git: set deterministic change id and add lore link to commits

Problem:
Non-deterministic change ids mean we get duplicate changes uploaded to
Gerrit for the same patch if the Bridge gets restarted.

When issue #14 is implemented, this will not matter as much, but there's
still a chance for updates not to be persisted in the DB.

Solution:
Use a deterministic change id, i.e. "I" + <commit sha of v1>.

If we try and create a Gerrit change for the same patch, I'm assuming
the collision with the pre-existing change id will prevent us from doing
so.

Alternative:
As noted in the file, we could consider using a `applypatch-msg` hook.

But we can do more if we edit the commit directly from Python, since we
have access to more information, e.g. the lore link.
Add in a "Lore-Link" footer while we're here to demonstrate this.

This iteration of the code is essentially a direct translation of what I
had originally prototyped in bash for the hook.

src/git.py

@@ -16,6 +16,8 @@
 import re
 import shutil
 import subprocess
+import tempfile
+from message import lore_link
 import message_dao
 from patch_parser import Patch, Patchset
 from absl import logging
@@ -36,6 +38,9 @@ class _Git(object):
     def __init__(self, git_dir: str) -> None:
         self._git_dir = git_dir
 
+    def __call__(self, verb: str, *args) -> str:
+        return _git(verb, *args, cwd=self._git_dir)
+
     def clone(self, remote, *args) -> str:
         return _git('clone', *args, '--', remote, self._git_dir)
 
@@ -72,8 +77,6 @@ def _parse_gerrit_patch_push(gerrit_result: str) -> str:
     logging.info('change_id = %s', change_id)
     return change_id
 
-CURL_CHANGE_ID_CMD = 'curl -Lo `git rev-parse --git-dir`/hooks/commit-msg https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; chmod +x `git rev-parse --git-dir`/hooks/commit-msg'
-
 class GerritGit(object):
     def __init__(self, git_dir: str, cookie_jar_path: str, url: str, project: str, branch: str) -> None:
         self._git_dir = git_dir
@@ -85,10 +88,7 @@ def __init__(self, git_dir: str, cookie_jar_path: str, url: str, project: str, b
     def shallow_clone(self, depth=1) -> str:
         return self._git.clone(self._remote, '--depth', str(depth), '--single-branch', '--branch', self._branch)
 
-    def amend_commit(self) -> str:
-        return self._git.commit('--amend', '--no-edit')
-
-    def apply_patch(self, patch: Patch) -> str:
+    def _apply_patch(self, patch: Patch) -> str:
         try:
             return self._git.am(patch.text_with_headers)
         except subprocess.CalledProcessError as e:
@@ -98,6 +98,28 @@ def apply_patch(self, patch: Patch) -> str:
             _git('am', '--abort', cwd=self._git_dir)
             raise
 
+    def _set_trailers(self, patch: Patch) -> str:
+        """Assuming `patch` is HEAD, edits the commit message before we upload to Gerrit.
+
+        Note: normally, we'd rely on a applypatch-msg hook for this.
+        But we have more context here, namely the message id so we can include a
+        link to the original message in Lore.
+
+        Args:
+            patch: the patch we've just applied.
+        """
+        commit_hash = self._git('rev-parse', 'HEAD').rstrip('\n')
+        original_message = self._git('log', '-1', '--format=%B').rstrip('\n')
+
+        change_id_trailer = f'Change-Id: I{commit_hash}'
+        lore_trailer = 'Lore-Link: ' + lore_link(patch.message_id)
+
+        with tempfile.NamedTemporaryFile(mode='wt') as f:
+          f.write(original_message)
+          self._git('interpret-trailers', '--in-place', '--if-exists=addIfDifferent',
+                    '--trailer', change_id_trailer,  '--trailer', lore_trailer, f.name)
+          return self._git.commit('--amend', '-F', f.name)
+
     def push_changes(self) -> str:
         try:
             return self._git.push(f'HEAD:refs/for/{self._branch}%notify=NONE')
@@ -106,8 +128,8 @@ def push_changes(self) -> str:
             raise
 
     def push_patch(self, patch: Patch) -> Patch:
-        self.apply_patch(patch)
-        self.amend_commit()
+        self._apply_patch(patch)
+        self._set_trailers(patch)
         gerrit_output = self.push_changes()
         change_id = _parse_gerrit_patch_push(gerrit_output)
         patch.change_id = change_id
@@ -120,7 +142,6 @@ def setup_git_dir(self, clone_depth=1) -> None:
         self._git.config('user.name', '"lkml-gerrit-bridge"')
         # TODO: Change config to use a service account instead of @willliu
         self._git.config('user.email', '"willliu@google.com"')
-        subprocess.run(CURL_CHANGE_ID_CMD, cwd=self._git_dir, shell=True)
 
     def cleanup_git_dir(self) -> None:
         shutil.rmtree(self._git_dir)

src/message.py

@@ -25,6 +25,9 @@ class Patchset(object):
     def __init__(self) -> None:
         self.patches = []  # type: List[Patch]
 
+def lore_link(message_id: str) -> str:
+    return 'https://lore.kernel.org/linux-kselftest/' + message_id[1:-1]
+
 class Message(object):
     def __init__(self, id, subject, from_, in_reply_to, content, archive_hash) -> None:
         self.id = id
@@ -61,12 +64,9 @@ def __str__(self) -> str:
     def __repr__(self) -> str:
         return str(self)
 
-    def lore_link(self) -> str:
-        return 'https://lore.kernel.org/linux-kselftest/' + self.id[1:-1]
-
     def debug_info(self) -> str:
         return (f'Message ID: {self.id}\n'
-                f'Lore Link: {self.lore_link()}\n'
+                f'Lore Link: {lore_link(self.id)}\n'
                 f'Commit Hash: {self.archive_hash}')
 
 def parse_message_from_str(raw_email: str, archive_hash: str) -> Message: