Fix race in ExecContext

When the context is cancelled, an interrupt should only be made if the
operation is still ongoing.

sqlite3.go

@@ -1171,9 +1171,13 @@ func (s *SQLiteStmt) exec(ctx context.Context, args []namedValue) (driver.Result
 	defer close(done)
 	go func(db *C.sqlite3) {
 		select {
-		case <-ctx.Done():
-			C.sqlite3_interrupt(db)
 		case <-done:
+		case <-ctx.Done():
+			select {
+			case <-done:
+			default:
+				C.sqlite3_interrupt(db)
+			}
 		}
 	}(s.c.db)
 

sqlite3_go18_test.go

@@ -134,3 +134,24 @@ func TestShortTimeout(t *testing.T) {
 		t.Fatal(ctx.Err())
 	}
 }
+
+func TestExecCancel(t *testing.T) {
+	db, err := sql.Open("sqlite3", ":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	if _, err = db.Exec("create table foo (id integer primary key)"); err != nil {
+		t.Fatal(err)
+	}
+
+	for n := 0; n < 100; n++ {
+		ctx, cancel := context.WithCancel(context.Background())
+		_, err = db.ExecContext(ctx, "insert into foo (id) values (?)", n)
+		cancel()
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}