Google Calendar sync: 409 errors when syncing from a CalDAV server to gcal and source event changes

When running vdirsyncer sync to regularly sync events from a CalDAV server to a Google Calendar, and the event on the CalDAV server changes after the initial sync, I will see an exception due to a HTTP 409 from Google Calendar.

Seems very similar to #613.

• Your vdirsyncer version: 0.18.0
• Your Python version: Python 3.9
• Your operating system: macOS 10.12 x86_64

config file:

[general]
status_path = "~/.cache/vdirsyncer/status/"

[storage my_caldav_calendar]
type = "caldav"
url = "[REDACTED]"
username = "[REDACTED]"
password = "[REDACTED]"
read_only = true
item_types = ["VEVENT"]

[storage my_remote_gcal]
type = "google_calendar"
token_file = "~/.cache/vdirsyncer/my_remote_gcal.token"
client_id = "[REDACTED]"
client_secret = "[REDACTED]"
item_types = ["VEVENT"]

[pair local_to_remote]
a = "my_caldav_calendar"
b = "my_remote_gcal"

# Robert work (new)
collections = [["lark_to_gcal", "[my-local-calendar-id]", "[my-google-calendar-id]@group.calendar.google.com"]]

conflict_resolution = "a wins"
partial_sync = "ignore"

vdirsyncer -vdebug sync:

Doing conflict resolution for item [my-event-id]...
Copying (updating) item [my-event-id] to gcal_robert_work/[my-google-calendar-id]@group.calendar.google.com
debug: Already normalized: '/caldav/v2/[my-google-calendar-id]@group.calendar.google.com/events/[my-event-id].ics'
debug: ====================
debug: PUT https://apidata.googleusercontent.com/caldav/v2/[my-google-calendar-id]@group.calendar.google.com/events/[my-event-id].ics
debug: {'User-Agent': 'my-google-app-id.apps.googleusercontent.com', 'Content-Type': 'text/calendar', 'If-Match': '"63778220399"'}
debug: [ICAL-EVENT-DETAILS-REDACTED]
debug: Sending request...
debug: 409
debug: {'Pragma': 'no-cache', 'Content-Type': 'text/xml; charset=UTF-8', 'Date': 'Tue, 18 Jan 2022 21:12:10 GMT', 'Expires': 'Mon, 01 Jan 1990 00:00:00 GMT', 'Cache-Control': 'no-cache, no-store, max-age=0, must-revalidate', 'Vary': 'Origin, X-Origin, Referer', 'Content-Encoding': 'gzip', 'Server': 'ESF', 'X-XSS-Protection': '0', 'X-Frame-Options': 'SAMEORIGIN', 'X-Content-Type-Options': 'nosniff', 'Alt-Svc': 'h3=":443"; ma=2592000,h3-29=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"', 'Transfer-Encoding': 'chunked'}
debug: b'<?xml version="1.0" encoding="UTF-8"?>\n<D:error xmlns:D="DAV:"/>\n'
error: Unknown error occurred for lark_to_gcal_robert_work/lark_to_gcal: 409 Client Error: Conflict for url: https://apidata.googleusercontent.com/caldav/v2/[my-google-calendar-id]@group.calendar.google.com/events/[my-event-id].ics
error: Use `-vdebug` to see the full traceback.
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/cli/utils.py", line 75, in handle_cli_error
debug:     raise e
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/sync/__init__.py", line 154, in sync
debug:     action.run(a_info, b_info, conflict_resolution, partial_sync)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/sync/__init__.py", line 276, in run
debug:     Update(new_item, b).run(a, b, conflict_resolution, partial_sync)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/sync/__init__.py", line 177, in run
debug:     self._run_impl(a, b)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/sync/__init__.py", line 229, in _run_impl
debug:     meta.etag = self.dest.storage.update(meta.href, self.item, meta.etag)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/base.py", line 13, in inner
debug:     return f(self, *args, **kwargs)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/dav.py", line 543, in update
debug:     href, etag = self._put(self._normalize_href(href), item, etag)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/dav.py", line 516, in _put
debug:     response = self.session.request(
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/storage/dav.py", line 397, in request
debug:     return http.request(method, url, session=self._session, **more)
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/vdirsyncer/http.py", line 174, in request
debug:     r.raise_for_status()
debug:   File "/Users/bytedance/.local/pipx/venvs/vdirsyncer/lib/python3.9/site-packages/requests/models.py", line 960, in raise_for_status
debug:     raise HTTPError(http_error_msg, response=self)
error: 1 out of 2 tasks failed.

Start with the update path in vdirsyncer/sync/init.py and follow it into storage/dav.py, especially update and _put, then inspect the request handling in vdirsyncer/http.py. Reproduce the CalDAV-to-Google Calendar update with the shown configuration and verify that a source event change no longer leaves the sync task failing on HTTP 409.