Preserve POSIX absolute bind mount paths on Windows

[sgolev]

On Windows, parse_short_mount() and assert_volume() unconditionally run os.path.abspath() on bind mount sources. For a POSIX absolute source like /etc/example, os.path.abspath() prepends the current Windows drive, producing C:\etc\example. As a result, when targeting a remote Linux host, podman-compose fails with:

statfs /mnt/c/etc/example: no such file or directory

The fix is to skip the rewrite when the source is POSIX absolute path on Windows. Relative, ~, .sock, and drive paths (C:\...) are unaffected.

This PR fixes #1352.

As a side note, assert_volume() doesn't work as intended when targeting a remote machine, because it ensures that directories exist on the local machine. I've tried to introduce minimal changes just to fix the Windows-to-remote-Linux workflow. When a POSIX absolute path is detected on Windows, both the rewrite and the local probe are skipped.

[p12tic]

Shouldn't the proper fix be to switch to posix path module for any operations on a remote Linux machine?

[sgolev]

If I understand you correctly, you're suggesting that we check whether we're running in remote mode and, in that case, switch to posixpath. But it helps only with absolute paths, so the behavior is essentially the same.

BTW, I found how Docker Compose handles paths: https://github.com/compose-spec/compose-go/blob/ace34ebd2aae2913524151b2b02e67790c88983c/paths/unix.go#L38. So they check whether the path is absolute in either POSIX or Windows notation leaving it unchanged if it is.

The corresponding Python code would be:

is_abs = os.path.isabs(mount_src) or secondarypathisabs(mount_src)
if not is_abs and (os.name != 'nt' or ".sock" not in mount_src):

[p12tic]

@sgolev

The corresponding Python code would be: <...>

Makes sense. I think we could do exactly as this, since this makes maximum compatibility with docker compose. Probably makes sense to introduce it as a function.