lib/mergeset: improve test coverage (#6118)

Add test to cover the code path with overflowing shards buffers and
triggering merge to partition.

This test covers the code path which leaded to
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5959

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

lib/mergeset/table.go

@@ -167,7 +167,7 @@ var rawItemsShardsPerTable = func() int {
 	return cpus * multiplier
 }()
 
-const maxBlocksPerShard = 256
+var maxBlocksPerShard = 256
 
 func (riss *rawItemsShards) init() {
 	riss.shards = make([]rawItemsShard, rawItemsShardsPerTable)

lib/mergeset/table_test.go

@@ -176,6 +176,66 @@ func TestTableCreateSnapshotAt(t *testing.T) {
 	_ = os.RemoveAll(path)
 }
 
+func TestTableAddItemsConcurrentStress(t *testing.T) {
+	const path = "TestTableAddItemsConcurrentStress"
+	if err := os.RemoveAll(path); err != nil {
+		t.Fatalf("cannot remove %q: %s", path, err)
+	}
+	defer func() {
+		_ = os.RemoveAll(path)
+	}()
+
+	rawItemsShardsPerTableOrig := rawItemsShardsPerTable
+	maxBlocksPerShardOrig := maxBlocksPerShard
+	rawItemsShardsPerTable = 10
+	maxBlocksPerShard = 3
+	defer func() {
+		rawItemsShardsPerTable = rawItemsShardsPerTableOrig
+		maxBlocksPerShard = maxBlocksPerShardOrig
+	}()
+
+	var flushes atomic.Uint64
+	flushCallback := func() {
+		flushes.Add(1)
+	}
+	prepareBlock := func(data []byte, items []Item) ([]byte, []Item) {
+		return data, items
+	}
+
+	blocksNeeded := rawItemsShardsPerTable * maxBlocksPerShard * 10
+	testAddItems := func(tb *Table) {
+		itemsBatch := make([][]byte, 0)
+
+		for j := 0; j < blocksNeeded; j++ {
+			item := bytes.Repeat([]byte{byte(j)}, maxInmemoryBlockSize-10)
+			itemsBatch = append(itemsBatch, item)
+		}
+		tb.AddItems(itemsBatch)
+	}
+
+	var isReadOnly atomic.Bool
+	tb := MustOpenTable(path, flushCallback, prepareBlock, &isReadOnly)
+
+	testAddItems(tb)
+
+	// Verify items count after pending items flush.
+	tb.DebugFlush()
+	if flushes.Load() == 0 {
+		t.Fatalf("unexpected zero flushes")
+	}
+
+	var m TableMetrics
+	tb.UpdateMetrics(&m)
+	if n := m.TotalItemsCount(); n != uint64(blocksNeeded) {
+		t.Fatalf("unexpected itemsCount; got %d; want %v", n, blocksNeeded)
+	}
+
+	tb.MustClose()
+
+	// Re-open the table and make sure itemsCount remains the same.
+	testReopenTable(t, path, blocksNeeded)
+}
+
 func TestTableAddItemsConcurrent(t *testing.T) {
 	const path = "TestTableAddItemsConcurrent"
 	if err := os.RemoveAll(path); err != nil {