Detect Git merges in rebase_migration

CHANGELOG.rst

@@ -2,6 +2,10 @@
 Changelog
 =========
 
+* Extend ``rebase_migration`` to detect Git in-progress merges and select the correct migration to rebase.
+
+  Thanks to Dmitry Sleptsov in `PR #260 <https://github.com/adamchainz/django-linear-migrations/pull/260>`__.
+
 2.7.0 (2023-02-25)
 ------------------
 

README.rst

@@ -149,14 +149,20 @@ Following a conflicted “rebase” operation in Git, run it with the name of th
 
     $ python manage.py rebase_migration <app_label>
 
-The command will use the conflict information in the ``max_migration.txt`` file to determine which migration to rebase.
-It will then rename the migration, edit it to depend on the new migration in your main branch, and update ``max_migration.txt``.
-If Black is installed, it will format the updated migration file with it, like Django’s built-in migration commands (from version 4.1+).
+The command uses the conflict information in the ``max_migration.txt`` file to determine which migration to rebase.
+It automatically detects whether a Git merge or rebase operation is in progress, assuming rebase if a Git repository cannot be found.
+The command then:
+
+1. renames the migration
+2. edits it to depend on the new migration from your main branch
+3. updates ``max_migration.txt``.
+
+If Black is installed, the command formats the updated migration file with it, like Django’s built-in migration commands do (from version 4.1+).
 See below for some examples and caveats.
 
 Note rebasing the migration might not always be the *correct* thing to do.
-If the migrations in main and feature branches have both affected the same models, rebasing the migration to the end may not make sense.
-However, such parallel changes would *normally* cause conflicts in your models files or other parts of the source code as well.
+If the migrations in your main and feature branches have both affected the same models, rebasing the migration to the end may not make sense.
+However, such parallel changes would *normally* cause conflicts in your model files or other parts of the source code as well.
 
 Worked Example
 ^^^^^^^^^^^^^^

src/django_linear_migrations/management/commands/rebase_migration.py

@@ -184,7 +184,29 @@ def find_migration_names(max_migration_lines: list[str]) -> tuple[str, str] | No
         return None
     if not lines[-1].startswith(">>>>>>>"):
         return None
-    return lines[1].strip(), lines[-2].strip()
+    migration_names = (lines[1].strip(), lines[-2].strip())
+    if is_merge_in_progress():
+        # During the merge 'ours' and 'theirs' are swapped in comparison with rebase
+        migration_names = (migration_names[1], migration_names[0])
+    return migration_names
+
+
+def is_merge_in_progress() -> bool:
+    try:
+        subprocess.run(
+            ["git", "rev-parse", "--verify", "MERGE_HEAD"],
+            capture_output=True,
+            check=True,
+            text=True,
+        )
+    except (FileNotFoundError, subprocess.SubprocessError):
+        # Either:
+        # - `git` is not available, or broken
+        # - there is no git repository
+        # - no merge head exists, so assume rebasing
+        return False
+    # Merged head exists, we are merging
+    return True
 
 
 def migration_applied(app_label: str, migration_name: str) -> bool:

tests/test_rebase_migration.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import os
+import subprocess
 import sys
 import time
 from functools import partial
@@ -438,7 +440,7 @@ def test_none_when_no_second_marker(self):
         result = module.find_migration_names(["<<<<<<<", "0002_author_nicknames"])
         assert result is None
 
-    def test_works_with_two_way_merge(self):
+    def test_works_with_two_way_merge_during_rebase(self):
         result = module.find_migration_names(
             [
                 "<<<<<<<",
@@ -450,7 +452,7 @@ def test_works_with_two_way_merge(self):
         )
         assert result == ("0002_author_nicknames", "0002_longer_titles")
 
-    def test_works_with_three_way_merge(self):
+    def test_works_with_three_way_merge_during_rebase(self):
         result = module.find_migration_names(
             [
                 "<<<<<<<",
@@ -464,6 +466,73 @@ def test_works_with_three_way_merge(self):
         )
         assert result == ("0002_author_nicknames", "0002_longer_titles")
 
+    def test_works_with_two_way_merge_during_merge(self):
+        with mock.patch.object(module, "is_merge_in_progress", return_value=True):
+            result = module.find_migration_names(
+                [
+                    "<<<<<<<",
+                    "0002_longer_titles",
+                    "=======",
+                    "0002_author_nicknames",
+                    ">>>>>>>",
+                ]
+            )
+        assert result == ("0002_author_nicknames", "0002_longer_titles")
+
+    def test_works_with_three_way_merge_during_merge(self):
+        with mock.patch.object(module, "is_merge_in_progress", return_value=True):
+            result = module.find_migration_names(
+                [
+                    "<<<<<<<",
+                    "0002_longer_titles",
+                    "|||||||",
+                    "0001_initial",
+                    "=======",
+                    "0002_author_nicknames",
+                    ">>>>>>>",
+                ]
+            )
+        assert result == ("0002_author_nicknames", "0002_longer_titles")
+
+
+class IsMergeInProgressTests(SimpleTestCase):
+    @pytest.fixture(autouse=True)
+    def tmp_path_fixture(self, tmp_path):
+        self.tmp_path = tmp_path
+        self.subprocess_run = partial(subprocess.run, cwd=tmp_path, check=True)
+
+    def setUp(self):
+        orig = os.getcwd()
+        os.chdir(self.tmp_path)
+        self.addCleanup(os.chdir, orig)
+
+    def test_no_git_command(self):
+        with mock.patch.dict(os.environ, {"PATH": ""}):
+            result = module.is_merge_in_progress()
+        assert result is False
+
+    def test_no_git_dir(self):
+        result = module.is_merge_in_progress()
+        assert result is False
+
+    def test_git_dir_no_merge(self):
+        self.subprocess_run(["git", "init"])
+        result = module.is_merge_in_progress()
+        assert result is False
+
+    def test_git_dir_merge(self):
+        self.subprocess_run(["git", "init", "-b", "main"])
+        self.subprocess_run(["git", "config", "user.email", "[email protected]"])
+        self.subprocess_run(["git", "config", "user.name", "A Hacker"])
+        self.subprocess_run(["git", "commit", "--allow-empty", "-m", "A"])
+        self.subprocess_run(["git", "switch", "--orphan", "other"])
+        self.subprocess_run(["git", "commit", "--allow-empty", "-m", "B"])
+        self.subprocess_run(
+            ["git", "merge", "--no-commit", "--allow-unrelated-histories", "main"]
+        )
+        result = module.is_merge_in_progress()
+        assert result is True
+
 
 class MigrationAppliedTests(TestCase):
     def test_table_does_not_exist(self):