Reuse more metadata buffers (#12955)

Reuse metadata buffers when no longer referenced.

Takes care of most of the happy paths.

cmd/metacache-server-pool.go

@@ -162,6 +162,7 @@ func (z *erasureServerPools) listPath(ctx context.Context, o *listPathOptions) (
 			if o.pool < len(z.serverPools) && o.set < len(z.serverPools[o.pool].sets) {
 				o.debugln("Resuming", o)
 				entries, err = z.serverPools[o.pool].sets[o.set].streamMetadataParts(ctx, *o)
+				entries.reuse = true // We read from stream and are not sharing results.
 				if err == nil {
 					return entries, nil
 				}
@@ -217,6 +218,7 @@ func (z *erasureServerPools) listPath(ctx context.Context, o *listPathOptions) (
 	if listErr != nil && !errors.Is(listErr, context.Canceled) {
 		return entries, listErr
 	}
+	entries.reuse = true
 	truncated := entries.len() > o.Limit || err == nil
 	entries.truncate(o.Limit)
 	if !o.Transient && truncated {
@@ -363,13 +365,33 @@ func (z *erasureServerPools) listAndSave(ctx context.Context, o *listPathOptions
 		o.debugln("listAndSave: listing", o.ID, "finished with ", err)
 	}(*o)
 
+	// Keep track of when we return since we no longer have to send entries to output.
+	var funcReturned bool
+	var funcReturnedMu sync.Mutex
+	defer func() {
+		funcReturnedMu.Lock()
+		funcReturned = true
+		funcReturnedMu.Unlock()
+	}()
 	// Write listing to results and saver.
 	go func() {
+		var returned bool
 		for entry := range inCh {
-			outCh <- entry
+			if !returned {
+				funcReturnedMu.Lock()
+				returned = funcReturned
+				funcReturnedMu.Unlock()
+				outCh <- entry
+				if returned {
+					close(outCh)
+				}
+			}
+			entry.reusable = returned
 			saveCh <- entry
 		}
-		close(outCh)
+		if !returned {
+			close(outCh)
+		}
 		close(saveCh)
 	}()