http3: add ConnContext to the server (#4230)

* Add ConnContext to http3.Server

ConnContext can be used to modify the context used by a new http
Request.

* Make linter happy

* Add nil check and integration test

* Add the ServerContextKey check to the ConnContext func

* Update integrationtests/self/http_test.go

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* Update http3/server.go

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

http3/server.go

@@ -211,6 +211,11 @@ type Server struct {
 	// In that case, the stream type will not be set.
 	UniStreamHijacker func(StreamType, quic.Connection, quic.ReceiveStream, error) (hijacked bool)
 
+	// ConnContext optionally specifies a function that modifies
+	// the context used for a new connection c. The provided ctx
+	// has a ServerContextKey value.
+	ConnContext func(ctx context.Context, c quic.Connection) context.Context
+
 	mutex     sync.RWMutex
 	listeners map[*QUICEarlyListener]listenerInfo
 
@@ -610,6 +615,12 @@ func (s *Server) handleRequest(conn quic.Connection, str quic.Stream, decoder *q
 	ctx = context.WithValue(ctx, ServerContextKey, s)
 	ctx = context.WithValue(ctx, http.LocalAddrContextKey, conn.LocalAddr())
 	ctx = context.WithValue(ctx, RemoteAddrContextKey, conn.RemoteAddr())
+	if s.ConnContext != nil {
+		ctx = s.ConnContext(ctx, conn)
+		if ctx == nil {
+			panic("http3: ConnContext returned nil")
+		}
+	}
 	req = req.WithContext(ctx)
 	r := newResponseWriter(str, conn, s.logger)
 	if req.Method == http.MethodHead {