Should S2N gracefully close connect on a user_canceled (90) error ?

[zz85]

Problem:

S2N_TLS_ALERT_CLOSE_NOTIFY currently marks a connection as close. should S2N_TLS_ALERT_USER_CANCELED do something similar?

Relates to #2213

Solution:

Requirements / Acceptance Criteria:

   user_canceled
      This handshake is being canceled for some reason unrelated to a
      protocol failure.  If the user cancels an operation after the
      handshake is complete, just closing the connection by sending a
      close_notify is more appropriate.  This alert should be followed
      by a close_notify.  This message is generally a warning.

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: Links to relevant RFC(s)
• Related Issues: Link any relevant issues
• Will the Usage Guide or other documentation need to be updated?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Will this change trigger SAW changes? Changes to the state machine, the s2n_handshake_io code that controls state transitions, the DRBG, or the corking/uncorking logic could trigger SAW failures.
  • Should this change be fuzz tested? Will it handle untrusted input? Create a separate issue to track the fuzzing work.

Out of scope:

Is there anything the solution will intentionally NOT address?