From dcda7bec63d555ad26ba8b9bf73a9610e9006fb8 Mon Sep 17 00:00:00 2001
From: Anna Shaleva <shaleva.ann@nspcc.ru>
Date: Tue, 19 Oct 2021 15:10:12 +0300
Subject: [PATCH] core: squash PS-seeking and merging routines in
 (*MemCachedStore).Seek

We don't need a separate routine to merge seek results.
---
 pkg/core/storage/memcached_store.go | 116 +++++++++++++++-------------
 1 file changed, 64 insertions(+), 52 deletions(-)

diff --git a/pkg/core/storage/memcached_store.go b/pkg/core/storage/memcached_store.go
index 5f3c168f6..ea137db8e 100644
--- a/pkg/core/storage/memcached_store.go
+++ b/pkg/core/storage/memcached_store.go
@@ -132,75 +132,87 @@ func (s *MemCachedStore) SeekAsync(ctx context.Context, key []byte, cutPrefix bo
 		return bytes.Compare(memRes[i].Key, memRes[j].Key) < 0
 	})
 
-	var (
-		data2   = make(chan KeyValue)
-		seekres = make(chan KeyValue)
-	)
+	var seekres = make(chan KeyValue)
 
 	// Seek over persistent store.
-	go func() {
-		var done bool
-		ps.Seek(key, func(k, v []byte) {
-			if done {
-				return
-			}
-			select {
-			case <-ctx.Done():
-				done = true
-			default:
-				// Must copy here, #1468.
-				data2 <- KeyValue{
-					Key:   slice.Copy(k),
-					Value: slice.Copy(v),
-				}
-			}
-		})
-		close(data2)
-	}()
-
-	// Merge results of seek operations in ascending order.
 	go func() {
 		var (
+			done    bool
+			iMem    int
 			kvMem   KeyValueExists
 			haveMem bool
-			iMem    int
 		)
 		if iMem < len(memRes) {
 			kvMem = memRes[iMem]
 			haveMem = true
 			iMem++
 		}
-		kvPs, havePs := <-data2
-		for {
-			if !haveMem && !havePs {
-				break
+
+		// Merge results of seek operations in ascending order.
+		ps.Seek(key, func(k, v []byte) {
+			if done {
+				return
 			}
-			var isMem = haveMem && (!havePs || (bytes.Compare(kvMem.Key, kvPs.Key) < 0))
-			if isMem {
-				if kvMem.Exists {
-					if cutPrefix {
-						kvMem.Key = kvMem.Key[len(key):]
-					}
-					seekres <- KeyValue{
-						Key:   kvMem.Key,
-						Value: kvMem.Value,
+			kvPs := KeyValue{
+				Key:   slice.Copy(k),
+				Value: slice.Copy(v),
+			}
+		loop:
+			for {
+				select {
+				case <-ctx.Done():
+					done = true
+					break loop
+				default:
+					var isMem = haveMem && (bytes.Compare(kvMem.Key, kvPs.Key) < 0)
+					if isMem {
+						if kvMem.Exists {
+							if cutPrefix {
+								kvMem.Key = kvMem.Key[len(key):]
+							}
+							seekres <- KeyValue{
+								Key:   kvMem.Key,
+								Value: kvMem.Value,
+							}
+						}
+						if iMem < len(memRes) {
+							kvMem = memRes[iMem]
+							haveMem = true
+							iMem++
+						} else {
+							haveMem = false
+						}
+					} else {
+						if !bytes.Equal(kvMem.Key, kvPs.Key) {
+							if cutPrefix {
+								kvPs.Key = kvPs.Key[len(key):]
+							}
+							seekres <- kvPs
+						}
+						break loop
 					}
 				}
-				if iMem < len(memRes) {
-					kvMem = memRes[iMem]
-					haveMem = true
-					iMem++
-				} else {
-					haveMem = false
-				}
-			} else {
-				if !bytes.Equal(kvMem.Key, kvPs.Key) {
-					if cutPrefix {
-						kvPs.Key = kvPs.Key[len(key):]
+			}
+
+		})
+		if !done && haveMem {
+		loop:
+			for i := iMem - 1; i < len(memRes); i++ {
+				select {
+				case <-ctx.Done():
+					break loop
+				default:
+					kvMem = memRes[i]
+					if kvMem.Exists {
+						if cutPrefix {
+							kvMem.Key = kvMem.Key[len(key):]
+						}
+						seekres <- KeyValue{
+							Key:   kvMem.Key,
+							Value: kvMem.Value,
+						}
 					}
-					seekres <- kvPs
 				}
-				kvPs, havePs = <-data2
 			}
 		}
 		close(seekres)