describe: make active branch always be the first choice of describe(#4)

fix: #4 and treeverse/dvc#6051

In dvc we get a random toggling branch name in exp show. It is because
when we iter branches the dulwich.refs.keys gives an unordered Set
of refs. Here we choose the active branch if we are `describe` its
commit rev.

1. Make active branch always in the first place.
2. Add a new test to it.

Author: karajan1001

scmrepo/git/__init__.py

@@ -314,7 +314,7 @@ def add_commit(
     _stash_push = partialmethod(_backend_func, "_stash_push")
     _stash_apply = partialmethod(_backend_func, "_stash_apply")
     _stash_drop = partialmethod(_backend_func, "_stash_drop")
-    describe = partialmethod(_backend_func, "describe")
+    _describe = partialmethod(_backend_func, "_describe")
     diff = partialmethod(_backend_func, "diff")
     reset = partialmethod(_backend_func, "reset")
     checkout_index = partialmethod(_backend_func, "checkout_index")
@@ -397,3 +397,17 @@ def stash_workspace(self, **kwargs):
 
     def _reset(self) -> None:
         self.backends.reset_all()
+
+    def describe(
+        self,
+        rev: str,
+        base: Optional[str] = None,
+        match: Optional[str] = None,
+        exclude: Optional[str] = None,
+    ) -> Optional[str]:
+        if base == "refs/heads" and self.get_rev() == rev:
+            try:
+                return base + "/" + self.active_branch()
+            except TypeError:
+                pass
+        return self._describe(rev, base, match, exclude)

scmrepo/git/backend/base.py

@@ -278,7 +278,7 @@ def _stash_drop(self, ref: str, index: int):
         """Drop the specified stash revision."""
 
     @abstractmethod
-    def describe(
+    def _describe(
         self,
         rev: str,
         base: Optional[str] = None,

scmrepo/git/backend/dulwich/__init__.py

@@ -590,7 +590,7 @@ def _stash_drop(self, ref: str, index: int):
         except ValueError as exc:
             raise SCMError("Failed to drop stash entry") from exc
 
-    def describe(
+    def _describe(
         self,
         rev: str,
         base: Optional[str] = None,

scmrepo/git/backend/gitpython.py

@@ -552,7 +552,7 @@ def _stash_drop(self, ref: str, index: int):
         except GitCommandError:
             self.remove_ref(ref)
 
-    def describe(
+    def _describe(
         self,
         rev: str,
         base: Optional[str] = None,

scmrepo/git/backend/pygit2.py

@@ -459,7 +459,7 @@ def _stash_drop(self, ref: str, index: int):
 
         self.repo.stash_drop(index)
 
-    def describe(
+    def _describe(
         self,
         rev: str,
         base: Optional[str] = None,

tests/test_dulwich.py

@@ -1,5 +1,8 @@
 import pytest
 from pytest_mock import MockerFixture
+from pytest_test_utils import TmpDir
+
+from scmrepo.git import Git
 
 
 @pytest.mark.parametrize(
@@ -26,3 +29,15 @@ def test_dulwich_github_compat(mocker: MockerFixture, algorithm: bytes):
     strings = iter((b"ssh-rsa", key_data))
     packet.get_string = lambda: next(strings)
     _process_public_key_ok_gh(auth, None, None, packet)
+
+
+def test_dulwich_describe(tmp_dir: TmpDir, scm: Git):
+
+    tmp_dir.gen({"foo": "foo"})
+    scm.add_commit("foo", message="foo")
+    rev = scm.get_rev()
+
+    scm.checkout("branch", create_new=True)
+    assert scm.describe(rev, "refs/heads") == "refs/heads/branch"
+    scm.checkout("master")
+    assert scm.describe(rev, "refs/heads") == "refs/heads/master"