LibPDF: Don't crash on fixed-width type 1 fonts that use /MissingWidth

Type 1 fonts usually have a m_font_program and no m_font -- they only
have m_font if we're using a replacement font for the fonts that
were built-in to PDFs before Acrobat 4.0 (and must still work to
show existing files).

However, SimpleFont::get_glyph_width() used to always return a
float, which in Type1Font was only implemented if m_font was set.

Per spec, we're supposed to just use /MissingWidth for fonts that
are missing an entry in the descriptor's /Width array. However, for
built-in fonts, no explicit /Width array is needed (PDF 1.7 spec,
Appendix H.3, 5.5.1). So if we just always use /MissingWidth,
then PDFs that use a built-in font draw all their text on top
of each other (e.g. 000333.pdf from stillhq.com-pdfdb).

So change get_glyph_width() to return Optional<float>, return
it only in Type1Font if m_font is set, and use MissingWidth
if it isn't set.

That way, replacement fonts still return a width, and real
fonts that are supposed to have /Width and use /MissingWidth
for missing entries do what they're supposed to too, instead
of crashing.

From 20 (6%) to 16 (5%) crashes on the 300 first PDFs, and from
39 (7.8%) to 31 (6.2%) on the 500-random PDFs test.

Userland/Libraries/LibPDF/Fonts/SimpleFont.cpp

@@ -53,8 +53,10 @@ PDFErrorOr<Gfx::FloatPoint> SimpleFont::draw_string(Gfx::Painter& painter, Gfx::
         float glyph_width;
         if (auto width = m_widths.get(char_code); width.has_value())
             glyph_width = font_size * width.value() / 1000.0f;
+        else if (auto width = get_glyph_width(char_code); width.has_value())
+            glyph_width = width.value();
         else
-            glyph_width = get_glyph_width(char_code);
+            glyph_width = m_missing_width;
 
         draw_glyph(painter, glyph_position, glyph_width, char_code, paint_color);
 

Userland/Libraries/LibPDF/Fonts/SimpleFont.h

@@ -17,7 +17,7 @@ public:
 
 protected:
     PDFErrorOr<void> initialize(Document* document, NonnullRefPtr<DictObject> const& dict, float font_size) override;
-    virtual float get_glyph_width(u8 char_code) const = 0;
+    virtual Optional<float> get_glyph_width(u8 char_code) const = 0;
     virtual void draw_glyph(Gfx::Painter& painter, Gfx::FloatPoint point, float width, u8 char_code, Color color) = 0;
     RefPtr<Encoding>& encoding() { return m_encoding; }
     RefPtr<Encoding> const& encoding() const { return m_encoding; }
@@ -26,7 +26,7 @@ private:
     RefPtr<Encoding> m_encoding;
     RefPtr<StreamObject> m_to_unicode;
     HashMap<u8, u16> m_widths;
-    u16 m_missing_width;
+    u16 m_missing_width { 0 };
 };
 
 }

Userland/Libraries/LibPDF/Fonts/TrueTypeFont.cpp

@@ -35,7 +35,7 @@ PDFErrorOr<void> TrueTypeFont::initialize(Document* document, NonnullRefPtr<Dict
     return {};
 }
 
-float TrueTypeFont::get_glyph_width(u8 char_code) const
+Optional<float> TrueTypeFont::get_glyph_width(u8 char_code) const
 {
     return m_font->glyph_width(char_code);
 }

Userland/Libraries/LibPDF/Fonts/TrueTypeFont.h

@@ -14,7 +14,7 @@ namespace PDF {
 
 class TrueTypeFont : public SimpleFont {
 public:
-    float get_glyph_width(u8 char_code) const override;
+    Optional<float> get_glyph_width(u8 char_code) const override;
     void set_font_size(float font_size) override;
     void draw_glyph(Gfx::Painter&, Gfx::FloatPoint, float, u8, Color) override;
     Type type() const override { return PDFFont::Type::TrueType; }

Userland/Libraries/LibPDF/Fonts/Type1Font.cpp

@@ -49,9 +49,11 @@ PDFErrorOr<void> Type1Font::initialize(Document* document, NonnullRefPtr<DictObj
     return {};
 }
 
-float Type1Font::get_glyph_width(u8 char_code) const
+Optional<float> Type1Font::get_glyph_width(u8 char_code) const
 {
-    return m_font->glyph_width(char_code);
+    if (m_font)
+        return m_font->glyph_width(char_code);
+    return OptionalNone {};
 }
 
 void Type1Font::set_font_size(float font_size)

Userland/Libraries/LibPDF/Fonts/Type1Font.h

@@ -22,7 +22,7 @@ struct Type1GlyphCacheKey {
 
 class Type1Font : public SimpleFont {
 public:
-    float get_glyph_width(u8 char_code) const override;
+    Optional<float> get_glyph_width(u8 char_code) const override;
     void set_font_size(float font_size) override;
     void draw_glyph(Gfx::Painter& painter, Gfx::FloatPoint point, float width, u8 char_code, Color color) override;
     Type type() const override { return PDFFont::Type::Type1; }