feat: implement semantically correct handling of shell/pty sessions across platforms

[mohammed90]

This PR resolves 3 issues:

1. The issue was recognized when a friend reported scp and rsync not working. I found the culprit to be not hooking up the session's/channel's I/O to spawned process, otherwise the new process assumed the null devices as its std{in,out,err}. In other words, the newly created process was not reading/writing to/from the client's shell, rather from, e.g., secondary tty device (pair of the pty device) ¹. Thus the I/O was not channeled from the client to the process, rather to the PTY session. The PR hooks the client's I/O directly as the newly spawned process' std{in,out}. This means when issuing a command which expects to read bytes from stdin, the stdin of the remote process is fed from the local client, not the remote tty. Now scp et. al. can work.

2. If you're on macOS, go look for your user in /etc/passwd. You will not find it. macOS uses Open Directory² to manage the users, so the users' {meta,}data are stored in the DirectoryService(8) database. In Go, implementation of user lookup in os/user pkg uses the native function getpwnam_r with cgo; but only parses /etc/passwd otherwise, except for lookups of current user which goes through syscall. Lookups return empty results for other users (see os/user: LookupUser() doesn't find users on macOS when compiled with CGO_ENABLED=0 golang/go#24383). To avoid cgo and still maintain robust and comprehensive offering, I had to shell out to dscacheutil on macOS to obtain full user details and parse /etc/passwd on other *nix platforms.

3. Change the Shell field in the Shell struct to ForceCommand, and use the opportunity to line-up the semantics to match OpenSSH behavior. The ForceCommand, if set, is executed using the user's shell, regardless of the command sent along the connection. The designated shell is always the user's default shell, which on *nix systems typically found in /etc/passwd and $SHELL.

TODO:

• update README examples

Footnotes

1. https://dev.to/napicella/linux-terminals-tty-pty-and-shell-192e ↩

2. https://en.wikipedia.org/wiki/Apple_Open_Directory ↩