Saving to locally sync'ed OneDrive folder

[davidjgraph]

There's clearly an issue saving diagrams to local folders that are sync'ed to OneDrive. It's still extremely rare, but it generally seems to affect larger files. This suggests that some operation on the sync client is happening while the save is ongoing, resulting in the file being cut off at some point.

We've looked at how we flush the write and we can't find any cases where we don't fully flush the buffer after a write. That, and the fact that (on recent builds) we're only hearing about this with OneDrive sync'ed folders, leads us to think the problem is the sync client, not us.

Searching, I found a number of threads discussing this specific problem with other apps, for example https://answers.microsoft.com/en-us/msoffice/forum/all/onedrive-corrupted-files/181006dd-6c36-483a-aca3-a2b8308f971a.

It's hard to say whether this is the same issue, but saving locally to a OneDrive sync'ed app seems like a bad idea. We cannot detect that you're saving to such a folder, so we can't generate a warning.

In terms of file recovery, we'd suggest three things:

1. Check whether the locally stored version is corrupted only. Download the online stored version to compare, it might be that version is complete.
2. If not, download the previous version of the file stored in OneDrive online. OneDrive has versioning for all files.
3. The draw.io format is an XML format. It's generally possible to tidy the end of the corrupted file and close the XML tags. If someone posts a corrupted file in this thread we can explain that on the example.