time.Time
}

type subRecord struct {
	time time.Time
	ch   chan *data
}

// cache implements Cache.
type cache struct {
	// Lock which guards all internal data structures.
	lock sync.RWMutex
	// Map that stores the pod statuses.
	pods map[string]*data
	// A global timestamp represents how fresh the cached data is. All
	// cache content is at the least newer than this timestamp. Note that the
	// timestamp is nil after initialization, and will only become non-nil when
	// it is ready to serve the cached statuses.
	timestamp *time.Time
	// Map that stores the subscriber records.
	subscribers map[string][]*subRecord
}

// NewCache creates a pod cache.
func NewCache() Cache {
	return &cache{
		pods:        map[string]*data{},
		subscribers: map[string][]*subRecord{},
	}
}

// Get returns the PodStatus for the pod; callers are expected not to
// modify the objects returned.
func (c *cache) Get(id string) (*PodStatus, error) {
	c.lock.RLock()
	defer c.lock.RUnlock()
	d := c.get(id)
	return d.status, d.err
}

func (c *cache) GetNewerThan(id string, minTime time.Time) (*PodStatus, error) {
	ch := c.subscribe(id, minTime)
	d := <-ch
	return d.status, d.err
}

// Set sets the PodStatus for the pod.
func (c *cache) Set(id string, status *PodStatus, err error, timestamp time.Time) {
	c.lock.Lock()
	defer c.lock.Unlock()
	defer c.notify(id, timestamp)
	c.pods[id] = &data{status: status, err: err, modified: timestamp}
}

// Delete removes the entry of the pod.
func (c *cache) Delete(id string) {
	c.lock.Lock()
	defer c.lock.Unlock()
	delete(c.pods, id)
}

// UpdateTime modifies the global timestamp of the cache and notify
// subscribers if needed.
func (c *cache) UpdateTime(timestamp time.Time) {
	c.lock.Lock()
	defer c.lock.Unlock()
	c.timestamp = &timestamp
	// Notify all the subscribers if the condition is met.
	for id := range c.subscribers {
		c.notify(id, *c.timestamp)
	}
}

func makeDefaultData(id string) *data {
	return &data{status: &PodStatus{ID: id}, err: nil}
}

func (c *cache) get(id string) *data {
	d, ok := c.pods[id]
	if !ok {
		// Cache should store *all* pod/container information known by the
		// container runtime. A cache miss indicates that there are no states
		// regarding the pod last time we queried the container runtime.
		// What this *really* means is that there are no visible pod/containers
		// associated with this pod. Simply return an default (mostly empty)
		// PodStatus to reflect this.
		return makeDefaultData(id)
	}
	return d
}

// getIfNewerThan returns the data it is newer than the given time.
// Otherwise, it returns nil. The caller should acquire the lock.
func (c *cache) getIfNewerThan(id string, minTime time.Time) *data {
	d, ok := c.pods[id]
	globalTimestampIsNewer := (c.timestamp != nil && c.timestamp.After(minTime))
	if !ok && globalTimestampIsNewer {
		// Status is not cached, but the global timestamp is newer than
		// minTime, return the default status.
		return makeDefaultData(id)
	}
	if ok && (d.modified.After(minTime) || globalTimestampIsNewer) {
		// Status is cached, return status if either of the following is true.
		//   * status was modified after minTime
		//   * the global timestamp of the cache is newer than minTime.
		return d
	}
	// The pod status is not ready.
	return nil
}

// notify sends notifications for pod with the given id, if the requirements
// are met. Note that the caller should acquire the lock.
func (c *cache) notify(id string, timestamp time.Time) {
	list, ok := c.subscribers[id]
	if !ok {
		// No one to notify.
		return
	}
	newList := []*subRecord{}
	for i, r := range list {
		if timestamp.Before(r.time) {
			// Doesn't meet the time requirement; keep the record.
			newList = append(newList, list[i])
			continue
		}
		r.ch <- c.get(id)
		close(r.ch)
	}
	if len(newList) == 0 {
		delete(c.subscribers, id)
	} else {
		c.subscribers[id] = newList
	}
}

func (c *cache) subscribe(id string, timestamp time.Time) chan *data {
	ch := make(chan *data, 1)
	c.lock.Lock()
	defer c.lock.Unlock()
	d := c.getIfNewerThan(id, timestamp)
	if d != nil {
		// If the cache entry is ready, send the data and return immediately.
		ch <- d
		return ch
	}
	// Add the subscription record.
	c.subscribers[id] = append(c.subscribers[id], &subRecord{time: timestamp, ch: ch})
	return ch
}