dvc: windows dir out containing files with / in their name will explode

Using master branch.

When we save dir cache we convert filenames in it to posix style in a lossy way:

"hey\there" -> "hey/there"
"hey/there" -> "hey/there"

which will lead to hey/there becoming hey\there after back transform, making filename contain path sep, which will lead to cache mismatch and other interesting behavior.

[Suor]

So we may just not permit having / in filenames on Windows or escape it somehow when converting it to posix. That latter could be messy though.

[daavoo]

Okay here’s a repro that breaks a plot when there’s a \ in the path. The plot path\fail.png doesn’t show up in rendered HTML of dvc plots show, while path/pass.png does. The same goes for the plot rendering in the DVC VSCode extension.

The repro is in main, then in fix-offending-plot the \ is swapped for / in the offending plot, then in remove-offending-plot the offending plot is commented out of dvc.yaml. Each branch has the output of dvc plots show committed as well.

Closing in favor of https://github.com/iterative/dvc/issues/8689