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diff --git a/git_trace2_event_log_base.py b/git_trace2_event_log_base.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 errno
+import json
+import os
+import socket
+import sys
+import tempfile
+import threading
+
+
+# BaseEventLog __init__ Counter that is consistent within the same process
+p_init_count = 0
+
+
+class BaseEventLog(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, repo_source_version=None, add_init_count=False
+    ):
+        """Initializes the event log."""
+        global p_init_count
+        p_init_count += 1
+        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
+
+        self.start = 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" % (
+            self.start.strftime("%Y%m%dT%H%M%SZ"),
+            os.getpid(),
+        )
+
+        if add_init_count:
+            self._sid = f"{self._sid}-{p_init_count}"
+
+        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()
+
+        if repo_source_version is not None:
+            # Add a version event to front of the log.
+            self._AddVersionEvent(repo_source_version)
+
+    @property
+    def full_sid(self):
+        return self._full_sid
+
+    def _AddVersionEvent(self, repo_source_version):
+        """Adds a 'version' event at the beginning of current log."""
+        version_event = self._CreateEventDict("version")
+        version_event["evt"] = "2"
+        version_event["exe"] = repo_source_version
+        self._log.insert(0, version_event)
+
+    def _CreateEventDict(self, event_name):
+        """Returns a dictionary with 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.current_thread().name,
+            "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
+        time_delta = datetime.datetime.utcnow() - self.start
+        exit_event["t_abs"] = time_delta.total_seconds()
+        self._log.append(exit_event)
+
+    def CommandEvent(self, name, subcommands):
+        """Append a 'command' event to the current log.
+
+        Args:
+            name: Name of the primary command (ex: repo, git)
+            subcommands: List of the sub-commands (ex: version, init, sync)
+        """
+        command_event = self._CreateEventDict("command")
+        command_event["name"] = name
+        command_event["subcommands"] = subcommands
+        self._log.append(command_event)
+
+    def LogConfigEvents(self, config, event_dict_name):
+        """Append a |event_dict_name| event for each config key in |config|.
+
+        Args:
+            config: Configuration dictionary.
+            event_dict_name: Name of the event dictionary for items to be logged
+                under.
+        """
+        for param, value in config.items():
+            event = self._CreateEventDict(event_dict_name)
+            event["param"] = param
+            event["value"] = value
+            self._log.append(event)
+
+    def DefParamRepoEvents(self, config):
+        """Append 'def_param' events for repo config keys to the current log.
+
+        This appends one event for each repo.* config key.
+
+        Args:
+            config: Repo configuration dictionary
+        """
+        # Only output the repo.* config parameters.
+        repo_config = {k: v for k, v in config.items() if k.startswith("repo.")}
+        self.LogConfigEvents(repo_config, "def_param")
+
+    def GetDataEventName(self, value):
+        """Returns 'data-json' if the value is an array else returns 'data'."""
+        return "data-json" if value[0] == "[" and value[-1] == "]" else "data"
+
+    def LogDataConfigEvents(self, config, prefix):
+        """Append a 'data' event for each entry in |config| to the current log.
+
+        For each keyX and valueX of the config, "key" field of the event is
+        '|prefix|/keyX' and the "value" of the "key" field is valueX.
+
+        Args:
+            config: Configuration dictionary.
+            prefix: Prefix for each key that is logged.
+        """
+        for key, value in config.items():
+            event = self._CreateEventDict(self.GetDataEventName(value))
+            event["key"] = f"{prefix}/{key}"
+            event["value"] = value
+            self._log.append(event)
+
+    def ErrorEvent(self, msg, fmt=None):
+        """Append a 'error' event to the current log."""
+        error_event = self._CreateEventDict("error")
+        if fmt is None:
+            fmt = msg
+        error_event["msg"] = f"RepoErrorEvent:{msg}"
+        error_event["fmt"] = f"RepoErrorEvent:{fmt}"
+        self._log.append(error_event)
+
+    def _WriteLog(self, write_fn):
+        """Writes the log out using a provided writer function.
+
+        Generate compact JSON output for each item in the log, and write it
+        using write_fn.
+
+        Args:
+            write_fn: A function that accepts byts and writes them to a
+                destination.
+        """
+
+        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
+            write_fn(
+                json.dumps(e, indent=None, separators=(",", ":")).encode(
+                    "utf-8"
+                )
+                + b"\n"
+            )
+
+    def Write(self, path=None):
+        """Writes the log out to a file or socket.
+
+        Log is only written if 'path' or 'git config --get trace2.eventtarget'
+        provide a valid path (or socket) 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. The path may have a
+                prefix of the form "af_unix:[{stream|dgram}:]", in which case
+                the path is treated as a Unix domain socket. See
+                https://git-scm.com/docs/api-trace2#_enabling_a_target for
+                details.
+
+        Returns:
+            log_path: Path to the log file or socket if log is written,
+                otherwise None
+        """
+        log_path = None
+        # If no logging path is specified, exit.
+        if path is None:
+            return None
+
+        path_is_socket = False
+        socket_type = None
+        if isinstance(path, str):
+            parts = path.split(":", 1)
+            if parts[0] == "af_unix" and len(parts) == 2:
+                path_is_socket = True
+                path = parts[1]
+                parts = path.split(":", 1)
+                if parts[0] == "stream" and len(parts) == 2:
+                    socket_type = socket.SOCK_STREAM
+                    path = parts[1]
+                elif parts[0] == "dgram" and len(parts) == 2:
+                    socket_type = socket.SOCK_DGRAM
+                    path = parts[1]
+            else:
+                # 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 (path_is_socket or os.path.isdir(path)):
+            return None
+
+        if path_is_socket:
+            if socket_type == socket.SOCK_STREAM or socket_type is None:
+                try:
+                    with socket.socket(
+                        socket.AF_UNIX, socket.SOCK_STREAM
+                    ) as sock:
+                        sock.connect(path)
+                        self._WriteLog(sock.sendall)
+                    return f"af_unix:stream:{path}"
+                except OSError as err:
+                    # If we tried to connect to a DGRAM socket using STREAM,
+                    # ignore the attempt and continue to DGRAM below. Otherwise,
+                    # issue a warning.
+                    if err.errno != errno.EPROTOTYPE:
+                        print(
+                            f"repo: warning: git trace2 logging failed: {err}",
+                            file=sys.stderr,
+                        )
+                        return None
+            if socket_type == socket.SOCK_DGRAM or socket_type is None:
+                try:
+                    with socket.socket(
+                        socket.AF_UNIX, socket.SOCK_DGRAM
+                    ) as sock:
+                        self._WriteLog(lambda bs: sock.sendto(bs, path))
+                        return f"af_unix:dgram:{path}"
+                except OSError as err:
+                    print(
+                        f"repo: warning: git trace2 logging failed: {err}",
+                        file=sys.stderr,
+                    )
+                    return None
+            # Tried to open a socket but couldn't connect (SOCK_STREAM) or write
+            # (SOCK_DGRAM).
+            print(
+                "repo: warning: git trace2 logging failed: could not write to "
+                "socket",
+                file=sys.stderr,
+            )
+            return None
+
+        # Path is an absolute path
+        # Use NamedTemporaryFile to generate a unique filename as required by
+        # git trace2.
+        try:
+            with tempfile.NamedTemporaryFile(
+                mode="xb", prefix=self._sid, dir=path, delete=False
+            ) as f:
+                # TODO(https://crbug.com/gerrit/13706): Support writing events
+                # as they occur.
+                self._WriteLog(f.write)
+                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