test concurrent value imports, fix race

fragment.go

@@ -1785,7 +1785,9 @@ func (f *fragment) importValue(columnIDs, values []uint64, bitDepth uint, clear
 		for i := uint(0); i < bitDepth+1; i++ {
 			rowSet[uint64(i)] = struct{}{}
 		}
+		f.mu.Lock()
 		err := f.importPositions(toSet, toClear, rowSet)
+		f.mu.Unlock()
 		return errors.Wrap(err, "importing positions")
 	}
 	err := f.snapshot()

fragment_internal_test.go

@@ -3024,3 +3024,24 @@ func TestSmallImportRestart(t *testing.T) {
 		t.Errorf("row 1 should be [1], but got %v", r1)
 	}
 }
+
+func TestImportValueConcurrent(t *testing.T) {
+	f := mustOpenFragment("i", "f", viewBSIGroupPrefix+"foo", 0, "none")
+	eg := &errgroup.Group{}
+	for i := 0; i < 4; i++ {
+		i := i
+		eg.Go(func() error {
+			for j := uint64(0); j < 10; j++ {
+				err := f.importValue([]uint64{j}, []uint64{uint64(rand.Int63n(1000))}, 10, i%2 == 0)
+				if err != nil {
+					return err
+				}
+			}
+			return nil
+		})
+	}
+	err := eg.Wait()
+	if err != nil {
+		t.Fatalf("concurrently importing values: %v", err)
+	}
+}