Check cap instead of len in bloblru

restic dump uses bloblru.Cache to keep buffers alive, but also reuses
evicted buffers. That means large buffers may be used to store small
blobs, causing the cache to think it's using less memory than it
actually does.

internal/bloblru/cache.go

@@ -47,7 +47,7 @@ func New(size int) *Cache {
 func (c *Cache) Add(id restic.ID, blob []byte) (old []byte) {
 	debug.Log("bloblru.Cache: add %v", id)
 
-	size := len(blob) + overhead
+	size := cap(blob) + overhead
 	if size > c.size {
 		return
 	}
@@ -66,7 +66,7 @@ func (c *Cache) Add(id restic.ID, blob []byte) (old []byte) {
 	for size > c.free {
 		_, val, _ := c.c.RemoveOldest()
 		b := val.([]byte)
-		if len(b) > len(old) {
+		if cap(b) > cap(old) {
 			// We can only return one buffer, so pick the largest.
 			old = b
 		}
@@ -91,6 +91,6 @@ func (c *Cache) Get(id restic.ID) ([]byte, bool) {
 
 func (c *Cache) evict(key, value interface{}) {
 	blob := value.([]byte)
-	debug.Log("bloblru.Cache: evict %v, %d bytes", key, len(blob))
-	c.free += len(blob) + overhead
+	debug.Log("bloblru.Cache: evict %v, %d bytes", key, cap(blob))
+	c.free += cap(blob) + overhead
 }

internal/bloblru/cache_test.go

@@ -28,9 +28,11 @@ func TestCache(t *testing.T) {
 		rtest.Equals(t, exp, blob)
 	}
 
-	addAndCheck(id1, make([]byte, 32*kiB))
-	addAndCheck(id2, make([]byte, 30*kiB))
-	addAndCheck(id3, make([]byte, 10*kiB))
+	// Our blobs have len 1 but larger cap. The cache should check the cap,
+	// since it more reliably indicates the amount of memory kept alive.
+	addAndCheck(id1, make([]byte, 1, 32*kiB))
+	addAndCheck(id2, make([]byte, 1, 30*kiB))
+	addAndCheck(id3, make([]byte, 1, 10*kiB))
 
 	_, ok := c.Get(id2)
 	rtest.Assert(t, ok, "blob %v not present", id2)