[tor-bugs] #33144 [Circumvention/Snowflake]: Overhaul websocketconn.Conn; implement net.Conn

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#33144: Overhaul websocketconn.Conn; implement net.Conn
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     Reporter:  dcf                      |      Owner:  dcf
         Type:  enhancement              |     Status:  assigned
     Priority:  Medium                   |  Milestone:
    Component:  Circumvention/Snowflake  |    Version:
     Severity:  Normal                   |   Keywords:
Actual Points:                           |  Parent ID:
       Points:                           |   Reviewer:
      Sponsor:                           |
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 In the turbotunnel branch I found it convenient to have
 `websocketconn.Conn` implement `net.Conn`, not only `io.ReadWriteCloser`.
 While implementing the additional methods, I found some ways that
 `websocketconn.Conn`'s existing methods do not satisfy the requirements of
 `net.Conn`:
  * there are data races when multiple goroutines call `Read` or `Write`.
    * this is partly because `Read` assigns to the shared field `conn.r`,
 and partly because [https://godoc.org/github.com/gorilla/websocket#hdr-
 Concurrency the underlying websocket.Conn doesn't permit concurrent
 access].
  * when a websocketconn.Conn is closed, it starts returning some kind of
 [https://godoc.org/github.com/gorilla/websocket#CloseError
 websocket.CloseError], not `io.EOF`.

 This branch
  1. adds tests that expose the above issues
  2. rewrites `websocketconn.Conn` to serialize `Read`s and `Write`s using
 my favorite `io.Pipe`-with-goroutine trick
  3. implements the remaining `net.Conn` methods

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33144>
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