diff: hanle repo with no commits (#3784)

`git diff` doesn't throw errors, so we could do the same.

Fixes #3398

dvc/repo/diff.py

@@ -52,6 +52,9 @@ def diff(self, a_rev="HEAD", b_rev=None):
             if _exists(output)
         }
 
+    if self.scm.no_commits:
+        return {}
+
     working_tree = self.tree
     a_tree = self.scm.get_tree(a_rev)
     b_tree = self.scm.get_tree(b_rev) if b_rev else working_tree

dvc/repo/metrics/diff.py

@@ -14,6 +14,9 @@ def _get_metrics(repo, *args, rev=None, **kwargs):
 
 
 def diff(repo, *args, a_rev=None, b_rev=None, **kwargs):
+    if repo.scm.no_commits:
+        return {}
+
     with_unchanged = kwargs.pop("all", False)
 
     old = _get_metrics(repo, *args, **kwargs, rev=(a_rev or "HEAD"))

dvc/repo/params/diff.py

@@ -13,6 +13,9 @@ def _get_params(repo, *args, rev=None, **kwargs):
 
 
 def diff(repo, *args, a_rev=None, b_rev=None, **kwargs):
+    if repo.scm.no_commits:
+        return {}
+
     with_unchanged = kwargs.pop("all", False)
 
     old = _get_params(repo, *args, **kwargs, rev=(a_rev or "HEAD"))

dvc/scm/git/__init__.py

@@ -439,3 +439,7 @@ class Git(Base):
         self._verify_hook("post-checkout")
         self._verify_hook("pre-commit")
         self._verify_hook("pre-push")
+
+    @property
+    def no_commits(self):
+        return not self.list_all_commits()

tests/dir_helpers.py

@@ -62,6 +62,7 @@ __all__ = [
     "erepo_dir",
     "git_dir",
     "setup_remote",
+    "git_init",
 ]
 
 
@@ -91,7 +92,7 @@ class TmpDir(pathlib.Path):
         str_path = os.fspath(self)
 
         if scm:
-            _git_init(str_path)
+            git_init(str_path)
         if dvc:
             self.dvc = Repo.init(
                 str_path, no_scm=not scm and not hasattr(self, "scm")
@@ -238,7 +239,7 @@ def dvc(tmp_dir):
     return tmp_dir.dvc
 
 
-def _git_init(path):
+def git_init(path):
     from git import Repo
     from git.exc import GitCommandNotFound
 

tests/func/metrics/test_diff.py

@@ -134,3 +134,14 @@ def test_metrics_diff_with_unchanged(tmp_dir, scm, dvc):
             "xyz": {"old": 10, "new": 10, "diff": 0},
         }
     }
+
+
+def test_no_commits(tmp_dir):
+    from dvc.repo import Repo
+    from dvc.scm.git import Git
+    from tests.dir_helpers import git_init
+
+    git_init(".")
+    assert Git().no_commits
+
+    assert Repo.init().metrics.diff() == {}

tests/func/params/test_diff.py

@@ -119,3 +119,14 @@ def test_pipeline_tracked_params(tmp_dir, scm, dvc, run_copy):
     assert dvc.params.diff(a_rev="HEAD~2") == {
         "params.yaml": {"foo": {"old": "bar", "new": "qux"}}
     }
+
+
+def test_no_commits(tmp_dir):
+    from dvc.repo import Repo
+    from dvc.scm.git import Git
+    from tests.dir_helpers import git_init
+
+    git_init(".")
+    assert Git().no_commits
+
+    assert Repo.init().params.diff() == {}

tests/func/test_diff.py

@@ -213,3 +213,14 @@ def test_diff_dirty(tmp_dir, scm, dvc):
 def test_no_changes(tmp_dir, scm, dvc):
     tmp_dir.dvc_gen("file", "first", commit="add a file")
     assert dvc.diff() == {}
+
+
+def test_no_commits(tmp_dir):
+    from dvc.repo import Repo
+    from dvc.scm.git import Git
+    from tests.dir_helpers import git_init
+
+    git_init(".")
+    assert Git().no_commits
+
+    assert Repo.init().diff() == {}

tests/unit/scm/test_git.py

@@ -69,3 +69,17 @@ def test_is_tracked_unicode(tmp_dir, scm):
     tmp_dir.gen("ṳṋṭṝḁḉḵḗḋ", "untracked")
     assert scm.is_tracked("ṭṝḁḉḵḗḋ")
     assert not scm.is_tracked("ṳṋṭṝḁḉḵḗḋ")
+
+
+def test_no_commits(tmp_dir):
+    from tests.dir_helpers import git_init
+    from dvc.scm.git import Git
+
+    git_init(".")
+    assert Git().no_commits
+
+    tmp_dir.gen("foo", "foo")
+    Git().add(["foo"])
+    Git().commit("foo")
+
+    assert not Git().no_commits