erepo: fix DependencyREPO schema

URL should be required and rev or rev_lock should be optional.

Related to #3087

dvc/dependency/repo.py

@@ -1,5 +1,7 @@
 import os
 
+from voluptuous import Required
+
 from .local import DependencyLOCAL
 from dvc.exceptions import OutputNotFoundError
 from dvc.path_info import PathInfo
@@ -11,7 +13,11 @@ class DependencyREPO(DependencyLOCAL):
     PARAM_REV = "rev"
     PARAM_REV_LOCK = "rev_lock"
 
-    REPO_SCHEMA = {PARAM_URL: str, PARAM_REV: str, PARAM_REV_LOCK: str}
+    REPO_SCHEMA = {
+        Required(PARAM_URL): str,
+        PARAM_REV: str,
+        PARAM_REV_LOCK: str,
+    }
 
     def __init__(self, def_repo, stage, *args, **kwargs):
         self.def_repo = def_repo
@@ -27,7 +33,8 @@ class DependencyREPO(DependencyLOCAL):
     @property
     def repo_pair(self):
         d = self.def_repo
-        return d[self.PARAM_URL], d[self.PARAM_REV_LOCK] or d[self.PARAM_REV]
+        rev = d.get(self.PARAM_REV_LOCK) or d.get(self.PARAM_REV)
+        return d[self.PARAM_URL], rev
 
     def __str__(self):
         return "{} ({})".format(self.def_path, self.def_repo[self.PARAM_URL])

tests/func/test_import.py

@@ -266,3 +266,21 @@ def test_import_non_existing(erepo_dir, tmp_dir, dvc):
     # https://github.com/iterative/dvc/pull/2837#discussion_r352123053
     with pytest.raises(PathMissingError):
         tmp_dir.dvc.imp(fspath(erepo_dir), "/root/", "root")
+
+
+def test_pull_no_rev_lock(erepo_dir, tmp_dir, dvc):
+    with erepo_dir.chdir():
+        erepo_dir.dvc_gen("foo", "contents", commit="create foo")
+
+    stage = dvc.imp(fspath(erepo_dir), "foo", "foo_imported")
+    assert "rev" not in stage.deps[0].def_repo
+    stage.deps[0].def_repo.pop("rev_lock")
+    stage.dump()
+
+    os.remove(stage.outs[0].cache_path)
+    (tmp_dir / "foo_imported").unlink()
+
+    dvc.pull([stage.path])
+
+    assert (tmp_dir / "foo_imported").is_file()
+    assert (tmp_dir / "foo_imported").read_text() == "contents"