cc-backend/pkg/lrucache/cache.go

// Copyright (C) 2022 NHR@FAU, University Erlangen-Nuremberg.
// All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.
package lrucache

import (
	"sync"
	"time"
)

// Type of the closure that must be passed to `Get` to
// compute the value in case it is not cached.
//
// returned values are the computed value to be stored in the cache,
// the duration until this value will expire and a size estimate.
type ComputeValue func() (value interface{}, ttl time.Duration, size int)

type cacheEntry struct {
	key   string
	value interface{}

	expiration            time.Time
	size                  int
	waitingForComputation int

	next, prev *cacheEntry
}

type Cache struct {
	mutex                 sync.Mutex
	cond                  *sync.Cond
	maxmemory, usedmemory int
	entries               map[string]*cacheEntry
	head, tail            *cacheEntry
}

// Return a new instance of a LRU In-Memory Cache.
// Read [the README](./README.md) for more information
// on what is going on with `maxmemory`.
func New(maxmemory int) *Cache {
	cache := &Cache{
		maxmemory: maxmemory,
		entries:   map[string]*cacheEntry{},
	}
	cache.cond = sync.NewCond(&cache.mutex)
	return cache
}

// Return the cached value for key `key` or call `computeValue` and
// store its return value in the cache. If called, the closure will be
// called synchronous and __shall not call methods on the same cache__
// or a deadlock might ocure. If `computeValue` is nil, the cache is checked
// and if no entry was found, nil is returned. If another goroutine is currently
// computing that value, the result is waited for.
func (c *Cache) Get(key string, computeValue ComputeValue) interface{} {
	now := time.Now()

	c.mutex.Lock()
	if entry, ok := c.entries[key]; ok {
		// The expiration not being set is what shows us that
		// the computation of that value is still ongoing.
		for entry.expiration.IsZero() {
			entry.waitingForComputation += 1
			c.cond.Wait()
			entry.waitingForComputation -= 1
		}

		if now.After(entry.expiration) {
			if !c.evictEntry(entry) {
				if entry.expiration.IsZero() {
					panic("cache entry that shoud have been waited for could not be evicted.")
				}
				c.mutex.Unlock()
				return entry.value
			}
		} else {
			if entry != c.head {
				c.unlinkEntry(entry)
				c.insertFront(entry)
			}
			c.mutex.Unlock()
			return entry.value
		}
	}

	if computeValue == nil {
		c.mutex.Unlock()
		return nil
	}

	entry := &cacheEntry{
		key:                   key,
		waitingForComputation: 1,
	}

	c.entries[key] = entry

	hasPaniced := true
	defer func() {
		if hasPaniced {
			c.mutex.Lock()
			delete(c.entries, key)
			entry.expiration = now
			entry.waitingForComputation -= 1
		}
		c.mutex.Unlock()
	}()

	c.mutex.Unlock()
	value, ttl, size := computeValue()
	c.mutex.Lock()
	hasPaniced = false

	entry.value = value
	entry.expiration = now.Add(ttl)
	entry.size = size
	entry.waitingForComputation -= 1

	// Only broadcast if other goroutines are actually waiting
	// for a result.
	if entry.waitingForComputation > 0 {
		// TODO: Have more than one condition variable so that there are
		// less unnecessary wakeups.
		c.cond.Broadcast()
	}

	c.usedmemory += size
	c.insertFront(entry)

	// Evict only entries with a size of more than zero.
	// This is the only loop in the implementation outside of the `Keys`
	// method.
	evictionCandidate := c.tail
	for c.usedmemory > c.maxmemory && evictionCandidate != nil {
		nextCandidate := evictionCandidate.prev
		if (evictionCandidate.size > 0 || now.After(evictionCandidate.expiration)) &&
			evictionCandidate.waitingForComputation == 0 {
			c.evictEntry(evictionCandidate)
		}
		evictionCandidate = nextCandidate
	}

	return value
}

// Put a new value in the cache. If another goroutine is calling `Get` and
// computing the value, this function waits for the computation to be done
// before it overwrites the value.
func (c *Cache) Put(key string, value interface{}, size int, ttl time.Duration) {
	now := time.Now()
	c.mutex.Lock()
	defer c.mutex.Unlock()

	if entry, ok := c.entries[key]; ok {
		for entry.expiration.IsZero() {
			entry.waitingForComputation += 1
			c.cond.Wait()
			entry.waitingForComputation -= 1
		}

		c.usedmemory -= entry.size
		entry.expiration = now.Add(ttl)
		entry.size = size
		entry.value = value
		c.usedmemory += entry.size

		c.unlinkEntry(entry)
		c.insertFront(entry)
		return
	}

	entry := &cacheEntry{
		key:        key,
		value:      value,
		expiration: now.Add(ttl),
	}
	c.entries[key] = entry
	c.insertFront(entry)
}

// Remove the value at key `key` from the cache.
// Return true if the key was in the cache and false
// otherwise. It is possible that true is returned even
// though the value already expired.
// It is possible that false is returned even though the value
// will show up in the cache if this function is called on a key
// while that key is beeing computed.
func (c *Cache) Del(key string) bool {
	c.mutex.Lock()
	defer c.mutex.Unlock()

	if entry, ok := c.entries[key]; ok {
		return c.evictEntry(entry)
	}
	return false
}

// Call f for every entry in the cache. Some sanity checks
// and eviction of expired keys are done as well.
// The cache is fully locked for the complete duration of this call!
func (c *Cache) Keys(f func(key string, val interface{})) {
	c.mutex.Lock()
	defer c.mutex.Unlock()

	now := time.Now()

	size := 0
	for key, e := range c.entries {
		if key != e.key {
			panic("key mismatch")
		}

		if now.After(e.expiration) {
			if c.evictEntry(e) {
				continue
			}
		}

		if e.prev != nil {
			if e.prev.next != e {
				panic("list corrupted")
			}
		}

		if e.next != nil {
			if e.next.prev != e {
				panic("list corrupted")
			}
		}

		size += e.size
		f(key, e.value)
	}

	if size != c.usedmemory {
		panic("size calculations failed")
	}

	if c.head != nil {
		if c.tail == nil || c.head.prev != nil {
			panic("head/tail corrupted")
		}
	}

	if c.tail != nil {
		if c.head == nil || c.tail.next != nil {
			panic("head/tail corrupted")
		}
	}
}

func (c *Cache) insertFront(e *cacheEntry) {
	e.next = c.head
	c.head = e

	e.prev = nil
	if e.next != nil {
		e.next.prev = e
	}

	if c.tail == nil {
		c.tail = e
	}
}

func (c *Cache) unlinkEntry(e *cacheEntry) {
	if e == c.head {
		c.head = e.next
	}
	if e.prev != nil {
		e.prev.next = e.next
	}
	if e.next != nil {
		e.next.prev = e.prev
	}
	if e == c.tail {
		c.tail = e.prev
	}
}

func (c *Cache) evictEntry(e *cacheEntry) bool {
	if e.waitingForComputation != 0 {
		// panic("cannot evict this entry as other goroutines need the value")
		return false
	}

	c.unlinkEntry(e)
	c.usedmemory -= e.size
	delete(c.entries, e.key)
	return true
}