sshpeer: defer pipe buffering and stderr sidechannel binding The doublepipe and bufferedinputpipe types facilitate polling multiple pipes without blocking and for automatically forwarding output from the SSH server's stderr pipe to the ui as "remote: " output. This all happens automatically and callers don't need to worry about reading from multiple pipes. An upcoming change to version 2 of the SSH wire protocol will eliminate the use of stderr and move side-channel output into the "main" pipe. The SSH wire protocol will use a pair of unidirectional pipes - just like the HTTP protocol. In this future world, the doublepipe primitive isn't necessary because the stderr pipe won't be used. To prepare for eventually not using doublepipe, we delay the construction of this primitive from immediately after connection establishment to inside construction of the peer instance. The handshake occurs between these two events. So we had to teach the handshake code to read from stderr so any stderr output from the server is still attended to early in the connection lifetime. Differential Revision: https://phab.mercurial-scm.org/D2383