Add method for getting all cache keys

Implemented the (*cache) Keys() method returning a sorted slice of all
keys in the cache.

cache.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"runtime"
+	"sort"
 	"sync"
 	"time"
 )
@@ -1052,6 +1053,20 @@ func (c *cache) Items() map[string]Item {
 	return m
 }
 
+// Keys returns a sorted slice of all the keys in the cache.
+func (c *cache) Keys() []string {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	keys := make([]string, len(c.items))
+	var i int
+	for k := range c.items {
+		keys[i] = k
+		i++
+	}
+	sort.Strings(keys)
+	return keys
+}
+
 // Returns the number of items in the cache. This may include items that have
 // expired, but have not yet been cleaned up.
 func (c *cache) ItemCount() int {

cache_test.go

@@ -68,6 +68,24 @@ func TestCache(t *testing.T) {
 	}
 }
 
+func TestCacheKeys(t *testing.T) {
+	tc := New(DefaultExpiration, 0)
+
+	tc.Set("a", 1, DefaultExpiration)
+	tc.Set("b", 2, DefaultExpiration)
+	tc.Set("c", 3, DefaultExpiration)
+
+	keys := tc.Keys()
+
+	if len(keys) != 3 {
+		t.Error("invalid number of cache keys received")
+	}
+
+	if keys[0] != "a" || keys[1] != "b" || keys[2] != "c" {
+		t.Error("invalid cache keys received")
+	}
+}
+
 func TestCacheTimes(t *testing.T) {
 	var found bool