Fix Windows path detection in cleanPathWithBase

[owenthereal]

Summary

Fix cleanPathWithBase to use filepath.IsAbs on the original path before converting to forward slashes. This correctly handles Windows paths like C:\foo which filepath.IsAbs recognizes but path.IsAbs (after ToSlash conversion to C:/foo) does not.

Problem

When using WithStartDirectory on Windows, absolute paths like C:\Users\foo are incorrectly treated as relative paths because:

1. filepath.ToSlash(filepath.Clean("C:\\Users\\foo")) → C:/Users/foo
2. path.IsAbs("C:/Users/foo") → false (only checks for / prefix)
3. Result: path gets joined with start directory, causing doubled paths

Solution

Check filepath.IsAbs(p) on the original path before the ToSlash conversion. filepath.IsAbs is platform-aware and correctly identifies Windows absolute paths.

Related

This fix resolves Windows SFTP path handling issues in owenthereal/upterm#440

[puellanivis]

Remote paths to windows servers require absolute paths to be encoded as /C:/path/to/file.

This is not a bug.

[puellanivis]

I keep thinking I’m missing something here.

I’m thinking I need this explained better. This code is only called from within the RequestServer, and only on paths coming from a remote client. Where, like I said, any absolute path coming from a client to a windows server must be encoded as /c:/path/to/file, but I can’t shake the feeling that I’m missing something that I should be able to see…