Release entryBufferPool once (#5324) (#5335)

* release entryBufferPool once

* reverseEntryIterator.Next is safe to call past expiry

Co-authored-by: Owen Diehl <ow.diehl@gmail.com>

pkg/iter/entry_iterator.go

@@ -717,9 +717,8 @@ func (i *reverseEntryIterator) load() {
 
 func (i *reverseEntryIterator) Next() bool {
 	i.load()
-	if len(i.buf.entries) == 0 {
-		entryBufferPool.Put(i.buf)
-		i.buf.entries = nil
+	if i.buf == nil || len(i.buf.entries) == 0 {
+		i.release()
 		return false
 	}
 	i.cur, i.buf.entries = i.buf.entries[len(i.buf.entries)-1], i.buf.entries[:len(i.buf.entries)-1]
@@ -736,12 +735,21 @@ func (i *reverseEntryIterator) Labels() string {
 
 func (i *reverseEntryIterator) Error() error { return nil }
 
-func (i *reverseEntryIterator) Close() error {
+func (i *reverseEntryIterator) release() {
+	if i.buf == nil {
+		return
+	}
+
 	if i.buf.entries != nil {
+		// preserve the underlying slice before releasing to pool
 		i.buf.entries = i.buf.entries[:0]
-		entryBufferPool.Put(i.buf)
-		i.buf.entries = nil
 	}
+	entryBufferPool.Put(i.buf)
+	i.buf = nil
+}
+
+func (i *reverseEntryIterator) Close() error {
+	i.release()
 	if !i.loaded {
 		return i.iter.Close()
 	}