Stream Shutdown does not produce Error on incomplete protocol shutdown

[jens-diewald]

As originally mentioned in #56, Asio.SSL has error::stream_truncated.
This StackOverflow Post does explain very well what it is good for: https://stackoverflow.com/questions/25587403/boost-asio-ssl-async-shutdown-always-finishes-with-an-error
(I think, that stream_truncated did not exist when that post was written and corresponds to the mentioned SSL_R_SHORT_READ.)

I have added #74 to show the current behavior of Asio.SSL and WinTLS in the relevant cases.

Looking at the Schannel Docs, i suspect that the sspi_shutdown class may have to be more complex than it currently is..?

As a sidenote: The StackOverflow post mentions that Asio should produce error::eof during shutdown in some cases.
I did not observe that in my tests. However, this is handled in the beast https sync/async examples and in the WinTLS async example, butn not in the WinTLS sync example.
Could it be, that the information is outdated and error::eof will not occur anymore?
At the very least, it seems inconsistent, that error::eof is handled in the async example, but not in the sync example.

[jens-diewald]

The beast https server examples have a helpful comment on this topic. For example here.
In particular: When using with HTTP (or another "self-terminated" protocol) it is not necessary to wait for the peers close_notify.
For better performance (or simpler code?), many implementations just close the underlying transport after sending their close_notify, without waiting for the peer.
This also seems to be, what the current sspi_shutdown code in wintls is doing.
Although i do not quite understand why it is okay to always use InitializeSecurityContext in sspi_shutdown::operator() and not AcceptSecurityContext when running as a server, as the documentation clearly states. @laudrup, can you comment on that?

Altogether, i think it may be reasonable to keep the current shutdown functions as a more efficient variant to use with HTTP or other "self-terminated" protocols.
But as this library is supposed to be more general than that, the default shutdown functions should make sure that the shutdown procedure was completed properly. I could not find an existing implementation as a good reference, but according to the documentation, the shutdown should be implemented very similarly to the handshake with a loop similar to the one in stream::handshake(handshake_type, error_code&). Then, a new error code stream_truncated should be introduced analogously to asio::ssl and it should explicitly be set, when asio::error::eof occurs when trying to write our close_notify to the stream or when trying to read the peers close_notify from the stream.

[jens-diewald]

As for a good reference implementation i have found https://github.com/steffengy/schannel-rs, which is rust but otherwise similar to wintls.
Notably, they use one function for the initial handshake and also for the shutdown. (https://github.com/steffengy/schannel-rs/blob/e93c3bcf588630717bc592fe9f36fc4b740589b5/src/tls_stream.rs#L583)
I believe that this is generally a good idea and that a lot of code can be share between the initial handshake and the shutdown.
As a first effort towards this end, i have added #77 to simplify the handshake code.