table: acquire active write block on compaction

Otherwise, compaction races with snapshots.

table.go

@@ -1113,6 +1113,12 @@ func (t *Table) externalParquetCompaction(writer io.Writer) func(compact []*part
 // compactParts will compact the given parts into a Parquet file written to w.
 // It returns the size in bytes of the compacted parts.
 func (t *Table) compactParts(w io.Writer, compact []*parts.Part) (int64, error) {
+	block, done, err := t.ActiveWriteBlock()
+	if err != nil {
+		return 0, err
+	}
+	defer done()
+
 	bufs := []dynparquet.DynamicRowGroup{}
 	var size int64
 	for _, part := range compact {
@@ -1128,7 +1134,7 @@ func (t *Table) compactParts(w io.Writer, compact []*parts.Part) (int64, error)
 		return 0, err
 	}
 	err = func() error {
-		p, err := t.active.rowWriter(w, merged.DynamicColumns())
+		p, err := block.rowWriter(w, merged.DynamicColumns())
 		if err != nil {
 			return err
 		}