[Feature]: QEMU Machine should support Windows Source paths for 9pfs on Windows hosts

[arixmkii]

Feature request description

There is a work in progress 9pfs support implementation for QEMU. There is a working demo of it #13006 (comment)

Currently it is impossible to Init a machine on Windows hosts with FS mounts configured via command line, because of the special meaning of : after drive letter in Windows paths. Current parser just splits string using : and is not in any way platform aware:

podman/pkg/machine/qemu/machine.go

Line 304 in f15b1f5

paths := strings.SplitN(volume, ":", 3)

Additionally there should be conversion mechanism predefined to generate some reasonable Unix path based on Windows source path. There is a tool in cygwin/msys2 called "cygpath" to convert Windows paths to Unix equivalents for their subsystem, but here it looks like unnecessary dependency.

Suggest potential solution

Solutions would be provided separately for the parser and for path conversion

Parser

The simplest way would be to change : to os.PathListSeparator, which is ":' on Unixes and ";" on Windows and is used when they define multiple records inside PATH environment variable. The downside of this is that there will be need to document and communicate that parsing rules differs between platforms.

Alternative would be to implement OS awareness like checking that there is a drive letter in Windows source path and combine it with path after (so, like split into one more, but then glue the first two to go to the previous state).

Not related to this feature, but it looks like it is better to add sanity checks that paths are absolute, because allowing relative paths complicates the things significantly (also, I don't think they would work, may be it is just unsupported by definition and doesn't need coverage - user will be responsible for the input).

Path conversion

		if runtime.GOOS == "windows" {
			target = "/" + strings.ReplaceAll(target, string(os.PathSeparator), "/")
			target = strings.ReplaceAll(target, ":", "")
		}

Looks good enough:

1. convert all PathSeparators to Unix, to represent the hierarchy;
2. append root, because we need absolute pat;
3. remove ":" after drive letter.

Example:
"C:\Temp\SubDir" would become "/C/Temp/SubDir"

Have you considered any alternatives?

Don't implement FS mounts on Windows hosts, but it cuts out a feature, which actually works (or will work, because now one needs to build from sources to enable it).

Additional context

Extracted from the larger scope of #13006

[arixmkii]

Example how it was implemented for the demo mentioned above arixmkii@330c74c

[github-actions]

A friendly reminder that this issue had no activity for 30 days.

[arixmkii]

#17113 implementing this was approved, but the merge was delay to after 4.4.0 release time point. I can rebase it, if needed.