Fix broken pipe when closing UNIX Socket in HTTP::Server

[j8r]

Solves #15886.

I noticed removing that line removes the issue altogether: https://github.com/crystal-lang/crystal/blob/180c4bc/src/http/server.cr#L507C10-L508C1.
I don't know if that's an option too, or putting sync = true after the handler has been called like

@handler.call(context)
if input.is_a?(IO::Buffered)
  io.sync = true
end

The latter is what has been done in this PR.

[ysbaddaden]

IO#sync= also flushes the buffer, so I don't see how the behavior could be different from flushing in IO#close 😕

Worse, if flush raises in IO#sync= then the socket won't be closed until collected by the GC, while IO#close makes sure that it is closed now.

[j8r]

What I know is setting IO#sync = true prevents the error from happening, maybe the flush is done differently compared to IO#close?
Why there is in the first place at https://github.com/crystal-lang/crystal/blob/180c4bc/src/http/server.cr#L507C10-L508C1

if io.is_a?(IO::Buffered)
    io.sync = false
end

Any better fix is welcomed, I'm not sure what's the best way to solve the issue.

[ysbaddaden]

Oh, this is flushing the socket buffer (output) before flushing the response buffer (response.output) to the socket buffer, and then flushing the socket buffer again :/

If there's anything to flush, then I guess you'll still get an exception at some point.

[j8r]

By the way if this can help, when using sockets input and output point to the same IO.