Bug 252668 – Calling FontCascade::glyphDataForCharacter triggers unexpected change in font loading behavior

• Component: Layout and Rendering
• Status: NEW

[zalan]

fast/text/font-promises-gc.html
imported/w3c/web-platform-tests/css/css-fonts/fallback-remote-to-data-url.html
imported/w3c/web-platform-tests/css/css-fonts/font-display/font-display-failure-fallback.html

they fail/pass depending on whether we issue a call to FontCascade::glyphDataForCharacter in RenderText::styleDidChange (see canUseSimplifiedTextMeasuring) or not.

[zalan]

it looks like imported/w3c/web-platform-tests/css/css-fonts/font-size-adjust-012.html also sensitive to this "first glyph call" (fails in WKTR, works fine when loading in MiniBrowser)

[Radar WebKit Bug Importer]

<rdar://problem/106033917>

[zalan]

Created attachment 465475 [details]
Early access

Running imported/w3c/web-platform-tests/css/css-fonts/font-size-adjust-012.html with glyph data being access at tree construction time.

[zalan]

Created attachment 465476 [details]
Delayed access

Running imported/w3c/web-platform-tests/css/css-fonts/font-size-adjust-012.html with glyph data being access right before layout.

[zalan]

Created attachment 465477 [details]
WebInspector timelines

[zalan]

Timeline recording shows that 
1. reading glyph data triggers font loading
2. delayed reading may cause intermediate rendering with blank glyphs

[zalan]

stacktrace for font loading (initiated by accessing glyph data)
 
  * frame #0: 0x000000014af5130c WebCore`WebCore::DocumentFontLoader::beginLoadingFontSoon(this=0x000000010f0f12b0, font=0x000000010f06df00) at DocumentFontLoader.cpp:66:9
    frame #1: 0x000000014af51284 WebCore`WebCore::Document::beginLoadingFontSoon(this={ origin = , url = , inMainFrame = Detached, backForwardCacheState = NotInBackForwardCache }, request=0x000000010f0f39c0) at Document.cpp:3203:19
    frame #2: 0x000000014aa9545c WebCore`WebCore::CSSFontFaceSource::load(this=0x000000010f126c00, document={ origin = , url = , inMainFrame = Detached, backForwardCacheState = None }) at CSSFontFaceSource.cpp:159:22
    frame #3: 0x000000014aa58260 WebCore`WebCore::CSSFontFace::pump(this=0x000000010f052220, policy=Allow) at CSSFontFace.cpp:644:25
    frame #4: 0x000000014aa59b34 WebCore`WebCore::CSSFontFace::font(this=0x000000010f052220, fontDescription=0x000000010f0cf628, syntheticBold=false, syntheticItalic=false, policy=Allow, fontPaletteValues=0x000000010f0cf6d0, fontFeatureValues=RefPtr<WebCore::FontFeatureValues, WTF::RawPtrTraits<WebCore::FontFeatureValues>, WTF::DefaultRefDerefTraits<WebCore::FontFeatureValues> > @ 0x000000016f931780) at CSSFontFace.cpp:708:25
    frame #5: 0x000000014ab08f34 WebCore`WebCore::CSSFontAccessor::font(this=0x000000010f0cf600, policy=Allow) const at CSSSegmentedFontFace.cpp:72:45
    frame #6: 0x000000014c3275ec WebCore`WebCore::FontRanges::Range::font(this=0x000000010f0a9528, policy=Allow) const at FontRanges.cpp:38:28
    frame #7: 0x000000014c2fa8d8 WebCore`WebCore::glyphPageFromFontRanges(pageNumber=7, fontRanges=0x000000010f0a9518) at FontCascadeFonts.cpp:465:26
    frame #8: 0x000000014c2f1290 WebCore`WebCore::FontCascadeFonts::glyphDataForCharacter(this=0x000000010f0a9500, c=120, description=0x000000010f0a23b0, variant=NormalVariant) at FontCascadeFonts.cpp:507:38
    frame #9: 0x000000014c2f0fb4 WebCore`WebCore::FontCascade::glyphDataForCharacter(this=0x000000010f0a23a0, c=120, mirror=false, variant=NormalVariant) const at FontCascade.cpp:333:21
    frame #10: 0x000000014cb6a9d0 WebCore`WebCore::initiateFontLoadingByAccessingGlyphData(textContent={ length = 1, contents = 'x' }, fontCascade=0x000000010f0a23a0) at RenderText.cpp:256:21
    frame #11: 0x000000014cb6a74c WebCore`WebCore::RenderText::styleDidChange(this=0x0000000111003af0, diff=Equal, oldStyle=0x0000000000000000) at RenderText.cpp:272:9
    frame #12: 0x000000014c9b1158 WebCore`WebCore::RenderElement::didAttachChild(this=0x0000000111003400, child=0x0000000111003af0, (null)=0x0000000000000000) at RenderElement.cpp:547:37
    frame #13: 0x000000014cd5fdac WebCore`WebCore::RenderTreeBuilder::attachToRenderElement(this=0x000000016f934c38, parent=0x0000000111003400, child=nullptr, beforeChild=0x0000000000000000) at RenderTreeBuilder.cpp:421:12
    frame #14: 0x000000014cd60144 WebCore`WebCore::RenderTreeBuilder::Inline::attachIgnoringContinuation(this=0x000000010f149910, parent=0x0000000111003400, child=nullptr, beforeChild=0x0000000000000000) at RenderTreeBuilderInline.cpp:194:15
    frame #15: 0x000000014cd5fbb8 WebCore`WebCore::RenderTreeBuilder::Inline::attach(this=0x000000010f149910, parent=0x0000000111003400, child=nullptr, beforeChild=0x0000000000000000) at RenderTreeBuilderInline.cpp:117:5
    frame #16: 0x000000014cd5edd4 WebCore`WebCore::RenderTreeBuilder::attachInternal(this=0x000000016f934c38, parent=0x0000000111003400, child=nullptr, beforeChild=0x0000000000000000) at RenderTreeBuilder.cpp:330:25
    frame #17: 0x000000014cd5e220 WebCore`WebCore::RenderTreeBuilder::attach(this=0x000000016f934c38, parent=0x0000000111003400, child=nullptr, beforeChild=0x0000000000000000) at RenderTreeBuilder.cpp:179:5
    frame #18: 0x000000014cd7c600 WebCore`WebCore::RenderTreeUpdater::createTextRenderer(this=0x000000016f934c10, textNode=0x0000000111009800, textUpdate=0x0000000000000000) at RenderTreeUpdater.cpp:504:15