WebDAV: Indicate final file size when doing chunk upload

[sntran]

What is your current rclone version (output from rclone version)?

rclone v1.65.1

• os/version: darwin 14.4.1 (64 bit)
• os/kernel: 23.4.0 (x86_64)
• os/type: darwin
• os/arch: amd64
• go/version: go1.21.5
• go/linking: dynamic
• go/tags: cmount

What problem are you are trying to solve?

Currently, when setting vendor to "nextcloud", rclone will MKCOL a temporary folder using the MD5 hash of the file path, prefixed with "rclone-chunked-upload-". While this works fine for Nextcloud, other implementations may not be able to take advantage of the chunking feature when the original file size is not know.

My use case is to be able to use rclone against storage systems that support chunked upload through API, but not WebDAV. That can be possible by writing a proxy on top of the API. However, some API requires knowing the file size beforehand before accepting chunks.

Example usage:

$ rclone copy \
  --webdav-vendor=nextcloud \
  --webdav-url=http://localhost:8000/dav/files/fixtures \
  fixtures/ :webdav: 

How do you think rclone should be changed to solve that?

Include the file size somewhere. One way is to use "Content-Range" header like Google Cloud Storage (https://cloud.google.com/storage/docs/performing-resumable-uploads#chunked-upload). However, that does not help when file size is needed in advance of the PUT.

Alternatively, we can append it to the name of the temporary upload folder, i.e. rclone-chunked-upload-:md5-:size. This way uploading to NextCloud still works normally.

Or we can do both.

I don't think it's a big change, and it's not breaking anything. I'd be happy to submit a PR if it's acceptable.

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