feat: Mark files with conflict markers as unmerged

Closes copier-org#1350

copier/main.py

@@ -901,6 +901,7 @@ def _apply_update(self):
                     apply_cmd = apply_cmd["--exclude", skip_pattern]
                 (apply_cmd << diff)(retcode=None)
                 if self.conflict == "inline":
+                    conflicted = []
                     status = git("status", "--porcelain").strip().splitlines()
                     for line in status:
                         # Filter merge rejections (part 1/2)
@@ -933,6 +934,23 @@ def _apply_update(self):
                         )
                         # Remove rejection witness
                         Path(f"{fname}.rej").unlink()
+                        # Store file name for marking it as unmerged after the loop
+                        conflicted.append(fname)
+                    # Forcefully mark files with conflict markers as unmerged,
+                    # see SO post: https://stackoverflow.com/questions/77391627/
+                    # and Git docs: https://git-scm.com/docs/git-update-index#_using_index_info.
+                    # For each file with conflict markers, we update the index to add
+                    # higher order versions of their paths, without entries for resolved contents.
+                    input_lines = []
+                    for line in (
+                        git("ls-files", "--stage", *conflicted).strip().splitlines()
+                    ):
+                        perms_sha_mode, path = line.split("\t")
+                        perms, sha, _ = perms_sha_mode.split()
+                        input_lines.append(f"0 {'0' * 40}\t{path}")
+                        for mode in (1, 2, 3):
+                            input_lines.append(f"{perms} {sha} {mode}\t{path}")
+                    (git["update-index", "--index-info"] << "\n".join(input_lines))()
             # Trigger recursive removal of deleted files in last template version
             _remove_old_files(subproject_top, compared)
 

tests/test_updatediff.py

@@ -21,6 +21,7 @@
     SUFFIX_TMPL,
     Spawn,
     build_file_tree,
+    git_init,
 )
 
 
@@ -983,3 +984,86 @@ def function_two():
             print("Previous line lied.")
         """
     )
+
+
+@pytest.mark.parametrize(
+    "filename",
+    [
+        "README.md",
+        "spa ces",
+        'qu`o"tes',
+        "m4â4ñ4a",
+    ],
+)
+def test_conflicted_files_are_marked_unmerged(
+    tmp_path_factory: pytest.TempPathFactory,
+    filename: str,
+) -> None:
+    # Template in v1 has a file with a single line;
+    # in v2 it changes that line.
+    # Meanwhile, downstream project appended contents to the first line.
+    src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))
+
+    # First, create the template with an initial file
+    build_file_tree(
+        {
+            (src / filename): "upstream version 1",
+            (src / "{{_copier_conf.answers_file}}.jinja"): (
+                "{{_copier_answers|to_nice_yaml}}"
+            ),
+        }
+    )
+    with local.cwd(src):
+        git_init("hello template")
+        git("tag", "v1")
+
+    # Generate the project a first time, assert the file exists
+    run_copy(str(src), dst, defaults=True, overwrite=True)
+    assert (dst / filename).exists()
+    assert "_commit: v1" in (dst / ".copier-answers.yml").read_text()
+
+    # Start versioning the generated project
+    with local.cwd(dst):
+        git_init("hello project")
+
+        # After first commit, change the file, commit again
+        Path(filename).write_text("upstream version 1 + downstream")
+        git("commit", "-am", "updated file")
+
+    # Now change the template
+    with local.cwd(src):
+        # Update the file
+        Path(filename).write_text("upstream version 2")
+
+        # Commit the changes
+        git("add", ".", "-A")
+        git("commit", "-m", "change line in file")
+        git("tag", "v2")
+
+    # Finally, update the generated project
+    run_update(dst_path=dst, defaults=True, overwrite=True, conflict="inline")
+    assert "_commit: v2" in (dst / ".copier-answers.yml").read_text()
+
+    # Assert that the file still exists, has inline markers,
+    # and is reported as "unmerged" by Git.
+    assert (dst / filename).exists()
+
+    expected_contents = dedent(
+        """\
+        <<<<<<< before updating
+        upstream version 1 + downstream
+        =======
+        upstream version 2
+        >>>>>>> after updating
+        """
+    )
+    assert (dst / filename).read_text().splitlines() == expected_contents.splitlines()
+    assert not (dst / f"{filename}.rej").exists()
+
+    with local.cwd(dst):
+        print(dst)
+        lines = git("ls-files", "--stage", filename).strip().splitlines()
+        assert len(lines) == 3
+        assert " 1\t" in lines[0]
+        assert " 2\t" in lines[1]
+        assert " 3\t" in lines[2]