Precondition failed (HTTP 412) when rerunning sync to mailbox.org

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Steps to reproduce

1. Sync from Google to local
2. Sync from local to Mailfence
3. Repair unsafe UID on Mailfence
4. Wipe out local (store and status)
5. Sync Mailfence to local
6. Sync local to Mailbox (with manual throttling through KeyboardInterrupt)

Expected behavior

• Items are uploaded to Mailbox until keyboard interrupt (Ctrl-C).
• Then, when sync is resumed (rerun), some more items are uploaded.
• Finally, there is nothing left to sync and it exits successfully.

Actual behavior

• Items are uploaded to Mailbox until keyboard interrupt.
• Then, when sync is resumed (rerun), there are conflicts, despite no concurrent modification from other clients.

System details

• Your vdirsyncer version: 0.16.4
• If applicable, which server software (and which version) you're using: Google, Mailbox, Mailfence
• Your Python version: 3.6.5
• Your operating system: Arch Linux \r (\l)
• Your config file: config.txt
• Use vdirsyncer -vdebug for debug output. vdebug.txt

Example

The item 00150f4f-5342-4cea-9fff-89d9af72ebcc.ics:

Copying (uploading) item 00150f4f-5342-4cea-9fff-89d9af72ebcc to mailbox_calendars/50
debug: Normalized URL from '00150f4f-5342-4cea-9fff-89d9af72ebcc.ics' to '/caldav/50/00150f4f-5342-4cea-9fff-89d9af72ebcc.ics'
debug: PUT https://dav.mailbox.org/caldav/50/00150f4f-5342-4cea-9fff-89d9af72ebcc.ics
debug: {'User-Agent': 'vdirsyncer/0.16.4', 'Content-Type': 'text/calendar', 'If-None-Match': '*'}
/.../
debug: Sending request...
debug: 412
debug: {'Date': 'Thu, 24 May 2018 21:20:11 GMT', 'Server': 'Openexchange WebDAV', 'X-Robots-Tag': 'none', 'Content-Length': '0', 'Strict-Transport-Security': 'max-age=15768000', 'Keep-Alive': 'timeout=5, max=98', 'Connection': 'Keep-Alive', 'Content-Type': 'text/calendar'}

error: Unknown error occured for calendars__local__mailbox/main: Precondition Failed

Resources

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/412

Start with the attached vdebug.txt and config.txt, tracing the interrupted upload and the resumed PUT that receives HTTP 412 with If-None-Match: *. Read the sync and WebDAV request-handling entry points implicated by that trace; done means an interrupted sync can be rerun without reporting false conflicts and eventually exits successfully.