Add simple in-memory cache (#2231)

* Add simple in-memory cache

* Support configuring cache expiration duration

* Support preinitializing cache with items

* Register cache

cache/cache.go

@@ -0,0 +1,65 @@
+package cache
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+var (
+	// DefaultCache is the default cache.
+	DefaultCache Cache = NewCache()
+	// DefaultExpiration is the default duration for items stored in
+	// the cache to expire.
+	DefaultExpiration time.Duration = 0
+
+	// ErrItemExpired is returned in Cache.Get when the item found in the cache
+	// has expired.
+	ErrItemExpired error = errors.New("item has expired")
+	// ErrKeyNotFound is returned in Cache.Get and Cache.Delete when the
+	// provided key could not be found in cache.
+	ErrKeyNotFound error = errors.New("key not found in cache")
+)
+
+// Cache is the interface that wraps the cache.
+//
+// Context specifies the context for the cache.
+// Get gets a cached value by key.
+// Put stores a key-value pair into cache.
+// Delete removes a key from cache.
+type Cache interface {
+	Context(ctx context.Context) Cache
+	Get(key string) (interface{}, time.Time, error)
+	Put(key string, val interface{}, d time.Duration) error
+	Delete(key string) error
+}
+
+// Item represents an item stored in the cache.
+type Item struct {
+	Value      interface{}
+	Expiration int64
+}
+
+// Expired returns true if the item has expired.
+func (i *Item) Expired() bool {
+	if i.Expiration == 0 {
+		return false
+	}
+
+	return time.Now().UnixNano() > i.Expiration
+}
+
+// NewCache returns a new cache.
+func NewCache(opts ...Option) Cache {
+	options := NewOptions(opts...)
+	items := make(map[string]Item)
+
+	if len(options.Items) > 0 {
+		items = options.Items
+	}
+
+	return &memCache{
+		opts:  options,
+		items: items,
+	}
+}