[scm] Add global config lock

Otherwise, when this is run concurrently by gclient, it causes race conflicts.

Bug: b/356935829
Change-Id: Ic7697415f7dbb82971be511c5058206f9195e896
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5784434
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Allen Li <ayatane@chromium.org>

scm.py

@@ -15,6 +15,7 @@ import threading
 
 from collections import defaultdict
 from itertools import chain
+from typing import Any
 from typing import Collection, Iterable, Iterator, Literal, Dict
 from typing import Optional, Sequence, Mapping
 
@@ -363,6 +364,8 @@ class GitConfigStateReal(GitConfigStateBase):
     the git configuration files on disk via GIT.Capture.
     """
 
+    _GLOBAL_LOCK = threading.Lock()
+
     def __init__(self, root: pathlib.Path):
         super().__init__()
         self.root = root
@@ -392,7 +395,8 @@ class GitConfigStateReal(GitConfigStateBase):
         args = ['config', f'--{scope}', key, value]
         if append:
             args.append('--add')
-        GIT.Capture(args, cwd=self.root)
+        with self._scope_lock(scope):
+            GIT.Capture(args, cwd=self.root)
 
     def set_config_multi(self, key: str, value: str, *,
                          value_pattern: Optional[str], scope: GitConfigScope):
@@ -433,6 +437,15 @@ class GitConfigStateReal(GitConfigStateBase):
                 raise GitConfigUnsetMissingValue(key, scope)
             raise
 
+    def _scope_lock(
+            self,
+            scope: GitConfigScope) -> contextlib.AbstractContextManager[Any]:
+        if scope == 'global':
+            return self._GLOBAL_LOCK
+        # TODO(ayatane): We should lock per local repo scope as well
+        # from a correctness perspective.
+        return contextlib.nullcontext()
+
 
 class GitConfigStateTest(GitConfigStateBase):
     """A fake implementation of GitConfigStateBase for testing.