repo_trace: Avoid race conditions with trace_file updating.

Change-Id: I0bc1bb3c8f60465dc6bee5081688a9f163dd8cf8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354515
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Joanna Wang <jojwang@google.com>

tests/test_repo_trace.py

@@ -0,0 +1,56 @@
+# Copyright 2022 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unittests for the repo_trace.py module."""
+
+import os
+import unittest
+from unittest import mock
+
+import repo_trace
+
+
+class TraceTests(unittest.TestCase):
+  """Check Trace behavior."""
+
+  def testTrace_MaxSizeEnforced(self):
+    content = 'git chicken'
+
+    with repo_trace.Trace(content, first_trace=True):
+      pass
+    first_trace_size = os.path.getsize(repo_trace._TRACE_FILE)
+
+    with repo_trace.Trace(content):
+      pass
+    self.assertGreater(
+        os.path.getsize(repo_trace._TRACE_FILE), first_trace_size)
+
+    # Check we clear everything is the last chunk is larger than _MAX_SIZE.
+    with mock.patch('repo_trace._MAX_SIZE', 0):
+      with repo_trace.Trace(content, first_trace=True):
+        pass
+      self.assertEqual(first_trace_size,
+                       os.path.getsize(repo_trace._TRACE_FILE))
+
+      # Check we only clear the chunks we need to.
+      repo_trace._MAX_SIZE = (first_trace_size + 1) / (1024 * 1024)
+      with repo_trace.Trace(content, first_trace=True):
+        pass
+      self.assertEqual(first_trace_size * 2,
+                       os.path.getsize(repo_trace._TRACE_FILE))
+
+      with repo_trace.Trace(content, first_trace=True):
+        pass
+      self.assertEqual(first_trace_size * 2,
+                       os.path.getsize(repo_trace._TRACE_FILE))