Conditionally prune more than itemsToPrune items

It's possible, though unlikely, that c.size will be larger than
c.maxSize by more than c.itemsToPrune. The most likely case that this
can happen is when using SetMaxSize to dynamically adjust the cache
size. The gc will now always clear to at least c.maxSize.

Makefile

@@ -1,5 +1,5 @@
 t:
-	go test ./...
+	go test ./... -race -count=1
 
 f:
 	go fmt ./...

cache.go

@@ -304,7 +304,13 @@ func (c *Cache) doPromote(item *Item) bool {
 func (c *Cache) gc() int {
 	dropped := 0
 	element := c.list.Back()
-	for i := 0; i < c.itemsToPrune; i++ {
+
+	itemsToPrune := int64(c.itemsToPrune)
+	if min := c.size - c.maxSize; min > itemsToPrune {
+		itemsToPrune = min
+	}
+
+	for i := int64(0); i < itemsToPrune; i++ {
 		if element == nil {
 			return dropped
 		}

cache_test.go

@@ -18,6 +18,7 @@ func Test_Cache(t *testing.T) {
 
 func (_ CacheTests) DeletesAValue() {
 	cache := New(Configure())
+	defer cache.Stop()
 	Expect(cache.ItemCount()).To.Equal(0)
 
 	cache.Set("spice", "flow", time.Minute)
@@ -32,6 +33,7 @@ func (_ CacheTests) DeletesAValue() {
 
 func (_ CacheTests) DeletesAPrefix() {
 	cache := New(Configure())
+	defer cache.Stop()
 	Expect(cache.ItemCount()).To.Equal(0)
 
 	cache.Set("aaa", "1", time.Minute)
@@ -55,6 +57,7 @@ func (_ CacheTests) DeletesAPrefix() {
 
 func (_ CacheTests) DeletesAFunc() {
 	cache := New(Configure())
+	defer cache.Stop()
 	Expect(cache.ItemCount()).To.Equal(0)
 
 	cache.Set("a", 1, time.Minute)

go.mod

@@ -3,6 +3,6 @@ module github.com/karlseguin/ccache/v2
 go 1.13
 
 require (
-	github.com/karlseguin/expect v1.0.2-0.20190806010014-778a5f0c6003
+	github.com/karlseguin/expect v1.0.7
 	github.com/wsxiaoys/terminal v0.0.0-20160513160801-0940f3fc43a0
 )

go.sum

@@ -2,5 +2,7 @@ github.com/karlseguin/expect v1.0.1 h1:z4wy4npwwHSWKjGWH85WNJO42VQhovxTCZDSzhjo8
 github.com/karlseguin/expect v1.0.1/go.mod h1:zNBxMY8P21owkeogJELCLeHIt+voOSduHYTFUbwRAV8=
 github.com/karlseguin/expect v1.0.2-0.20190806010014-778a5f0c6003 h1:vJ0Snvo+SLMY72r5J4sEfkuE7AFbixEP2qRbEcum/wA=
 github.com/karlseguin/expect v1.0.2-0.20190806010014-778a5f0c6003/go.mod h1:zNBxMY8P21owkeogJELCLeHIt+voOSduHYTFUbwRAV8=
+github.com/karlseguin/expect v1.0.7 h1:OF4mqjblc450v8nKARBS5Q0AweBNR0A+O3VjjpxwBrg=
+github.com/karlseguin/expect v1.0.7/go.mod h1:lXdI8iGiQhmzpnnmU/EGA60vqKs8NbRNFnhhrJGoD5g=
 github.com/wsxiaoys/terminal v0.0.0-20160513160801-0940f3fc43a0 h1:3UeQBvD0TFrlVjOeLOBz+CPAI8dnbqNSVwUwRrkp7vQ=
 github.com/wsxiaoys/terminal v0.0.0-20160513160801-0940f3fc43a0/go.mod h1:IXCdmsXIht47RaVFLEdVnh1t+pgYtTAhQGj73kz+2DM=

layeredcache.go

@@ -286,7 +286,13 @@ func (c *LayeredCache) doPromote(item *Item) bool {
 func (c *LayeredCache) gc() int {
 	element := c.list.Back()
 	dropped := 0
-	for i := 0; i < c.itemsToPrune; i++ {
+	itemsToPrune := int64(c.itemsToPrune)
+
+	if min := c.size - c.maxSize; min > itemsToPrune {
+		itemsToPrune = min
+	}
+
+	for i := int64(0); i < itemsToPrune; i++ {
 		if element == nil {
 			return dropped
 		}

layeredcache_test.go

@@ -235,8 +235,9 @@ func (_ LayeredCacheTests) ResizeOnTheFly() {
 	for i := 0; i < 5; i++ {
 		cache.Set(strconv.Itoa(i), "a", i, time.Minute)
 	}
+	time.Sleep(time.Millisecond * 20)
 	cache.SetMaxSize(3)
-	time.Sleep(time.Millisecond * 10)
+	time.Sleep(time.Millisecond * 20)
 	Expect(cache.GetDropped()).To.Equal(2)
 	Expect(cache.Get("0", "a")).To.Equal(nil)
 	Expect(cache.Get("1", "a")).To.Equal(nil)
@@ -245,7 +246,7 @@ func (_ LayeredCacheTests) ResizeOnTheFly() {
 	Expect(cache.Get("4", "a").Value()).To.Equal(4)
 
 	cache.Set("5", "a", 5, time.Minute)
-	time.Sleep(time.Millisecond * 5)
+	time.Sleep(time.Millisecond * 10)
 	Expect(cache.GetDropped()).To.Equal(1)
 	Expect(cache.Get("2", "a")).To.Equal(nil)
 	Expect(cache.Get("3", "a").Value()).To.Equal(3)