Use linux sendfile for PhysicalFile response

[maknapp]

Is your feature request related to a problem? Please describe.

I was expecting PhysicalFile to use the sendfile system call in linux but all it does is stream the file using this fallback:

https://github.com/aspnet/AspNetCore/blob/7ae6d1e3791e09db1fe2b032044208b04d4cc3b1/src/Http/Http/src/SendFileFallback.cs

This fallback uses much more CPU and memory to serve files than using the sendfile system call.

Describe the solution you'd like

I'd like an option to enable sendfile responses, either in setup or for each response.

[Tratcher]

Prior discussion here:
aspnet/KestrelHttpServer#1593
aspnet/KestrelHttpServer#1673

Summary:

• It wouldn't work with SSL
• The linux APIs are not async

[maknapp]

For reference, the linux kernel has TLS support (for this purpose): torvalds/linux@3c4d755 and
https://www.kernel.org/doc/html/latest/networking/tls.html

[Tratcher]

For reference, the linux kernel has TLS support (for this purpose): torvalds/linux@3c4d755 and
https://www.kernel.org/doc/html/latest/networking/tls.html

That would require re-designing Kestrel's TLS support to take advantage of.

[analogrelay]

For reference, the linux kernel has TLS support (for this purpose)

This would require a change in the runtime to support this directly on Socket.

• The linux APIs are not async

This is another main reason we don't plan to do this.

Closing as we don't plan to add this support.