get/import: check target path before downloading

[jorgeorpinel]

λ dvc version
DVC version: 0.93.0
Python version: 3.7.5
Platform: Windows-10-10.0.18362-SP0
Binary: True
Package: exe
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Cache: reflink - not supported, hardlink - supported, symlink - not supported
Filesystem type (cache directory): ('NTFS', 'C:\\')
Repo: dvc, git
Filesystem type (workspace): ('NTFS', 'C:\\')

λ echo data > data.xml
λ la
...
-rw-r--r-- 1 usd grp        5 Apr 23 21:31 data.xml
λ dvc get https://github.com/iterative/dataset-registry get-started/data.xml -o data.xml
# HERE I SEE THE DOWNLOAD BAR FIRST, and the whole transfer is done up to 100%
ERROR: unexpected error - [WinError 5] Access is denied: 'C:\\Users\\poj12\\DVC-repos\\dvc-get-started\\data.xml.Cf4LfW3t5gZ7aK9F2qXquP'
λ la
...
-r--r--r-- 1 usd grp 37916850 Apr 23 21:26 data.xml
-rw-r--r-- 1 usd grp 37916850 Apr 23 21:26 data.xml.Cf4LfW3t5gZ7aK9F2qXquP

The result is I end up with the download having overwritten the small data.xml I created, but with an error message, and with the tmp file not deleted either, as it can be seen above.

[efiop]

Not able to reproduce. Probably was related to some workspace issues. Feel free to reopen if you are still able to reproduce.

[efiop]

Ok, there was a misunderstanding on my side, the error itself is irrelevant, as it is related to some setup issues, but the fact that the thing is downloaded before checking if file is already there is questionable. The temporary file is from the link failure, probably @jorgeorpinel had the symlink configured in global/system config without the rights to use them. Would've been great if we had a verbose log for the error 🙁

Keeping open for the further research, as we might want to revisit ExternalRepo.get_external to not download beforehand, but actually do that dynamically when checking out, so existing target could be checked for before.

Regarding the link issue, the logic change proposal from above is not really related to it, because we couldn't really know that the link will fail before trying to link the file in an atomic way. So need to give it a thought.

[jorgeorpinel]

This is fixed now (tested on DVC 2.3.0).