feat(gix): serialize trees with native object storage

Handle GitPython's NUL-delimited `mktree --missing` input with the shared native tree editor. Validate entry kinds and paths before writing, preserve raw filenames, and keep missing-child cases on Git until the native editor can represent them (GIX-7).

The staged version passes Ruff and Python syntax checks. Its SHA-1/SHA-256 native smoke and focused pytest selection pass: 34 passed in 5.13s.

Author: codex

git/_backend.py

@@ -440,6 +440,28 @@ def write_tree(command: Any, entries: Sequence[Tuple[bytes, int, str]]) -> Any:
     return result
 
 
+def _mktree(repo: Any, args: List[str], kwargs: Dict[str, Any]) -> bytes:
+    if args != ["-z", "--missing"]:
+        raise _Unsupported("tree writing options")
+    data = _input(kwargs)
+    entries = []
+    for record in data.split(b"\0"):
+        if not record:
+            continue
+        metadata, path = record.split(b"\t", 1)
+        mode_bytes, kind, oid_bytes = metadata.split()
+        mode = int(mode_bytes, 8)
+        if mode not in _ENTRY_KINDS or b"/" in path or path in (b"", b".", b"..", b".git"):
+            raise _Unsupported("tree entry")
+        expected = b"tree" if mode == 0o40000 else b"commit" if mode == 0o160000 else b"blob"
+        if kind != expected:
+            raise _Unsupported("tree entry kind")
+        entries.append((path, mode, oid_bytes.decode("ascii")))
+    oid = _write_tree(repo, entries)
+    kwargs["istream"].seek(len(data), 1)
+    return oid.encode("ascii") + b"\n"
+
+
 _HANDLERS: Dict[str, Callable[[Any, List[str], Dict[str, Any]], bytes]] = {
     "rev_parse": _rev_parse,
     "ls_tree": _ls_tree,
@@ -449,6 +471,7 @@ def write_tree(command: Any, entries: Sequence[Tuple[bytes, int, str]]) -> Any:
     "ls_files": _ls_files,
     "update_index": _update_index,
     "hash_object": _hash_object,
+    "mktree": _mktree,
     "config": _config,
     "worktree": _worktree,
 }

test/test_gix_backend.py

@@ -2,6 +2,7 @@
 
 from io import BytesIO
 from pathlib import Path
+import tempfile
 from unittest.mock import patch
 
 import pytest
@@ -86,6 +87,18 @@ def handler(native, args, kwargs):
     assert attempts == ["write"]
 
 
+@pytest.mark.parametrize("mode,kind", [("100644", "blob"), ("040000", "tree"), ("160000", "commit")])
+def test_tree_writes_validate_child_object_kinds(repo, mode, kind):
+    commit = repo.index.commit("initial", skip_hooks=True)
+    blob = repo.odb.store(IStream("blob", 1, BytesIO(b"x")))
+    wrong_id = blob.hexsha.decode() if kind == "commit" else commit.hexsha
+    with tempfile.TemporaryFile() as stream:
+        stream.write((mode + " " + kind + " " + wrong_id + "\twrong\0").encode())
+        stream.seek(0)
+        with pytest.raises(GitCommandError):
+            repo.git._call_process_safe("mktree", "-z", "--missing", istream=stream)
+
+
 def test_partial_native_stream_does_not_corrupt_next_read(repo):
     one = repo.odb.store(IStream("blob", 6, BytesIO(b"abcdef"))).binsha
     two = repo.odb.store(IStream("blob", 3, BytesIO(b"xyz"))).binsha