[tor-bugs] #29125 [- Select a component]: Make websocket server tolerant of HTTP/2

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#29125: Make websocket server tolerant of HTTP/2
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     Reporter:  dcf                   |      Owner:  (none)
         Type:  defect                |     Status:  new
     Priority:  Medium                |  Milestone:
    Component:  - Select a component  |    Version:
     Severity:  Normal                |   Keywords:
Actual Points:                        |  Parent ID:
       Points:                        |   Reviewer:
      Sponsor:                        |
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 The Snowflake websocket server, inherited from flash proxy,
 [https://gitweb.torproject.org/pluggable-
 transports/websocket.git/tree/websocket/websocket.go?h=0.3#n326 assumes]
 that the incoming http.ResponseWriter implements the
 [https://golang.org/pkg/net/http/#Hijacker http.Hijacker] interface, which
 HTTP/2 connections do not.

 It causes a panic in the log when you connect to the server with e.g.
 curl:
 {{{
 2019/01/18 18:56:29 http2: panic serving X.X.X.X:YYYY: interface
 conversion: *http2.responseWriter is not http.Hijacker: missing method
 Hijack
 }}}

 I'm not sure if WebSocket connections from browsers use HTTP/2 yet or not,
 but in any case we should handle it gracefully.

 My first inclination is to see if porting to
 [https://godoc.org/github.com/gorilla/websocket gorilla/websocket] fixes
 this and #28726.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29125>
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