'dotnet new android' produces invalid images

[jonathanpeppers]

Our templates: microsoft.android.templates.30.0.100-preview.6.56.nupkg.zip

If you dotnet new android you get a broken image that is coincidentally ~8kb:

This broken image causes dotnet build to fail when calling the underlying Android tooling:

Resources\mipmap-xxxhdpi\ic_launcher_round.png error APT2000: failed reading from input: PNG chunk type 49444154 is too large: chunk length is 10836 but chunk starts at byte 45/8187.

If I unzip this file myself (with a tool like 7-zip), it is actually larger than ~8kb:

I suspect if a template has a file larger than ~8kb you'll hit this issue.

This started happening in Maestro bumps here:

• [main] Update dependencies from dotnet/installer xamarin/xamarin-android#6029
• [release/6.0.1xx-preview6] Update dependencies from dotnet/installer xamarin/xamarin-android#6034

So likely the issue started somewhere bumping .NET 6.0.100-preview.6.21313.2 to 6.0.100-preview.6.21321.13.

[vlada-shubina]

@jonathanpeppers I tried to reproduce this bug, but was not able two - the files in template and in generated output are identical.
We were not touching core template engine (the assembly that generates the files) since preview 5. Anything else that can affect this?

In general I would recommend to use "copy-only" mode for files without replacements (see the link). This will avoid any file modification, the file will be copied from template content, and also should be faster as template engine will not scan the file content for variables.

---

Update: I was able to reproduce the issue, however only using dotnet/installer to install dotnet new. We should look on what has been changed in dotnet-sdk or dotnet/installer in these timeframe which started causing the issue.

[jonathanpeppers]

I can confirm adding this to our template.json works around the issue:

  "sources": [
      {
        "source": "./",
        "target": "./",
        "copyOnly": "**/Resources/**/*.png"
      }
  ],

I'll set this up on any iOS/Android templates for .png files, thanks. It should improve performance as well.

[vlada-shubina]

The cause of the issue is: dotnet/runtime#53502, dotnet/docs#24649.
It affects only .NET core running on runtime later then .NET 6 preview 6, it does not affect full framework versions.
We will need to rewrite portions of the code in TemplateEngine.Core using stream.Read in a wrong way (about 20 occurrences).
This is tentatively scheduled for preview 7, might slip to rc1.
For those affected, please consider to use copyOnly (see #3325 (comment)) in case the files should be just copied to target folder without any modifications, especially in case of binary files; and see if it solves the problem.

It is strongly recommended to use copyOnly for all binary files in your templates (even without the context of the bug) as template engine is scanning files byte wise for making replacements. In case of the match the byte content will be replaced even in binary files, and this leads to corrupted file after template generation. Using copyOnly in your template for the files that should be copied without modifications also improves performance when instantiating the template.