honor canceled context and do not leak on mergeChannels (minio#15034)

mergeEntryChannels has the potential to perpetually
wait on the results channel, context might be closed
and we did not honor the caller context canceling.

cmd/metacache-entries.go

@@ -686,8 +686,12 @@ func mergeEntryChannels(ctx context.Context, in []chan metaCacheEntry, out chan<
 			}
 		}
 		if best.name > last {
-			out <- *best
-			last = best.name
+			select {
+			case <-ctxDone:
+				return ctx.Err()
+			case out <- *best:
+				last = best.name
+			}
 		} else if serverDebugLog {
 			console.Debugln("mergeEntryChannels: discarding duplicate", best.name, "<=", last)
 		}

cmd/metacache-server-pool.go

@@ -273,11 +273,11 @@ func (z *erasureServerPools) listMerged(ctx context.Context, o listPathOptions,
 	for _, pool := range z.serverPools {
 		for _, set := range pool.sets {
 			wg.Add(1)
-			results := make(chan metaCacheEntry, 100)
-			inputs = append(inputs, results)
+			innerResults := make(chan metaCacheEntry, 100)
+			inputs = append(inputs, innerResults)
 			go func(i int, set *erasureObjects) {
 				defer wg.Done()
-				err := set.listPath(listCtx, o, results)
+				err := set.listPath(listCtx, o, innerResults)
 				mu.Lock()
 				defer mu.Unlock()
 				if err == nil {
@@ -328,6 +328,7 @@ func (z *erasureServerPools) listMerged(ctx context.Context, o listPathOptions,
 	if err != nil {
 		return err
 	}
+
 	if contextCanceled(ctx) {
 		return ctx.Err()
 	}