sd_login: work on it

17 files changed, 283 insertions, 475 deletions

diff --git a/readme.go b/readme.go
index 269e504..080c367 100644
--- a/readme.go
+++ b/readme.go
@@ -25,7 +25,7 @@
 //     sd_event    | N/A    | Not implemented; not nescessary in Go.
 //     sd_id128    | DONE   | Package sd_id128 reads, generates, and processes 128-bit ID values.
 //     sd_journal  | TODO   | TODO.
-//     sd_login    | TODO   | TODO.
+//     sd_login    | WIP    | Package sd_login TODO.
 //     sd_messages | DONE   | Package sd_messages is a table of constant message IDs understood by journald.
 //     ------------+--------+-------------------------------------------------------------------------------
 package libsystemd
diff --git a/sd_login/.gitignore b/sd_login/.gitignore
index 84030d6..8050442 100644
--- a/sd_login/.gitignore
+++ b/sd_login/.gitignore
@@ -1,2 +1 @@
 /type_pid.go
-/cgroup_skip_gen.go
diff --git a/sd_login/Makefile b/sd_login/Makefile
index 055ee05..daf076d 100644
--- a/sd_login/Makefile
+++ b/sd_login/Makefile
@@ -13,7 +13,6 @@
 # limitations under the License.
 
 files.src.gen += type_pid.go
-files.src.gen += cgroup_skip_gen.go
 
 files.generate: $(files.src.gen)
 maintainer-clean:
@@ -23,6 +22,4 @@ maintainer-clean:
 %.go: %.go.gen
 	./$^ > $@
 
