Async IO methods should use `FileOptions.Asynchronous`

[Neme12]

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

3.0.1

Environment (Operating system, version and so on)

Windows 11 Version 22H2

.NET Framework version

.NET 7.0.304

Description

Async methods that read/write from the file system, such as Image.LoadAsync and Image.SaveAsync, use async methods on FileStream but don't instantiate the FileStream with FileOptions.Asynchronous, as should be done when using async operations. This means that the implementation isn't actually truly async and instead uses synchronous IO on a new thread, needlessly using more resources.

There is a simple workaround for this by creating the FileStream manually and giving ImageSharp the stream as opposed to the file path, e.g.:

await using (var fileStream = new FileStream(path, new FileStreamOptions
{
    Mode = FileMode.Open,
    Access = FileAccess.Read,
    Share = FileShare.Read,
    Options = FileOptions.Asynchronous,
}))
using (var image = await Image.LoadAsync(fileStream))
{
// ...
}

but I feel like this is a bug and should be fixed in ImageSharp itself. The method is misleading, suggesting that it uses async IO while actually doesn't.

[antonfirsov]

This makes sense, here is what BCL is doing for File.ReadAllTextAsync:

https://github.com/dotnet/runtime/blob/2f208128f257b2390a8edefa0a19e0eaad6bd419/src/libraries/System.Private.CoreLib/src/System/IO/File.cs#L853-L856

I guess SequentialScan would make things worse in our case, because BufferedReadStream does Seek a lot.

@Neme12 interested creating a PR?

[Neme12]

Sure, I can give it a try.