Implement lazy loading on /sync (#2346)

* Initial work on lazyloading

* Partially implement lazy loading on /sync

* Rename methods

* Make missing tests pass

* Preallocate slice, even if it will end up with fewer values

* Let the cache handle the user mapping

* Linter

* Cap cache growth

internal/caching/cache_lazy_load_members.go

@@ -0,0 +1,86 @@
+package caching
+
+import (
+	"fmt"
+	"time"
+
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+)
+
+const (
+	LazyLoadCacheName           = "lazy_load_members"
+	LazyLoadCacheMaxEntries     = 128
+	LazyLoadCacheMaxUserEntries = 128
+	LazyLoadCacheMutable        = true
+	LazyLoadCacheMaxAge         = time.Minute * 30
+)
+
+type LazyLoadCache struct {
+	// InMemoryLRUCachePartition containing other InMemoryLRUCachePartitions
+	// with the actual cached members
+	userCaches *InMemoryLRUCachePartition
+}
+
+// NewLazyLoadCache creates a new LazyLoadCache.
+func NewLazyLoadCache() (*LazyLoadCache, error) {
+	cache, err := NewInMemoryLRUCachePartition(
+		LazyLoadCacheName,
+		LazyLoadCacheMutable,
+		LazyLoadCacheMaxEntries,
+		LazyLoadCacheMaxAge,
+		true,
+	)
+	if err != nil {
+		return nil, err
+	}
+	go cacheCleaner(cache)
+	return &LazyLoadCache{
+		userCaches: cache,
+	}, nil
+}
+
+func (c *LazyLoadCache) lazyLoadCacheForUser(device *userapi.Device) (*InMemoryLRUCachePartition, error) {
+	cacheName := fmt.Sprintf("%s/%s", device.UserID, device.ID)
+	userCache, ok := c.userCaches.Get(cacheName)
+	if ok && userCache != nil {
+		if cache, ok := userCache.(*InMemoryLRUCachePartition); ok {
+			return cache, nil
+		}
+	}
+	cache, err := NewInMemoryLRUCachePartition(
+		LazyLoadCacheName,
+		LazyLoadCacheMutable,
+		LazyLoadCacheMaxUserEntries,
+		LazyLoadCacheMaxAge,
+		false,
+	)
+	if err != nil {
+		return nil, err
+	}
+	c.userCaches.Set(cacheName, cache)
+	go cacheCleaner(cache)
+	return cache, nil
+}
+
+func (c *LazyLoadCache) StoreLazyLoadedUser(device *userapi.Device, roomID, userID, eventID string) {
+	cache, err := c.lazyLoadCacheForUser(device)
+	if err != nil {
+		return
+	}
+	cacheKey := fmt.Sprintf("%s/%s/%s/%s", device.UserID, device.ID, roomID, userID)
+	cache.Set(cacheKey, eventID)
+}
+
+func (c *LazyLoadCache) IsLazyLoadedUserCached(device *userapi.Device, roomID, userID string) (string, bool) {
+	cache, err := c.lazyLoadCacheForUser(device)
+	if err != nil {
+		return "", false
+	}
+
+	cacheKey := fmt.Sprintf("%s/%s/%s/%s", device.UserID, device.ID, roomID, userID)
+	val, ok := cache.Get(cacheKey)
+	if !ok {
+		return "", ok
+	}
+	return val.(string), ok
+}