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diff --git a/git_trace2_event_log.py b/git_trace2_event_log.py
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+# Copyright (C) 2020 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.
+
+"""Provide event logging in the git trace2 EVENT format.
+
+The git trace2 EVENT format is defined at:
+https://www.kernel.org/pub/software/scm/git/docs/technical/api-trace2.html#_event_format
+https://git-scm.com/docs/api-trace2#_the_event_format_target
+
+  Usage:
+
+  git_trace_log = EventLog()
+  git_trace_log.StartEvent()
+  ...
+  git_trace_log.ExitEvent()
+  git_trace_log.Write()
+"""
+
+
+import datetime
+import json
+import os
+import sys
+import tempfile
+import threading
+
+from git_command import GitCommand, RepoSourceVersion
+
+
+class EventLog(object):
+  """Event log that records events that occurred during a repo invocation.
+
+  Events are written to the log as a consecutive JSON entries, one per line.
+  Entries follow the git trace2 EVENT format.
+
+  Each entry contains the following common keys:
+  - event: The event name
+  - sid: session-id - Unique string to allow process instance to be identified.
+  - thread: The thread name.
+  - time: is the UTC time of the event.
+
+  Valid 'event' names and event specific fields are documented here:
+  https://git-scm.com/docs/api-trace2#_event_format
+  """
+
+  def __init__(self, env=None):
+    """Initializes the event log."""
+    self._log = []
+    # Try to get session-id (sid) from environment (setup in repo launcher).
+    KEY = 'GIT_TRACE2_PARENT_SID'
+    if env is None:
+      env = os.environ
+
+    now = datetime.datetime.utcnow()
+
+    # Save both our sid component and the complete sid.
+    # We use our sid component (self._sid) as the unique filename prefix and
+    # the full sid (self._full_sid) in the log itself.
+    self._sid = 'repo-%s-P%08x' % (now.strftime('%Y%m%dT%H%M%SZ'), os.getpid())
+    parent_sid = env.get(KEY)
+    # Append our sid component to the parent sid (if it exists).
+    if parent_sid is not None:
+      self._full_sid = parent_sid + '/' + self._sid
+    else:
+      self._full_sid = self._sid
+
+    # Set/update the environment variable.
+    # Environment handling across systems is messy.
+    try:
+      env[KEY] = self._full_sid
+    except UnicodeEncodeError:
+      env[KEY] = self._full_sid.encode()
+
+    # Add a version event to front of the log.
+    self._AddVersionEvent()
+
+  @property
+  def full_sid(self):
+    return self._full_sid
+
+  def _AddVersionEvent(self):
+    """Adds a 'version' event at the beginning of current log."""
+    version_event = self._CreateEventDict('version')
+    version_event['evt'] = 2
+    version_event['exe'] = RepoSourceVersion()
+    self._log.insert(0, version_event)
+
+  def _CreateEventDict(self, event_name):
+    """Returns a dictionary with the common keys/values for git trace2 events.
+
+    Args:
+      event_name: The event name.
+
+    Returns:
+      Dictionary with the common event fields populated.
+    """
+    return {
+        'event': event_name,
+        'sid': self._full_sid,
+        'thread': threading.currentThread().getName(),
+        'time': datetime.datetime.utcnow().isoformat() + 'Z',
+    }
+
+  def StartEvent(self):
+    """Append a 'start' event to the current log."""
+    start_event = self._CreateEventDict('start')
+    start_event['argv'] = sys.argv
+    self._log.append(start_event)
+
+  def ExitEvent(self, result):
+    """Append an 'exit' event to the current log.
+
+    Args:
+      result: Exit code of the event
+    """
+    exit_event = self._CreateEventDict('exit')
+
+    # Consider 'None' success (consistent with event_log result handling).
+    if result is None:
+      result = 0
+    exit_event['code'] = result
+    self._log.append(exit_event)
+
+  def Write(self, path=None):
+    """Writes the log out to a file.
+
+    Log is only written if 'path' or 'git config --get trace2.eventtarget'
+    provide a valid path to write logs to.
+
+    Logging filename format follows the git trace2 style of being a unique
+    (exclusive writable) file.
+
+    Args:
+      path: Path to where logs should be written.
+
+    Returns:
+      log_path: Path to the log file if log is written, otherwise None
+    """
+    log_path = None
+    # If no logging path is specified, get the path from 'trace2.eventtarget'.
+    if path is None:
+      cmd = ['config', '--get', 'trace2.eventtarget']
+      # TODO(https://crbug.com/gerrit/13706): Use GitConfig when it supports
+      # system git config variables.
+      p = GitCommand(None, cmd, capture_stdout=True, capture_stderr=True,
+                     bare=True)
+      retval = p.Wait()
+      if retval == 0:
+        # Strip trailing carriage-return in path.
+        path = p.stdout.rstrip('\n')
+      elif retval != 1:
+        # `git config --get` is documented to produce an exit status of `1` if
+        # the requested variable is not present in the configuration. Report any
+        # other return value as an error.
+        print("repo: error: 'git config --get' call failed with return code: %r, stderr: %r" % (
+            retval, p.stderr), file=sys.stderr)
+
+    if isinstance(path, str):
+      # Get absolute path.
+      path = os.path.abspath(os.path.expanduser(path))
+    else:
+      raise TypeError('path: str required but got %s.' % type(path))
+
+    # Git trace2 requires a directory to write log to.
+
+    # TODO(https://crbug.com/gerrit/13706): Support file (append) mode also.
+    if not os.path.isdir(path):
+      return None
+    # Use NamedTemporaryFile to generate a unique filename as required by git trace2.
+    try:
+      with tempfile.NamedTemporaryFile(mode='x', prefix=self._sid, dir=path,
+                                       delete=False) as f:
+        # TODO(https://crbug.com/gerrit/13706): Support writing events as they
+        # occur.
+        for e in self._log:
+          # Dump in compact encoding mode.
+          # See 'Compact encoding' in Python docs:
+          # https://docs.python.org/3/library/json.html#module-json
+          json.dump(e, f, indent=None, separators=(',', ':'))
+          f.write('\n')
+        log_path = f.name
+    except FileExistsError as err:
+      print('repo: warning: git trace2 logging failed: %r' % err,
+            file=sys.stderr)
+      return None
+    return log_path