webdav: nextcloud: new chunking code causing a panic

[ncw]

It was reported on the forum that rclone v1.63.1 is panicing in the Go http standard library

https://forum.rclone.org/t/panic-after-updating-to-v1-63-1/40226

With a bit of tracing the Go stdlib code I managed to work out that it is caused by the nil, nil return of this code

rclone/backend/webdav/chunking.go

Lines 122 to 126 in 70cbcef

	getBody := func() (io.ReadCloser, error) {
	// RepeatableReader{} plays well with accounting so rewinding doesn't make the progress buggy
	if _, err := in.Seek(0, io.SeekStart); err == nil {
	return nil, err
	}

GetBody must not return a nil body with a nil error.

@devnoname120 do you have any ideas about this? I think this is probably a type which should say err != nil not err == nil but that means we've been taking the wrong branch all along so a bit of testing may be needed.

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[devnoname120]

@ncw Sorry for the issue and thanks for the ping, I'll try to look into it later this week.

From rough memory I remember that I specifically added and tested this retry mechanism because it was preventing me from uploading large files (50 GB) with a large number of chunks (~ 1000).

There was almost always at least a chunk that failed to upload and would trigger the restart of the entire transfer. So it definitely worked at some point because I personally needed this to work. I may have pushed the wrong commit or something at that time however, so I will investigate...

[ncw]

I'm pretty sure changing err == nil to err != nil will fix it but great if you can confirm.

[devnoname120]

@ncw Your suggestion looks good to me. It indeed looks like a mistake — maybe I botched up at some point when refactoring my pull request?

I opened a new pull request, it also fixes an additional segmentation violation: #7195

[ncw]

I've merged this to master now which means it will be in the latest beta in 15-30 minutes and released in v1.64 (or v1.63.2 if we make one)

Thank you @devnoname120

[devnoname120]

@ncw I'm not entirely sure how your release schedule works but do you plan to publish a new release in the near future? Imho those fixes are significant.

[ncw]

@devnoname120 check out the milestone :-) In about a week is the plan!

[ncw]

This is released into v1.64 now :-)