Windows: fonts on shared folders can't be loaded by DirectWrite/WPF (Z: not registered with Mount Manager) #51

Description

In a Windows guest, fonts stored on a VMPal shared folder (Z:) can't be loaded by DirectWrite, so WPF apps that ship their own font files can't use them when run from the share. Text falls back to a generic font, and any element that references the font file directly (e.g. WPF Glyphs) throws an unhandled FileNotFoundException and crashes the app. The same files copied to C:\ work fine.

The cause appears to be that the VirtIO-FS volume isn't registered with the Windows Mount Manager. GetFinalPathNameByHandle fails with ERROR_UNRECOGNIZED_VOLUME (1005) for every file on Z: unless VOLUME_NAME_NT is requested, and DirectWrite's local font file loader depends on that call. Ordinary reads and memory-mapping of the same files succeed.

Steps to Reproduce

1. Share a Mac folder with a Windows 11 VM (it mounts as Z:) and put any .ttf in it, e.g. Z:\Fonts\Roboto-Regular.ttf.
2. In the guest, open PowerShell (powershell.exe -STA) and run:

   Add-Type -AssemblyName PresentationCore
   New-Object System.Windows.Media.GlyphTypeface([Uri]'Z:\Fonts\Roboto-Regular.ttf')

3. Copy the same file to C:\Temp\ and run the same command against that path.
4. Optionally run mountvol and look for Z:.

Current Behavior

• Step 2 fails with FileNotFoundException: Unable to find the specified file. Calling DirectWrite directly, IDWriteFactory::CreateFontFileReference succeeds but IDWriteFontFile::Analyze returns 0x88985003 (DWRITE_E_FILENOTFOUND).

• Step 3 succeeds.

• mountvol doesn't list Z:, and GetVolumeNameForVolumeMountPoint("Z:\") fails with error 1005.

• GetFinalPathNameByHandle on an open handle to a file on Z:

  • VOLUME_NAME_DOS: error 1005
  • VOLUME_NAME_GUID: error 1005
  • VOLUME_NAME_NT: \Device\Volume{…}\Fonts\Roboto-Regular.ttf

  For a file on C:, all three return a path.

In a real WPF app (.NET 10, run from Z:), the crash looks like this in the Application event log:

System.IO.FileNotFoundException: Unable to find the specified file.
   at MS.Internal.Text.TextInterface.DWriteUtil.ConvertHresultToException(Int32 hr)
   at MS.Internal.Text.TextInterface.Factory.CreateFontFace(Uri filePathUri, UInt32 faceIndex, FontSimulations fontSimulationFlags)
   at System.Windows.Media.GlyphTypeface.Initialize(Uri typefaceSource, StyleSimulations styleSimulations)
   at System.Windows.Documents.Glyphs.ParseGlyphRunProperties()
   ...

Expected Behavior

Files on a shared folder should behave like files on a local disk for path-resolution APIs, so DirectWrite and WPF can load fonts from them. Registering the volume with the Mount Manager would probably fix it; WinFsp does this when the mount point is given as \\.\Z: rather than Z:. virtiofs.exe currently accepts only X: or *, and the VMPal Shared Folders service starts it without options, so there's no way to change this from inside the guest.

Environment

• App version: VMPal 0.80, VMPal Tools 0.54 (also seen on the previous version)
• Mac: MacBook Air, Apple M5, 32 GB
• macOS version: macOS 27.0.1 (26A434)
• VM's OS: Windows 11 24H2 ARM64 (build 26100.4349); WinFsp 2025 (2.1.25156); Z: reported as a fixed disk (DriveType 3), file system VirtIO-FS

Log

No VMPal crash. The guest app crash is shown above.