-cgroup_skip_gen.go: cgroup_skip.go
-
 .DELETE_ON_ERROR:
diff --git a/sd_login/cgroup.go b/sd_login/cgroup.go
new file mode 100644
index 0000000..0ceef37
--- /dev/null
+++ b/sd_login/cgroup.go
@@ -0,0 +1,16 @@
+package sd_login
+
+type _Cgroup interface {
+	MustSkipSystemPrefix() _Cgroup
+	GetSession() Session
+	GetOwnerUser() User
+	GetMachine() Machine
+
+	GetUserSlice() string
+	GetUserUnit() string
+
+	GetSlice() string
+	GetUnit() string
+}
+
+func (pid PID) getCgroup() (_Cgroup, error)
diff --git a/sd_login/cgroup_generic.go b/sd_login/cgroup_generic.go
deleted file mode 100644
index 3842c64..0000000
--- a/sd_login/cgroup_generic.go
+++ /dev/null
@@ -1,163 +0,0 @@
-// 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
-
-import (
-	"bufio"
-	"fmt"
-	"io"
-	"os"
-	"strings"
-
-	"golang.org/x/sys/unix"
-)
-
-type _Cgroup string
-
-var cgVersion_cache uint
-
-func cgVersion() uint {
-	if cgVersion_cache == 0 {
-		fs, err := statfs("/sys/fs/cgroup/")
-		if err != nil {
-			return 0
-		}
-		if fs.Type == magic_CGROUP2_SUPER {
-			cgVersion_cache = 2
-		} else if fs.Type == magic_TMPFS {
-			// XXX: systemd-specific cgroup v1 logic
-			fs, err = statfs("/sys/fs/cgroup/systemd/")
-			if err != nil {
-				return 0
-			}
-			if fs.Type == magic_CGROUP2_SUPER {
-				cgVersion_cache = 2
-			} else {
-				cgVersion_cache = 1
-			}
-		}
-	}
-	return cgVersion_cache
-}
-
-// getCgroup returns the cgroup path of the process, relative to the
-// root of the cgroup hierarchy.
-//
-// In cgroup v2, there is a only one cgroup hierarchy, so the behavior
-// is obvious.
-//
-// However, in cgroup v1, there were multiple cgroup hierarchies, so
-// this function must decide which hierarchy to use.  We chooses the
-// first hierarchy with either the "name=systemd" controller or the
-// "name=elogind" controller attached to it[1].  If no such hierarchy
-// exists, then an error is returned.
-//
-// However, it is possible to generally use cgroup v1, but use a
-// single (named) v2 hierarchy alongside many v1 hierarchies.  In this
-// case, we use the v2 hierarchy iff it is named "systemd", otherwise
-// we use the cgroup v1 behavior.
-//
-// [1]: The "first" in that sentence is worrying; shouldn't the choice
-// of hierarchy not depend on the undefined order that controllers are
-// listed in?  Well, a controller may be attached to only one
-// hierarchy at a time.  So there is only an ambiguity for "first" to
-// come in if both "name=systemd" and "name=elogind" controllers
-// exist.  Systemd and elogind cannot be used together, so this isn't
-// a concern.
-//
-// BUG(lukeshu): PID.getCgroup: Has systemd-specific logic.  However,
-// it is only for "legacy" cgroup v1 compatibility; the cgroup v2
-// logic is totally implementation-agnostic.  Unfortunately(?), no
-// distro seems to be using cgroup v2 (introduced in Linux 4.5) yet by
-// default.
-func (pid PID) getCgroup() (_Cgroup, error) {
-	cgVer := cgVersion()
-
-	var cgroupFilename string
-	if pid == 0 {
-		cgroupFilename = "/proc/self/cgroup"
-	} else {
-		cgroupFilename = fmt.Sprintf("/proc/%d/cgroup", pid)
-	}
-
-	f, err := os.Open(cgroupFilename)
-	if err != nil {
-		return "", err
-	}
-	defer f.Close()
-
-	bf := bufio.NewReader(f)
-
-	for {
-		line, err := bf.ReadString('\n')
-		if err == io.EOF {
-			break
-		}
-		if err != nil {
-			return "", err
-		}
-		line = strings.TrimSuffix(line, "\n")
-
-		parts := strings.SplitN(line, ":", 3)
-		if len(parts) != 3 {
-			continue
-		}
-
-		hierarchy := parts[0]
-		controllers := parts[1]
-		path := _Cgroup(parts[2])
-
-		switch cgVer {
-		case 1:
-			for _, controller := range strings.Split(controllers, ",") {
-				if controller == "name=systemd" || controller == "name=elogind" {
-					return path, nil
-				}
-			}
-		case 2:
-			if hierarchy != "0" {
-				continue
-			}
-			return path, nil
-		}
-	}
-	return "", unix.ENODATA
-}
-
-// cgGetRootPath determines the cgroup that all other cgroups belong
-// to.  The common case is just "/", but it could be something else if
-// we are inside of a container, but have a view of the entier cgroup
-// hierarchy.
-//
-// BUG(lukeshu): cgGetRootPath: works correctly on systemd and
-// elogind, but I'm not sure it's general.
-func cgGetRootPath() (_Cgroup, error) {
-	cgpath, err := PID(1).getCgroup()
-	if err != nil {
-		return "/", err
-	}
-
-	cgpath = _Cgroup(trimOneSuffix(string(cgpath),
-		"/init.scope",   // modern systemd
-		"/system.slice", // legacy systemd
-		"/system",       // even more legacy systemd
-	))
-
-	return cgpath, nil
-}
-
-func cgUnescape(s string) string {
-	return strings.TrimPrefix(s, "_")
-}
diff --git a/sd_login/cgroup_skip.go b/sd_login/cgroup_skip.go
deleted file mode 100644
index 7eb2be8..0000000
--- a/sd_login/cgroup_skip.go
+++ /dev/null
@@ -1,98 +0,0 @@
-// 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
-
-import (
-	"strconv"
-	"strings"
-)
-
-func (cgroup _Cgroup) SkipPath(prefix _Cgroup) (_Cgroup, bool) {
-	//skip: SkipPath(prefix _Cgroup) : SkipPath(prefix)
-	rest, ok := path_startswith(string(cgroup), string(prefix))
-	if ok {
-		return _Cgroup(rest), true
-	} else {
-		return cgroup, false
-	}
-}
-
-// Skip (*.slice){1,}
-func (cgroup _Cgroup) SkipSlices() (_Cgroup, bool) {
-	//skip: SkipSlices() : SkipSlices()
-	cg := string(cgroup)
-	skipped := false
-	for {
-		cg = strings.TrimLeft(cg, "/")
-		part, rest := split2(cg, '/')
-		if !valid_slice_name(part) {
-			return _Cgroup(cg), skipped
-		}
-		skipped = true
-		cg = rest
-	}
-}
-
-// Skip user@*.service
-func (cgroup _Cgroup) SkipUserManager() (_Cgroup, bool) {
-	//skip: SkipUserManager() : SkipUserManager()
-	part, rest := split2(strings.TrimLeft(string(cgroup), "/"), '/')
-	uid_str, ok := trimPrefixSuffix(part, "user@", ".service")
-	if !ok {
-		return cgroup, false
-	}
-	_, err := strconv.Atoi(uid_str)
-	if err != nil {
-		return cgroup, false
-	}
-	return _Cgroup(rest), true
-}
-
-// Skip session-*.scope
-func (cgroup _Cgroup) SkipSession() (_Cgroup, bool) {
-	//skip: SkipSession() : SkipSession()
