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- read&write must only raise EOFError
- read must raise if not all data read

was also emitted when the state did not actually change

see: https://github.com/cherrypy/cherrypy/issues/1625

this fixes a problem when the session was reset and the poll socket url
was redirected to the login page which was then served as the "response"
to the ajax callback which caused infinit poll socket reloads without
delay - this is one of the causes of high server load