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+#!/usr/bin/env python3
+
+# Copyright (c) 2021 Egor Tensin <Egor.Tensin@gmail.com>
+# This file is part of the "linux-status" project.
+# For details, see https://github.com/egor-tensin/linux-status.
+# Distributed under the MIT License.
+
+# Initially it was just index.html and a bunch of shell scripts in cgi-bin.
+# The web server could be launched using just `python -m http.server --cgi`,
+# and all was well.  Until I decided that I want to package this little app
+# of mine so that it can be easily installed and run on a variety of systems.
+# There was a problem with that, however: the app was designed to be run by
+# a regular user, not root (mostly due to all the `systemctl --user` calls),
+# and it didn't play nicely with the whole package idea.  Plus, I've long
+# wanted to make it show all the systemd user instances, not just the running
+# user's.  In addition, I wanted to dedupe the small chunks of code in cgi-bin.
+#
+# So this script was born.  It could handle a lot more complexity than the
+# shell scripts; it could be run as root and as a regular user, etc.  It was
+# still a CGI script (and it still is BTW), and a separate Python instance was
+# used by the web server to execute it.
+#
+# There was a problem, of course.  The thing is, I for once had a very strict
+# performance requirement for this script: it had to work nicely on my rather
+# old Raspberry Pi 1 Model B+.  It, being a Python script, didn't.  After
+# some investigation, it was obvious that the main problem were the imports
+# (especially those of the cgi & logging modules); they would take more time
+# than the interval between requests.  So I had to find a way to make it more
+# performant.
+#
+# I played with Lua for a bit, but couldn't quite make it work in a
+# satisfactory way.  Then I had an idea: I could use http.server's HTTP server
+# classes, overriding them to handle the "CGI" requests using the code in this
+# script.  That way, the imports would only be done once for the entire life
+# of the server.  In addition, no more tricks with sudo were required
+# (http.server runs CGI scripts as user nobody, which is a no-no for this app;
+# some trickery like whitelisting nobody in /etc/sudoers.d were employed).
+#
+# So that's where we are now, and it works!  Doesn't load my good ol' Raspberry
+# no more.
+
+import abc
+import cgi
+from collections import namedtuple
+from concurrent.futures import ThreadPoolExecutor
+from enum import Enum
+import http.server
+import json
+import os
+import pwd
+import shlex
+import socket
+import subprocess
+from subprocess import DEVNULL, PIPE, STDOUT
+
+
+def split_by(xs, sep):
+    group = []
+    for x in xs:
+        if x == sep:
+            yield group
+            group = []
+        else:
+            group.append(x)
+    yield group
+
+
+class Response:
+    DEFAULT_STATUS = http.server.HTTPStatus.OK
+
+    @staticmethod
+    def body_from_json(body):
+        return json.dumps(body, ensure_ascii=False, indent=4)
+
+    def __init__(self, body, status=None):
+        if status is None:
+            status = Response.DEFAULT_STATUS
+        self.status = status
+        self.body = body
+
+    def headers(self):
+        yield 'Content-Type', 'text/html; charset=utf-8'
+
+    def encode_body(self):
+        return self.body.encode(errors='replace')
+
+    def write_as_cgi_script(self):
+        self.write_headers_as_cgi_script()
+        self.write_body_as_cgi_script()
+
+    def write_headers_as_cgi_script(self):
+        for name, val in self.headers():
+            print(f'{name}: {val}')
+        print()
+
+    def write_body_as_cgi_script(self):
+        if self.body is not None:
+            print(self.body)
+
+    def write_to_request_handler(self, handler):
+        handler.send_response(self.status)
+        self.write_headers_to_request_handler(handler)
+        self.write_body_to_request_handler(handler)
+
+    def write_headers_to_request_handler(self, handler):
+        for name, val in self.headers():
+            handler.send_header(name, val)
+        handler.end_headers()
+
+    def write_body_to_request_handler(self, handler):
+        if self.body is not None:
+            handler.wfile.write(self.encode_body())
+
+
+def run_do(*args, **kwargs):
+    output = subprocess.run(args, stdin=DEVNULL, stdout=PIPE, stderr=STDOUT, universal_newlines=True, **kwargs)
+    # Include the output in the exception's message:
+    try:
+        output.check_returncode()
+    except Exception as e:
+        raise RuntimeError("Command's output was this:\n" + output.stdout) from e
+    return output.stdout
+
+
+class Task(abc.ABC):
+    def complete(self):
+        self.run()
+        return Response(Response.body_from_json(self.result()))
+
+    @abc.abstractmethod
+    def run(self):
+        pass
+
+    @abc.abstractmethod
+    def result(self):
+        pass
+
+
+class TaskList(Task):
+    def __init__(self, tasks):
+        self.tasks = tasks
+
+    def add(self, name, task):
+        if name in self.tasks:
+            raise RuntimeError(f'duplicate task name: {name}')
+        self.tasks[name] = task
+
+    def run(self):
+        for task in self.tasks.values():
+            task.run()
+
+    def result(self):
+        return {name: task.result() for name, task in self.tasks.items()}
+
+
+pool = ThreadPoolExecutor()
+
+
+def run(*args, **kwargs):
+    return pool.submit(run_do, *args, **kwargs)
+
+
+class Command(Task):
+    def __init__(self, *args):
+        self.args = args
+        self.env = None
+
+    def run(self):
+        self.task = run(*self.args, env=self.env)
+        return self.task
+
+    def result(self):
+        return self.task.result()
+
+    def now(self):
+        return self.run().result()
+
+
+class Systemd(Command):
+    def __init__(self, *args):
+        super().__init__(*args)
+        self.env = self.make_env()
+
+    @staticmethod
+    def make_env():
+        env = os.environ.copy()
+        env['SYSTEMD_PAGER'] = ''
+        env['SYSTEMD_COLORS'] = 'no'
+        return env
+
+    @staticmethod
+    def su(user, cmd):
+        new = Systemd('su', '-c', shlex.join(cmd.args), user.name)
+        new.env = Systemd.fix_su_env(user, cmd.env.copy())
+        return new
+
+    @staticmethod
+    def fix_su_env(user, env):
+        # https://unix.stackexchange.com/q/483948
+        # https://unix.stackexchange.com/q/346841
+        # https://unix.stackexchange.com/q/423632
+        # https://unix.stackexchange.com/q/245768
+        # https://unix.stackexchange.com/q/434494
+        env['XDG_RUNTIME_DIR'] = user.runtime_dir
+        # I'm not sure the bus part works everywhere.
+        bus_path = os.path.join(user.runtime_dir, 'bus')
+        env['DBUS_SESSION_BUS_ADDRESS'] = 'unix:path=' + bus_path
+        return env
+
+
+class Ctl(Systemd):
+    def __init__(self, executable, *args):
+        super().__init__(executable, *args, '--full')
+
+    @classmethod
+    def system(cls, *args):
+        return cls('--system', *args)
+
+    @classmethod
+    def user(cls, *args):
+        return cls('--user', *args)
+
+
+class Systemctl(Ctl):
+    def __init__(self, *args):
+        super().__init__('systemctl', *args)
+
+
+class Journalctl(Ctl):
+    def __init__(self, *args):
+        super().__init__('journalctl', *args)
+
+
+class Loginctl(Systemd):
+    def __init__(self, *args):
+        super().__init__('loginctl', *args, '--full')
+
+
+class Docker(Command):
+    def __init__(self, *args):
+        super().__init__('docker', *args)
+
+
+class DockerVersion(Docker):
+    def __init__(self, *args):
+        super().__init__('version')
+
+    @staticmethod
+    def is_daemon_running():
+        try:
+            DockerVersion().now()
+            return True
+        except:
+            return False
+
+
+class DockerPs(Docker):
+    def __init__(self, *args):
+        super().__init__('ps', *args)
+
+    @staticmethod
+    def quiet(*args):
+        return DockerPs('--quiet', *args)
+
+    @staticmethod
+    def get_all_ids():
+        cmd = DockerPs.quiet('--all')
+        return cmd.now().splitlines()
+
+
+class DockerInspect(Docker):
+    # This is pretty cool.  I wanted to separate container entries with \0, and
+    # this is the best I could come up with.  Note that a newline still gets
+    # added at the end.
+    FORMAT = '{{printf "%s%c" (json .) 0}}'
+
+    def __init__(self, *args):
+        super().__init__('inspect', f'--format={DockerInspect.FORMAT}', *args)
+
+
+class DockerStatus(DockerInspect):
+    def __init__(self):
+        self.containers = DockerPs.get_all_ids()
+        super().__init__(*self.containers)
+
+    def run(self):
+        if not self.containers:
+            # `docker inspect` requires at least one container argument.
+            return ''
+        return super().run()
+
+    def result(self):
+        if not self.containers:
+            # `docker inspect` requires at least one container argument.
+            return []
+        result = super().result()
+        result = result.split('\0')
+        result = [json.loads(info) for info in result if info.strip()]
+        result = [DockerStatus.filter_info(info) for info in result]
+        return result
+
+    @staticmethod
+    def filter_info(info):
+        assert info['Name'][0] == '/'
+        return {
+            'exit_code': info['State']['ExitCode'],
+            'health': info['State'].get('Health', {}).get('Status', None),
+            'image': info['Config']['Image'],
+            # Strip the leading /:
+            'name': info['Name'][1:],
+            'started_at': info['State']['StartedAt'],
+            'status': info['State']['Status'],
+        }
+
+class Hostname(Task):
+    def run(self):
+        pass
+
+    def result(self):
+        return socket.gethostname()
+
+
+class Top(Command):
+    COMMAND = None
+
+    def __init__(self):
+        super().__init__(*Top.get_command())
+
+    @staticmethod
+    def get_command():
+        # On more modern versions of top, we want to enable memory scaling
+        # from the command line (another option is the rc file, but that's too
+        # complicated).  For that, we simply run `top -h` once, and check if
+        # the output contains the flags we want to use.
+        if Top.COMMAND is not None:
+            return Top.COMMAND
+        help_output = run_do('top', '-h')
+        args = ['top', '-b', '-n', '1', '-w', '512']
+        if 'Ee' in help_output:
+            args += ['-E', 'm', '-e', 'm']
+        Top.COMMAND = args
+        return Top.COMMAND
+
+
+class Reboot(Command):
+    def __init__(self):
+        super().__init__('systemctl', 'reboot')
+
+
+class Poweroff(Command):
+    def __init__(self):
+        super().__init__('systemctl', 'poweroff')
+
+
+class InstanceStatus(TaskList):
+    def __init__(self, systemctl, journalctl):
+        tasks = {
+            'overview': systemctl('status'),
+            'failed': systemctl('list-units', '--failed'),
+            'timers': systemctl('list-timers', '--all'),
+            'journal': journalctl('-b', '--lines=20'),
+        }
+        super().__init__(tasks)
+
+
+class SystemStatus(InstanceStatus):
+    def __init__(self):
+        super().__init__(Systemctl.system, Journalctl.system)
+        if DockerVersion.is_daemon_running():
+            self.add('docker', DockerStatus())
+
+
+class UserStatus(InstanceStatus):
+    def __init__(self, systemctl=Systemctl.user, journalctl=Journalctl.user):
+        super().__init__(systemctl, journalctl)
+
+    @staticmethod
+    def su(user):
+        systemctl = lambda *args: Systemd.su(user, Systemctl.user(*args))
+        journalctl = lambda *args: Systemd.su(user, Journalctl.user(*args))
+        return UserStatus(systemctl, journalctl)
+
+
+class UserStatusList(TaskList):
+    def __init__(self):
+        if running_as_root():
+            # As root, we can query all the user instances.
+            tasks = {user.name: UserStatus.su(user) for user in systemd_users()}
+        else:
+            # As a regular user, we can only query ourselves.
+            tasks = {}
+            user = get_current_user()
+            if user_instance_active(user):
+                tasks[user.name] = UserStatus()
+        super().__init__(tasks)
+
+
+class Status(TaskList):
+    def __init__(self):
+        tasks = {
+            'hostname': Hostname(),
+            'system': SystemStatus(),
+            'user': UserStatusList(),
+        }
+        super().__init__(tasks)
+
+
+User = namedtuple('User', ['uid', 'name'])
+SystemdUser = namedtuple('SystemdUser', ['uid', 'name', 'runtime_dir'])
+
+
+def running_as_root():
+    # AFAIK, Python's http.server drops root privileges and executes the scripts
+    # as user nobody.
+    # It does no such thing if run as a regular user though.
+    return os.geteuid() == 0 or running_as_nobody()
+
+
+def running_as_nobody():
+    for user in users():
+        if user.name == 'nobody':
+            return user.uid == os.geteuid()
+    return False
+
+
+def get_current_user():
+    uid = os.getuid()
+    entry = pwd.getpwuid(uid)
+    return User(entry.pw_uid, entry.pw_name)
+
+
+def user_instance_active(user):
+    # I'm pretty sure this is the way to determine if the user instance is
+    # running?
+    # Source: https://www.freedesktop.org/software/systemd/man/user@.service.html
+    unit_name = f'user@{user.uid}.service'
+    cmd = Systemctl.system('is-active', unit_name, '--quiet')
+    try:
+        cmd.now()
+        return True
+    except Exception:
+        return False
+
+
+# A pitiful attempt to find a list of possibly-systemd-enabled users follows
+# (i.e. users that might be running a per-user systemd instance).
+# I don't know of a better way than probing /run/user/UID.
+# Maybe running something like `loginctl list-sessions` would be better?
+
+# These are the default values, see [1].
+# The actual values are specified in /etc/login.defs.
+# Still, they can be overridden on the command line, so I don't think there
+# actually is a way to list non-system users (imma call them "human").
+#
+#   [1]: https://man7.org/linux/man-pages/man8/useradd.8.html
+UID_MIN = 1000
+UID_MAX = 60000
+
+
+def users():
+    for entry in pwd.getpwall():
+        yield User(entry.pw_uid, entry.pw_name)
+
+
+def human_users():
+    for user in users():
+        if user.uid < UID_MIN:
+            continue
+        if user.uid > UID_MAX:
+            continue
+        yield user
+
+
+# You know what, loginctl might just be the answer.
+# Namely, `loginctl list-users`: during my testing, it did only list the users
+# that were running a systemd instance.
+def systemd_users():
+    def list_users():
+        output = Loginctl('list-users', '--no-legend').now()
+        lines = output.splitlines()
+        if not lines:
+            return
+        for line in lines:
+            # This assumes user names cannot contain spaces.
+            # loginctl list-users output must be in the UID NAME format.
+            info = line.lstrip().split(' ', 2)
+            if len(info) < 2:
+                raise RuntimeError(f'invalid `loginctl list-users` output:\n{output}')
+            uid, user = info[0], info[1]
+            yield User(uid, user)
+
+    def show_users(users):
+        user_args = [user.name for user in users]
+        if not user_args:
+            return None
+        properties = 'UID', 'Name', 'RuntimePath'
+        prop_args = (arg for prop in properties for arg in ('-p', prop))
+        output = Loginctl('show-user', *prop_args, '--value', *user_args).now()
+        lines = output.splitlines()
+        # Assuming that for muptiple users, the properties will be separated by
+        # an empty string.
+        groups = split_by(lines, '')
+        for group in groups:
+            if len(group) != len(properties):
+                raise RuntimeError(f'invalid `loginctl show-user` output:\n{output}')
+            yield SystemdUser(int(group[0]), group[1], group[2])
+
+    return show_users(list_users())
+
+
+class Request(Enum):
+    STATUS = 'status'
+    TOP = 'top'
+    REBOOT = 'reboot'
+    POWEROFF = 'poweroff'
+
+    def __str__(self):
+        return self.value
+
+    @staticmethod
+    def from_http_path(path):
+        if not path or path[0] != '/':
+            raise ValueError('HTTP path must start with a forward slash /')
+        return Request(path[1:])
+
+    def process(self):
+        if self is Request.STATUS:
+            return Status().complete()
+        if self is Request.TOP:
+            return Top().complete()
+        if self is Request.REBOOT:
+            return Reboot().complete()
+        if self is Request.POWEROFF:
+            return Poweroff().complete()
+        raise NotImplementedError(f'unknown request: {self}')
+
+
+def process_cgi_request():
+    params = cgi.FieldStorage()
+    what = params['what'].value
+    Request(what).process().write_as_cgi_script()
+
+
+def main():
+    process_cgi_request()
+
+
+if __name__ == '__main__':
+    main()