When working with a MainLoop, if the file reaches the EOF it will
immediately fire the read handle callback. We cannot readily determine
if the file is at EOF or if the file is pointing to a socket/pipe that
has consumed all the current data in the buffer but the remote end has
not yet hung up. This is causing JSONTransport to continuously fire the
OnRead callback trigging repeated calls to the
`MessageHandler::OnClose`.
Since MainLoop does not perform the actual read, we need to adjust the
behavior of JSONTransport to fully own the read handle.
This change moves the ownership of the `MainLoop::ReadHandleUP` and
additionally own a reference to the `MainLoop` itself to ensure the loop
outlives the JSONTransport object.
This allows us to remove the handle immediately when we detect an EOF /
hang up has occurred.
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