// Copyright (C) 2016-2017 Luke Shumaker <lukeshu@sbcglobal.net>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package sd_login introspects session and login information from
// systemd, logind, and machined.
//
// There are 5 basic object types that these three system services
// manage and are introspected by this package.
//
// The machine manager maintains a list of "machines":
//
//  1. machine: A local container or virtual machine
//
// The host system, and machines hosted on it, may each run their own
// login manager, which keeps track of "seats", "sessions", and
// "users":
//
//  2. seat: A set of hardware devices for a workspace; i.e. a screen
//     and keyboard
//
//  3. session: A login session; tied to 0 or 1 seats (0 for things
//     like SSH login)
//
//  4. user: Keeps track of logged in users; that is, users with 1 or
//     more sessions
//
// Finally, sessions and users can be used to group together
// "processes", which are kept track of by cgroup manager:
//
//  5. process: A process may belong to a session, or directly to a
//     user if it is not part of a session.  This "belonging" is
//     separate accounting by the cgroup manager; it is NOT the same
//     as the EUID/RUID.
//
// How these relations look in the types implemented here:
//
//    ............................................
//    .                                          .
//    .                 +-------------+          .
//    .                 | MachineName |          .
//    .                 +-------------+          .
//    .                        ^                 .
//    .                        |                 .
//    .                        |GetMachine()     .
//    .                        |                 .
//    .                        ^                 .
//    .                  +-----------+           .
//    .                  | ProcessID |           .
//    .                  +-----------+           .
//    .                     v     v              .
//    .                     |     |              .
//    .     GetSession()    |     |   GetOwner() .
//    .            ,--------'     '-------,      .
//    .            |                      |      .
//    .            v                      v      .
//    .     +-------------+ GetUser() +--------+ .
//    .     | SessionName |>--------->| UserID | .
//    .     +-------------+           +--------+ .
//    .          v^^    ^ GetSessions() v        .
//    .          |||    `---------------'        .
//    .          |||                             .
//    . GetSeat()||GetSessions()                 .
//    .          |||GetActive()                  .
//    .          v^^                             .
//    .      +----------+                        .
//    .      | SeatName |                        .
//    .      +----------+                        .
//    .                                          .
//    ............................................
//
// Missing from the above diagram:
//
//  - As an optimization, SeatName.GetSessions() and .GetActive()
//    return both the session name, and the user ID that owns the
//    session.  This is equivalent to just calling .GetUser() on the
//    returned session.
//
//  - There is also a UserID.GetSeats() that is equivalent to calling
//    UserID.GetSessions(), and calling .GetSeat() on each returned
//    session.
//
// TODO: Better describe how machines fit in; the high-level
// description doesn't really match the interface exposed (as evident
// by a glance at the diagram).
//
// Finally, if you would like to perform some action whenever a
// machine, seat, session, or logged-in-user is added or removed,
// there is a Monitor that will help you avoid polling.
package sd_login