Bugs in the error handling while uploading a file to the backend could
cause incomplete files, e.g. https://github.com/golang/go/issues/42400
which could affect the local backend.
Proactively add sanity checks which will treat an upload as failed if
the reported upload size does not match the actual file size.
This code is more strict in what it expects to find in the backend:
depending on the layout, either a directory full of files or a directory
full of such directories.
Due to the return if !isFile, the IsDir branch in List was never taken
and subdirectories were traversed recursively.
Also replaced isFile by an IsRegular check, which has been equivalent
since Go 1.12 (golang/go@a2a3dd00c9).
The file is already created with the proper permissions, thus the chmod
call is not critical. However, some file systems have don't allow
modifications of the file permissions. Similarly the chmod call in the Remove
operation should not prevent it from working.
As mentioned in issue [#1560](https://github.com/restic/restic/pull/1560#issuecomment-364689346)
this changes the signature for `backend.Save()`. It now takes a
parameter of interface type `RewindReader`, so that the backend
implementations or our `RetryBackend` middleware can reset the reader to
the beginning and then retry an upload operation.
The `RewindReader` interface also provides a `Length()` method, which is
used in the backend to get the size of the data to be saved. This
removes several ugly hacks we had to do to pull the size back out of the
`io.Reader` passed to `Save()` before. In the `s3` and `rest` backend
this is actively used.
- be explicit when discarding returned errors from .Close(), etc.
- remove named return values from funcs when naked return not used
- fix some "err" shadowing when redeclaration not needed
