dvc: add Repo(skip_checks) flag

dvc/repo/__init__.py

@@ -174,7 +174,20 @@ def _ignore(self):
 
     def check_modified_graph(self, new_stages):
         """Generate graph including the new stage to check for errors"""
-        self._collect_graph(self.stages + new_stages)
+        # Building graph might be costly for the ones with many DVC-files,
+        # so we provide this undocumented hack to skip it. See [1] for
+        # more details. The hack can be used as:
+        #
+        #     repo = Repo(...)
+        #     repo._skip_graph_checks = True
+        #     repo.add(...)
+        #
+        # A user should care about not duplicating outs and not adding cycles,
+        # otherwise DVC might have an undefined behaviour.
+        #
+        # [1] https://github.com/iterative/dvc/issues/2671
+        if not getattr(self, "_skip_graph_checks", False):
+            self._collect_graph(self.stages + new_stages)
 
     def collect(self, target, with_deps=False, recursive=False, graph=None):
         import networkx as nx

tests/unit/repo/test_repo.py

@@ -82,3 +82,30 @@ def test_collect_optimization(tmp_dir, dvc, mocker):
     # Should read stage directly instead of collecting the whole graph
     dvc.collect(stage.path)
     dvc.collect_granular(stage.path)
+
+
+def test_skip_graph_checks(tmp_dir, dvc, mocker, run_copy):
+    # See https://github.com/iterative/dvc/issues/2671 for more info
+    mock_collect_graph = mocker.patch("dvc.repo.Repo._collect_graph")
+
+    # sanity check
+    tmp_dir.gen("foo", "foo text")
+    dvc.add("foo")
+    run_copy("foo", "bar")
+    assert mock_collect_graph.called
+
+    # check that our hack can be enabled
+    mock_collect_graph.reset_mock()
+    dvc._skip_graph_checks = True
+    tmp_dir.gen("baz", "baz text")
+    dvc.add("baz")
+    run_copy("baz", "qux")
+    assert not mock_collect_graph.called
+
+    # check that our hack can be disabled
+    mock_collect_graph.reset_mock()
+    dvc._skip_graph_checks = False
+    tmp_dir.gen("quux", "quux text")
+    dvc.add("quux")
+    run_copy("quux", "quuz")
+    assert mock_collect_graph.called