select: do not fetch the key following the last one from the engine

space:select(key, {limit = N}) limits the output to N keys, but it still
fetches the (N+1)-th key from the engine. This is pointless. Besides,
this can result in a conflict in Vinyl as Vinyl adds all keys returned
by iterator to the conflict manager.

src/box/engine.cc

@@ -197,14 +197,13 @@ Handler::executeSelect(struct txn *, struct space *space,
 	index->initIterator(it, type, key, part_count);
 
 	struct tuple *tuple;
-	while ((tuple = it->next(it)) != NULL) {
+	while (found < limit && (tuple = it->next(it)) != NULL) {
 		if (offset > 0) {
 			offset--;
 			continue;
 		}
-		if (limit == found++)
-			break;
 		port_add_tuple_xc(port, tuple);
+		found++;
 	}
 }
 

test/vinyl/mvcc.result

@@ -3865,6 +3865,62 @@ c1:commit()
 t:truncate()
 ---
 ...
+--
+-- Check that select() does not add the key following
+-- the last returned key to the conflict manager.
+--
+t:replace{1}
+---
+- [1]
+...
+t:replace{2}
+---
+- [2]
+...
+c1:begin()
+---
+- 
+...
+c1("t:select({}, {limit = 0})") -- none
+---
+- - []
+...
+c2:begin()
+---
+- 
+...
+c2("t:replace{1, 'new'}")
+---
+- - [1, 'new']
+...
+c2:commit()
+---
+- 
+...
+c1("t:select({}, {limit = 1})") -- {1, 'new'}
+---
+- - [[1, 'new']]
+...
+c2:begin()
+---
+- 
+...
+c2("t:replace{2, 'new'}")
+---
+- - [2, 'new']
+...
+c2:commit()
+---
+- 
+...
+c1("t:select()") -- {1, 'new'}, {2, 'new'}
+---
+- - [[1, 'new'], [2, 'new']]
+...
+c1:commit()
+---
+- 
+...
 -- *************************************************************************
 -- 1.7 cleanup marker: end of tests cleanup
 -- *************************************************************************

test/vinyl/mvcc.test.lua

@@ -1536,6 +1536,24 @@ c1("t.index.pk:count()") -- 2
 c1:commit()
 t:truncate()
 
+--
+-- Check that select() does not add the key following
+-- the last returned key to the conflict manager.
+--
+t:replace{1}
+t:replace{2}
+c1:begin()
+c1("t:select({}, {limit = 0})") -- none
+c2:begin()
+c2("t:replace{1, 'new'}")
+c2:commit()
+c1("t:select({}, {limit = 1})") -- {1, 'new'}
+c2:begin()
+c2("t:replace{2, 'new'}")
+c2:commit()
+c1("t:select()") -- {1, 'new'}, {2, 'new'}
+c1:commit()
+
 -- *************************************************************************
 -- 1.7 cleanup marker: end of tests cleanup
 -- *************************************************************************