alter: init space truncate_count after recovering snapshot

The replace trigger of _truncate system space (on_replace_dd_truncate)
does nothing on insertion into or deletion from the space - it only
updates space truncate_count when a tuple gets updated. As a result,
space truncate_count isn't initialized properly after recovering
snapshot. This does no harm to memtx, because it doesn't use space
truncate_count at all, but it breaks the assumption made by vinyl that
if space truncate_count is less than index truncate_count (which is
loaded from vylog), the space will be truncated during WAL recovery and
hence there's no point in applying statements to the space (see
vy_is_committed_one). As a result, all statements inserted into a vinyl
space after snapshot following truncation of the space, are ignored on
WAL recovery. To fix that, we must initialize space truncate_count when
a tuple is inserted into _truncate system space.

Closes #2521

src/box/alter.cc

@@ -1400,20 +1400,26 @@ on_replace_dd_truncate(struct trigger * /* trigger */, void *event)
 	struct tuple *old_tuple = stmt->old_tuple;
 	struct tuple *new_tuple = stmt->new_tuple;
 
-	if (old_tuple == NULL || new_tuple == NULL) {
-		/* Space create or drop. */
+	if (new_tuple == NULL) {
+		/* Space drop - nothing to do. */
 		return;
 	}
 
-	/*
-	 * Truncate counter is updated - truncate the space.
-	 */
 	uint32_t space_id =
 		tuple_field_u32_xc(new_tuple, BOX_TRUNCATE_FIELD_SPACE_ID);
 	uint64_t truncate_count =
 		tuple_field_u64_xc(new_tuple, BOX_TRUNCATE_FIELD_COUNT);
 	struct space *old_space = space_cache_find(space_id);
 
+	if (old_tuple == NULL) {
+		/* Space create - initialize truncate_count. */
+		old_space->truncate_count = truncate_count;
+		return;
+	}
+
+	/*
+	 * Truncate counter is updated - truncate the space.
+	 */
 	struct truncate_space *truncate =
 		region_calloc_object_xc(&fiber()->gc, struct truncate_space);
 

test/engine/truncate.result

@@ -248,6 +248,9 @@ box.snapshot()
 ---
 - ok
 ...
+_ = s1:insert{321, 123}
+---
+...
 s2:truncate()
 ---
 ...
@@ -291,10 +294,12 @@ s3 = box.space.test3
 s1.index.i1:select()
 ---
 - - [123, 321]
+  - [321, 123]
 ...
 s1.index.i2:select()
 ---
-- - [123, 321]
+- - [321, 123]
+  - [123, 321]
 ...
 s2.index.i1:select()
 ---

test/engine/truncate.test.lua

@@ -112,6 +112,7 @@ _ = s2:insert{10, 30}
 _ = s2:insert{20, 20}
 _ = s2:insert{30, 10}
 box.snapshot()
+_ = s1:insert{321, 123}
 s2:truncate()
 _ = s2:insert{456, 654}
 s3 = box.schema.create_space('test3', {engine = engine})