-	part, rest := split2(strings.TrimLeft(string(cgroup), "/"), '/')
-	session, ok := trimPrefixSuffix(part, "session-", ".scope")
-	if !ok {
-		return cgroup, false
-	}
-	if !valid_session_name(session) {
-		return cgroup, false
-	}
-	return _Cgroup(rest), true
-}
-
-// Skip (/*.slice){0,}/(user@*.service|session-*.scope)
-func (cgroup _Cgroup) SkipUserPrefix() (_Cgroup, bool) {
-	//skip: SkipUserPrefix() : SkipUserPrefix()
-	cgroup, _ = cgroup.SkipSlices()
-	cgroup, ok := cgroup.SkipUserManager()
-	if ok {
-		return cgroup, ok
-	}
-	return cgroup.SkipSession()
-}
-
-// Skip cgGetRootPath
-func (cgroup _Cgroup) SkipSystemPrefix() (_Cgroup, bool) {
-	//skip: SkipSystemPrefix() : SkipSystemPrefix()
-
-	rootpath, err := cgGetRootPath()
-	if err != nil {
-		return cgroup, false
-	}
-
-	return cgroup.SkipPath(rootpath)
-}
diff --git a/sd_login/cgroup_skip_gen.go.gen b/sd_login/cgroup_skip_gen.go.gen
deleted file mode 100755
index 26c515a..0000000
--- a/sd_login/cgroup_skip_gen.go.gen
+++ /dev/null
@@ -1,41 +0,0 @@
-#!/usr/bin/env bash
-# Copyright (C) 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.
-
-{
-	printf '//'
-	printf ' %q' "$0" "$@"
-	printf '\n// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT\n\n'
-
-	echo package sd_login
-
-	grep -o '//skip:.*' "$1" | cut -d: -f2- | while read -r line; do
-		sig=$(echo $(cut -d: -f1 <<<"$line"))
-		cal=$(echo $(cut -d: -f2 <<<"$line"))
-		cat <<EOF
-
-func (cgroup _Cgroup) Maybe${sig} _Cgroup {
-	cgroup, _ = cgroup.${cal}
-	return cgroup
-}
-func (cgroup _Cgroup) Must${sig} _Cgroup {
-	cgroup, ok := cgroup.${cal}
-	if !ok {
-		return ""
-	}
-	return cgroup
-}
-EOF
-	done
-} | gofmt
diff --git a/sd_login/cgroup_systemd.go b/sd_login/cgroup_systemd.go
deleted file mode 100644
index bbef9d3..0000000
--- a/sd_login/cgroup_systemd.go
+++ /dev/null
@@ -1,107 +0,0 @@
-// 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
-
-import (
-	"os"
-	"strconv"
-	"strings"
-)
-
-// XXX: logind
-func (cgroup _Cgroup) GetSession() Session {
-	unit := cgroup.GetUnit()
-
-	session, ok := trimPrefixSuffix(unit, "session-", ".scope")
-	if !ok || !valid_session_name(session) {
-		return ""
-	}
-
-	return Session(session)
-}
-
-// XXX: who owns this, systemd or logind?  elogind doesn't implement
-// it, but idk
-func (cgroup _Cgroup) GetOwnerUser() User {
-	slice := cgroup.GetSlice()
-
-	uid_str, ok := trimPrefixSuffix(slice, "user-", ".slice")
-	if !ok {
-		return -1
-	}
-
-	uid, err := strconv.Atoi(uid_str)
-	if err != nil {
-		return -1
-	}
-
-	return User(uid)
-}
-
-// XXX: machined
-func (cgroup _Cgroup) GetMachine() Machine {
-	unit := cgroup.GetUnit()
-	if unit == "" {
-		return ""
-	}
-
-	machine, err := os.Readlink("/run/systemd/machines/unit:" + unit)
-	if err != nil {
-		return ""
-	}
-	return Machine(machine)
-}
-
-// XXX: systemd
-func (cgroup _Cgroup) decodeUnit() string {
-	unit, _ := split2(string(cgroup), '/')
-	if len(unit) < 3 {
-		return ""
-	}
-	unit = cgUnescape(unit)
-	if valid_unit_name(unit)&(unit_name_plain|unit_name_instance) == 0 {
-		return ""
-	}
-	return unit
-}
-
-func (cgroup _Cgroup) GetUnit() string {
-	unit := cgroup.MaybeSkipSlices().decodeUnit()
-	if strings.HasSuffix(unit, ".slice") {
-		return ""
-	}
-	return unit
-}
-func (cgroup _Cgroup) GetUserUnit() string {
-	return cgroup.MustSkipUserPrefix().GetUnit()
-}
-func (cgroup _Cgroup) GetSlice() string {
-	cg := string(cgroup)
-	n := 0
-	for {
-		cg = strings.TrimLeft(cg, "/")
-		part, rest := split2(cg, '/')
-		if !valid_slice_name(part) {
-			if n == 0 {
-				return "-.slice"
-			}
-			return _Cgroup(cg).decodeUnit()
-		}
-		cg = rest
-	}
-}
-func (cgroup _Cgroup) GetUserSlice() string {
-	return cgroup.MustSkipUserPrefix().GetSlice()
-}
diff --git a/sd_login/doc.go b/sd_login/doc.go
index ba1a0f7..33c31be 100644
--- a/sd_login/doc.go
+++ b/sd_login/doc.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -12,27 +12,34 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package sd_login TODO
+//go:generate make
+
+// 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.
 //
-// The login manager manages 4 basic object types
+// The machine manager maintains a list of "machines":
 //
-//  - machine: A container or virtual machine
+//  1. machine: A local container or virtual machine
 //
-//  - seat: A set of hardware devices for a workspace; ie a screen
-//    and keyboard
+// The host system, and machines hosted on it, may each run their own
+// login manager, which keeps track of seats, sessions, and users:
 //
-//  - session: A login session; tied to 0 or 1 seats (0 for things
-//    like SSH login)
+//  2. seat: A set of hardware devices for a workspace; i.e. a screen
+//     and keyboard
 //
-//  - user: Keeps track of logged in users; that is, users with 1 or
-//    more sessions
+//  3. session: A login session; tied to 0 or 1 seats (0 for things
+//     like SSH login)
 //
-// TODO: why are machines part of this?
+//  4. user: Keeps track of logged in users; that is, users with 1 or
+//     more sessions
 //
-//  - pid: 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 login manager; it is NOT the same as the
-//    EUID/RUID.
+// Finally, sessions and users can be used to group together
+// processes, which are kept track of by cgroup manager:
 //
-//  - peer: A peer is a process on the other end of a AF_UNIX socket.
+//  5. pid: 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 login manager; it is NOT the same as the
+//     EUID/RUID.
 package sd_login
diff --git a/sd_login/gen.go b/sd_login/gen.go
deleted file mode 100644
index 3e5b5ac..0000000
--- a/sd_login/gen.go
+++ /dev/null
@@ -1,3 +0,0 @@
-//go:generate make
-
-package sd_login
diff --git a/sd_login/type_machine.go b/sd_login/type_machine.go
index 5892017..d89eaad 100644
--- a/sd_login/type_machine.go
+++ b/sd_login/type_machine.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -21,9 +21,44 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// A Machine is a name representing a locally running container or
+// virtual machine that is registerd with (systemd-)machined.
 type Machine string
 
