I have changed my opinion of the “peek hack” from <https://bugs.python.org/issue3566#msg231413>. It would be useful when doing non-idempotent requests like POST, to avoid sending a request when we know it is going to fail. I looked into how to implement it so that it works for SSL (which does not support MSG_PEEK), and the neatest solution I could think of would require changing the non-blocking behaviour of BufferedReader.peek(), as described in Issue 13322. So I will leave that for later.

Adding ConnectionClosed.v3.patch; main changes:

* Removed the connection_reused flag to HTTPResponse
* ConnectionClosed raised even for the first request of a connection
* Added HTTPConnection.closed flag, which the user may check before a request to see if a fresh connection will be made, or an existing connection will be reused
* ConnectionClosed now subclasses both BadStatusLine and ConnectionError
* Fixed http.client.__all__ and added a somewhat automated test for it

BTW these patches kind of depend on Issue 5811 to confirm that BufferedReader.peek() will definitely return at least one byte unless at EOF.