2d canvas font code panics on default system font on HL2

[jdm]

From a log on a user's device:

RUST: ERROR - canvas::canvas_data - error getting font handle for style Font { font_family: FontFamily { families: FontFamilyList([FamilyName(FamilyName { name: Atom('Helvetica' type=dynamic), syntax: Identifiers }), FamilyName(FamilyName { name: Atom('Arial' type=inline), syntax: Identifiers }), Generic(SansSerif)]), is_system_font: false }, font_style: Normal, font_variant_caps: Normal, font_weight: FontWeight(700.0), font_size: FontSize { size: NonNegative(18.0 px), keyword_info: KeywordInfo { kw: None, factor: 1.0, offset: 0.0 px } }, font_stretch: FontStretch(NonNegative(Percentage(1.0))), hash: 7779769241958551862 }: no font found
\00thread 'CanvasThread' panicked at 'error getting font handle for default system font: NotFound', components\canvas\canvas_data.rs:1432:10

[jdm]

We'll need to make this optional and just ignore text operations when we can't find fonts to avoid panics like this; we should also figure out how to load the default fallback font on UWP as well.

[jdm]

Using https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Drawing_text, I see the "Hello world" text on desktop macOS and Windows nightlies, and a blank canvas in the UWP nightly. I'm going to try and fix that.

[jdm]

Ok, so the path we follow gets us to https://github.com/servo/font-kit/blob/efb64bcd3fdaf21a46a0edb086a4f157264a54ea/src/source.rs#L146 before we encounter any errors. We have a FontHandle that corresponds to the serif generic font (which is Times New Roman on windows), and the corresponding path we have is C:\WINDOWS\FONTS\TIMES.TTF.

Font::from_handle takes us to https://github.com/servo/font-kit/blob/3f5c183ecfbbba9af388307e495031c6c0b7cf47/src/loader.rs#L69, where we take the path branch and end up in https://github.com/servo/font-kit/blob/3f5c183ecfbbba9af388307e495031c6c0b7cf47/src/loader.rs#L62. At this point, we:

• open the path without any problems
• call Loader::from_file with the new opened font file
• this takes us here, where GetFinalPathNameByHandleW returns 0

It's not really clear to me why we open a file from a path, then try to determine the path to that file handle. I also need to figure out what the winapi says the actual underlying error is at this point, since the docs claim that's a valid function to call from UWP apps.

[jdm]

Ok, not super surprisingly, the underlying error is Io(Os { code: 5, kind: PermissionDenied, message: "Access is denied." }). It's interesting that we're allowed to open that file path but not not go from the file handle to the path.

[jdm]

#27184 will make the crash go away. I've submitted servo/font-kit#156 to fix the underlying problem, which will allow the actual text to appear when we update to a version of font-kit that includes that change.