-// running VMs/containers
+// isValid returns whether the Machine name is valid.
+func (m Machine) isValid() bool {
+	if len(m) > host_name_max {
+		// Note that Linux HOST_NAME_MAX is 64, but DNS allows
+		// 255, so names from DNS might be invalid.
+		return false
+	}
+
+	n_dots := 0
+	dot := true
+	for _, c := range m {
+		if c == '.' {
+			if dot {
+				return false
+			}
+			dot = true
+			n_dots++
+		} else {
+			if !strings.ContainsRune(letters+digits+"-_.", c) {
+				return false
+			}
+			dot = false
+		}
+	}
+
+	if dot { // trailing dot or empty
+		return false
+	}
+
+	return true
+}
+
+// GetMachines returns a list of currently running containers/virtual
+// machines.
 func GetMachines() ([]Machine, error) {
 	strs, err := get_files_in_directory("/run/systemd/machines/")
 	if err != nil {
@@ -31,7 +66,8 @@ func GetMachines() ([]Machine, error) {
 	}
 	var machines []Machine
 	for _, str := range strs {
-		if strings.HasPrefix(str, "unit:") || !valid_machine_name(str) {
+		machine := Machine(str)
+		if strings.HasPrefix(str, "unit:") || machine.isValid() {
 			continue
 		}
 		machines = append(machines, Machine(str))
@@ -39,7 +75,14 @@ func GetMachines() ([]Machine, error) {
 	return machines, nil
 }
 
+// GetClass returns the class of a locally running machine.  The class
+// is either "vm" or "container", depending on if the machine is an
+// virtual machine or a container.
 func (m Machine) GetClass() (string, error) {
+	if !m.isValid() {
+		return "", unix.EINVAL
+	}
+
 	env, err := parse_env_file("/run/systemd/machines/" + string(m))
 	if err != nil {
 		return "", err
@@ -51,7 +94,13 @@ func (m Machine) GetClass() (string, error) {
 	return class, nil
 }
 
+// GetIfIndices returns the numeric indices of the network interfaces
+// on the host that are pointing toward this machine.
 func (m Machine) GetIfIndices() ([]int, error) {
+	if !m.isValid() {
+		return nil, unix.EINVAL
+	}
+
 	env, err := parse_env_file("/run/systemd/machines/" + string(m))
 	if err != nil {
 		return nil, err
diff --git a/sd_login/type_monitor.go b/sd_login/type_monitor.go
index 50f601a..2422f06 100644
--- a/sd_login/type_monitor.go
+++ b/sd_login/type_monitor.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -19,7 +19,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-// Monitor monitors several types of events, so you don't have to do
+// A Monitor monitors several types of events, so you don't have to do
 // busy-polling.
 //
 // However, Monitor won't actually give you the event that happened,
@@ -84,10 +84,14 @@ func NewMonitor(categories MonitorCategory) (*Monitor, error) {
 	return &Monitor{in: in}, nil
 }
 
+// Close closes the monitor.  After calling Close, further calls to
+// the monitor will fail.
 func (m *Monitor) Close() error {
 	return m.in.Close()
 }
 
+// Flush discards any events that have happened since the last Flush
+// or Wait call.
 func (m *Monitor) Flush() error {
 	for {
 		ev, err := m.in.ReadNonblock()
@@ -100,6 +104,10 @@ func (m *Monitor) Flush() error {
 	}
 }
 
+// Wait blocks until a watched for event has happened.  A single call
+// to Wait discards only discards first event; if multiple events
+// happen at once, multiple calls to Wait will immediately return
+// unless Flush is called.
 func (m *Monitor) Wait() error {
 	_, err := m.in.ReadBlock()
 	return err
diff --git a/sd_login/type_pid.go.gen b/sd_login/type_pid.go.gen
index ec1bfe0..8d5e5fb 100755
--- a/sd_login/type_pid.go.gen
+++ b/sd_login/type_pid.go.gen
@@ -21,13 +21,21 @@
 	cat <<EOF
 package sd_login
 
-import "net"
+import (
+	"net"
 
-// PID represents a process.
+	"golang.org/x/sys/unix"
+)
+
+// A PID represents a process.
 //
 // As a special case, PID(0) refers to the current process.
 type PID int
 
+func (pid PID) isValid() bool {
+	return pid >= 0
+}
+
 func GetPeer(conn *net.UnixConn) (PID, error) {
 	ucred, err := getpeercred(conn)
 	if err != nil {
@@ -35,6 +43,7 @@ func GetPeer(conn *net.UnixConn) (PID, error) {
 	}
 	return PID(ucred.Pid), nil
 }
+
 EOF
 
 	data=(
@@ -52,6 +61,10 @@ EOF
 		cat <<EOF
 
 func (pid PID) ${Func}() (v ${Type}, err error) {
+	if !pid.isValid() {
+		err = unix.EINVAL
+		return
+	}
 	cgroup, err := pid.getCgroup()
 	if err != nil {
 		return
diff --git a/sd_login/type_seat.go b/sd_login/type_seat.go
index fd86663..a62effc 100644
--- a/sd_login/type_seat.go
+++ b/sd_login/type_seat.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -14,9 +14,25 @@
 
 package sd_login
 
+import (
+	"strconv"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// A Seat represents a set of hardware devices for a workspace; i.e. a
+// screen and keyboard.
+//
+// Multiple sessions may be associated with a seat, but only one
+// session may be active on a seat at a time.
 type Seat string
 
-// GetSeats returns a list of all currently avialable local seats.
+func (seat Seat) isValid() bool {
+	return valid_filename(string(seat))
+}
+
+// GetSeats returns a list of all currently available local seats.
 func GetSeats() ([]Seat, error) {
 	strs, err := get_files_in_directory("/run/systemd/seats/")
 	if err != nil {
@@ -29,9 +45,95 @@ func GetSeats() ([]Seat, error) {
 	return seats, nil
 }
 
-func (seat Seat) GetActive() (Session, User, error)
-func (seat Seat) GetSessions() ([]Session, []User, error)
+// GetActive returns which session is currently active on this seat,
+// or nil if there is no currently active session.
+func (seat Seat) GetActive() (sess AnnotatedSession, err error) {
+	if !seat.isValid() {
+		err = unix.EINVAL
+		return
+	}
+	env, err := parse_env_file("/run/systemd/seats/" + string(seat))
+	if err != nil {
+		return
+	}
+
+	strName, ok := env["ACTIVE"]
+	if !ok {
+		err = unix.ENXIO
+		return
+	}
+	strUid, ok := env["ACTIVE_UID"]
+	if !ok {
+		err = unix.ENXIO
+		return
+	}
+	intUid, err := strconv.Atoi(strUid)
+	if err != nil {
+		return
+	}
+	sess.Session = Session(strName)
+	sess.User = User(intUid)
+
+	return
+}
+
+// GetSessions returns a list of all sessions associated with the
+// seat, whether they are active or not.
+func (seat Seat) GetSessions() ([]AnnotatedSession, error) {
+	if !seat.isValid() {
+		return nil, unix.EINVAL
+	}
+	env, err := parse_env_file("/run/systemd/seats/" + string(seat))
+	if err != nil {
+		return nil, err
+	}
+
+	strSessions, ok := env["SESSIONS"]
+	if !ok {
+		return nil, unix.ENXIO
+	}
+	strUids, ok := env["UIDS"]
+	if !ok {
+		return nil, unix.ENXIO
+	}
+
+	arySessions := strings.Fields(strSessions)
+	aryUids := strings.Fields(strUids)
+	if len(arySessions) != len(aryUids) {
+		return nil, unix.ENXIO
+	}
+	ret := make([]AnnotatedSession, len(arySessions))
+	for i := 0; i < len(arySessions); i++ {
+		uid, err := strconv.Atoi(aryUids[i])
+		if err != nil {
+			return nil, err
+		}
+		ret[i].Session = Session(arySessions[i])
+		ret[i].User = User(uid)
+	}
+
+	return ret, nil
+}
+
+func (seat Seat) can(cap string) (bool, error) {
+	if !seat.isValid() {
+		return false, unix.EINVAL
+	}
+	env, err := parse_env_file("/run/systemd/seats/" + string(seat))
+	if err != nil {
+		return false, err
+	}
+	return parse_boolean(env["CAN_"+cap])
+}
 
-func (seat Seat) CanMultiSession() bool
-func (seat Seat) CanTTY() bool
-func (seat Seat) CanGraphical() bool
+func (seat Seat) CanMultiSession() (bool, error) {
+	return seat.can("MULTI_SESSION")
+}
+
+func (seat Seat) CanTTY() (bool, error) {
+	return seat.can("TTY")
+}
+
+func (seat Seat) CanGraphical() (bool, error) {
+	return seat.can("GRAPHICAL")
+}
diff --git a/sd_login/type_session.go b/sd_login/type_session.go
index 47856de..e692426 100644
--- a/sd_login/type_session.go
+++ b/sd_login/type_session.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -16,6 +16,15 @@ package sd_login
 
 type Session string
 
+type AnnotatedSession struct {
+	Session
+	User User
+}
+
+func (sess AnnotatedSession) GetUser() (User, error) {
+	return sess.User, nil
+}
+
 type SessionState int
 
 const (
diff --git a/sd_login/util.go b/sd_login/util.go
index 45df0e0..2497aef 100644
--- a/sd_login/util.go
+++ b/sd_login/util.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -19,6 +19,8 @@ import (
 	"os"
 	"path"
 	"strings"
+	
+	"golang.org/x/sys/unix"
 )
 
 const (
@@ -97,6 +99,27 @@ func get_files_in_directory(apath string) ([]string, error) {
 	return ret, nil
 }
 
+func parse_boolean(v string) (bool, error) {
+	switch {
+	case v == "1",
+		strings.EqualFold(v, "yes"),
+		strings.EqualFold(v, "y"),
+		strings.EqualFold(v, "true"),
+		strings.EqualFold(v, "t"),
+		strings.EqualFold(v, "on"):
+		return true, nil
+	case v == "0",
+		strings.EqualFold(v, "no"),
+		strings.EqualFold(v, "n"),
+		strings.EqualFold(v, "false"),
+		strings.EqualFold(v, "f"),
+		strings.EqualFold(v, "off"):
+		return false, nil
+	default:
+		return false, unix.EINVAL
+	}
+}
+
 func parse_env_file(filename string) (map[string]string, error)
 
 /*
diff --git a/sd_login/util_valid.go b/sd_login/util_valid.go
index e0dd906..46493b3 100644
--- a/sd_login/util_valid.go
+++ b/sd_login/util_valid.go
@@ -1,4 +1,4 @@
-// Copyright (C) 2016 Luke Shumaker <lukeshu@sbcglobal.net>
+// 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.
@@ -30,34 +30,14 @@ func valid_slice_name(s string) bool {
 	return strings.HasSuffix(s, ".slice") && valid_unit_name(s) == unit_name_plain
 }
 
-func valid_machine_name(s string) bool {
-	if len(s) > host_name_max {
-		// Note that Linux HOST_NAME_MAX is 64, but DNS allows
-		// 255, so names from DNS might be invalid.
+func valid_filename(s string) bool {
+	switch s {
+	case "", ".", "..":
 		return false
 	}
-
-	n_dots := 0
-	dot := true
-	for _, c := range s {
-		if c == '.' {
-			if dot {
-				return false
-			}
-			dot = true
-			n_dots++
-		} else {
-			if !strings.ContainsRune(letters+digits+"-_.", c) {
-				return false
-			}
-			dot = false
-		}
-	}
-
-	if dot { // trailing dot or empty
+	if strings.ContainsRune(s, '/') {
 		return false
 	}
-
 	return true
 }
 
@@ -66,9 +46,26 @@ const (
 	unit_name_plain    = 1 << 0 // foo.service
 	unit_name_instance = 1 << 1 // foo@bar.service
 	unit_name_template = 1 << 2 // foo@.service
-	unit_name_any      = unit_name_plain | unit_name_instance | unit_name_template
 )
 
+// valid_unit_name returns returns which type of unit the given unit
+// name is valid for.
+//
+// To simply check whether a unit name is valid, without caring about
+// the type, you can simply check that it doesn't return the "invalid"
+// type:
+//
+//     is_valid := valid_unit_name(unitname) != unit_name_invalid
+//
+// To check whether it matches a specific type (in this example,
+// "template"), you can test equality against that type:
+//
+//     is_valid_template := valid_unit_name(unitname) == unit_name_template
+//
+// If there are several acceptable types, you can treat multiple types
+// as bitmasks, and use the usual bitfield checks:
+//
+//     is_valid_plain_or_instance := valid_unit_name(unitname)&(unit_name_plain|unit_name_instance) != 0
 func valid_unit_name(unit string) int {
 	const_unit_name_max := 256
 	const_unit_types := []string{