Add boundary check to HarfBuzzTextShaper (#20716)

* feat: add boundary check.

* feat: add empty string handling and unit tests for HarfBuzzTextShaper

src/HarfBuzz/Avalonia.HarfBuzz/HarfBuzzTextShaper.cs

@@ -24,6 +24,9 @@ namespace Avalonia.Harfbuzz
         {
             var textSpan = text.Span;
 
+            if (text.Length == 0)
+                return new ShapedBuffer(text, 0, options.GlyphTypeface, options.FontRenderingEmSize, options.BidiLevel);
+
             var glyphTypeface = options.GlyphTypeface;
 
             if (glyphTypeface.TextShaperTypeface is not HarfBuzzTypeface harfBuzzTypeface)
@@ -121,6 +124,8 @@ namespace Avalonia.Harfbuzz
         {
             var length = buffer.Length;
 
+            if (length == 0) return;
+
             var glyphInfos = buffer.GetGlyphInfoSpan();
 
             var second = glyphInfos[length - 1];

tests/Avalonia.Base.UnitTests/Media/Fonts/Tables/HeadTableTests.cs

@@ -191,8 +191,9 @@ namespace Avalonia.Base.UnitTests.Media.Fonts.Tables
             Assert.True(HeadTable.TryLoad(typeface, out var headTable));
             Assert.Equal(FontDirectionHint.LeftToRightWithNeutrals, headTable.FontDirectionHint);
         }
+    }
     
-        private class CustomPlatformTypeface : IPlatformTypeface
+    internal class CustomPlatformTypeface : IPlatformTypeface
     {
         private readonly UnmanagedFontMemory _fontMemory;
 
@@ -253,4 +254,3 @@ namespace Avalonia.Base.UnitTests.Media.Fonts.Tables
         public bool TryGetTable(OpenTypeTag tag, out ReadOnlyMemory<byte> table) => _fontMemory.TryGetTable(tag, out table);
     }
 }
-}

tests/Avalonia.Base.UnitTests/Media/TextFormatting/HarfbuzzTextShaperTests.cs

@@ -0,0 +1,120 @@
+using System;
+using System.Linq;
+using Avalonia.Base.UnitTests.Media.Fonts.Tables;
+using Avalonia.Harfbuzz;
+using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Avalonia.Platform;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Media.TextFormatting;
+
+public class HarfBuzzTextShaperTests
+{
+    private static readonly string s_InterFontUri =
+        "resm:Avalonia.Base.UnitTests.Assets.Inter-Regular.ttf?assembly=Avalonia.Base.UnitTests";
+
+    private readonly HarfBuzzTextShaper _shaper;
+    
+    private TestServices Services => TestServices.MockThreadingInterface.With(
+        textShaperImpl: _shaper);
+
+    public HarfBuzzTextShaperTests()
+    {
+        _shaper = new HarfBuzzTextShaper();
+    }
+
+    [Fact]
+    public void ShapeText_WithValidInput_ReturnsShapedBuffer()
+    {
+        using (UnitTestApplication.Start(Services))
+        {
+            var text = "Hello World".AsMemory();
+            var options = CreateTextShaperOptions();
+            
+            var result = _shaper.ShapeText(text, options);
+            
+            Assert.NotNull(result);
+            Assert.Equal(text.Length, result.Length);
+        }
+    }
+
+    [Fact]
+    public void ShapeText_WithEmptyString_ReturnsEmptyShapedBuffer()
+    {
+        using (UnitTestApplication.Start(Services))
+        {
+            var text = "".AsMemory();
+            var options = CreateTextShaperOptions();
+            
+            var result = _shaper.ShapeText(text, options);
+            
+            Assert.NotNull(result);
+            Assert.Equal(0, result.Length);
+        }
+    }
+
+    [Fact]
+    public void ShapeText_WithTabCharacter_ReplacesWithSpace()
+    {
+        using (UnitTestApplication.Start(Services))
+        {
+            var text = "Hello\tWorld".AsMemory();
+            var options = CreateTextShaperOptions();
+            
+            var result = _shaper.ShapeText(text, options);
+            
+            Assert.NotNull(result);
+            Assert.True(result.Length == 11);
+        }
+    }
+
+    [Fact]
+    public void ShapeText_WithCRLF_MergesBreakPair()
+    {
+        using (UnitTestApplication.Start(Services))
+        {
+            var text = "Line1\r\nLine2".AsMemory();
+            var options = CreateTextShaperOptions();
+            
+            var result = _shaper.ShapeText(text, options);
+            
+            Assert.NotNull(result);
+            Assert.NotEqual(0.0, result[5].GlyphAdvance);
+        }
+    }
+    
+    [Fact]
+    public void ShapeText_EndWithCRLF_MergesBreakPair()
+    {
+        using (UnitTestApplication.Start(Services))
+        {
+            var text = "Line1\r\n".AsMemory();
+            var options = CreateTextShaperOptions();
+            
+            var result = _shaper.ShapeText(text, options);
+            
+            Assert.NotNull(result);
+            Assert.Equal(0.0, result[5].GlyphAdvance);
+        }
+    }
+
+    private TextShaperOptions CreateTextShaperOptions(
+        sbyte bidiLevel = 0,
+        double letterSpacing = 0,
+        double fontSize = 16)
+    {
+        var assetLoader = new StandardAssetLoader();
+
+        using var stream = assetLoader.Open(new Uri(s_InterFontUri));
+
+        var typeface = new GlyphTypeface(new CustomPlatformTypeface(stream));
+
+        return new TextShaperOptions(
+            typeface,
+            fontSize,
+            bidiLevel,
+            letterSpacing: letterSpacing);
+    }
+}