Race-condition when writing 32-bit png images with text in parallel.

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

ImageSharp version

3.0.0-alpha.0.73

Other ImageSharp packages and versions

SixLabors.ImageSharp.Drawing: 1.0.0-beta15.3, SixLabors.Fonts: 1.0.0-beta19.5

Environment (Operating system, version and so on)

Windows 10, 22H2, Build 19045.2486

.NET Framework version

.NET 7

Description

In unit test I experienced that when running test locally, the text in images would sometimes move a tiny bit, but only when running the tests in parallel, which made me think there was a race-condition somewhere.

Below I have created the smallest reproducing example I could come up with and commented with what changes removes what looks like a race-condition to me.

To repeat to conditions here:

• Running in parallel
• Using Rgba32
• Writing some text with a space and some character, e.g. " A".
• Saving to gif, tga, or png.

Steps to Reproduce

using SixLabors.Fonts;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Drawing.Processing;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Processing;
using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

// Works for: single thread
// Fails for: two threads
const int NumberOfThreads = 2;

Parallel.ForEach(Enumerable.Range(0, NumberOfThreads), _ =>
{
    var imageLength0 = Generate();
    var imageLength1 = Generate();

    if (imageLength0 != imageLength1)
        throw new InvalidOperationException($"Expected {imageLength0} to equal {imageLength1}");
});

static long Generate()
{
    using var memoryStream = new MemoryStream();

    // Works for: Rgb24
    // Fails for: Rgba32
    using var image = new Image<Rgba32>(30, 15);

    // Works for: " ", "  ", "A" or "AA"
    // Fails for: " A", "A "
    const string Text = " A";
    image.Mutate(x => x.DrawText(Text, SystemFonts.CreateFont("Arial", 15), Color.Black, new PointF(0, 0)));

    // Works for bmp, jpeg
    // Fails for png, tga, gif
    image.SaveAsPng(memoryStream);
    return memoryStream.Length;
}

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net7.0</TargetFramework>
    </PropertyGroup>
    <ItemGroup>
        <PackageReference Include="SixLabors.ImageSharp" Version="3.0.0-alpha.0.73" />
        <PackageReference Include="SixLabors.ImageSharp.Drawing" Version="1.0.0-beta15.3" />
        <PackageReference Include="SixLabors.Fonts" Version="1.0.0-beta19.5" />
    </ItemGroup>
</Project>

Images

No response

[jnyrup]

Some more findings.

Removing ascender -= metric.TopSideBearing * scaleY; from https://github.com/SixLabors/Fonts/commit/39773e680338b229cc285b52a3ba97ef9bf63182 fixes my problem and all SixLabor.Fonts test at that commit pass.

Similarly removing ascender -= tsbOffset * scaleY; from current main fixes my problem and all SixLabor.Fonts test at that commit pass.

[jnyrup]

Sure! See the difference in the amount of vertical space above the text.