"""
Contains base Spawner class & default implementation
"""

# Copyright (c) Jupyter Development Team.
# Distributed under the terms of the Modified BSD License.

import errno
import os
import pipes
import shutil
import signal
import sys
import warnings
from subprocess import Popen
from tempfile import mkdtemp

from sqlalchemy import inspect

from tornado import gen
from tornado.ioloop import PeriodicCallback

from traitlets.config import LoggingConfigurable
from traitlets import (
    Any, Bool, Dict, Instance, Integer, Float, List, Unicode,
    observe, validate,
)

from .objects import Server
from .traitlets import Command, ByteSpecification
from .utils import random_port, url_path_join, exponential_backoff


class Spawner(LoggingConfigurable):
    """Base class for spawning single-user notebook servers.

    Subclass this, and override the following methods:

    - load_state
    - get_state
    - start
    - stop
    - poll

    As JupyterHub supports multiple users, an instance of the Spawner subclass
    is created for each user. If there are 20 JupyterHub users, there will be 20
    instances of the subclass.
    """
    
    # private attributes for tracking status
    _spawn_pending = False
    _start_pending = False
    _stop_pending = False
    _proxy_pending = False
    _waiting_for_response = False
    _jupyterhub_version = None
    _spawn_future = None

    @property
    def _log_name(self):
        """Return username:servername or username

        Used in logging for consistency with named servers.
        """
        if self.name:
            return '%s:%s' % (self.user.name, self.name)
        else:
            return self.user.name

    @property
    def pending(self):
        """Return the current pending event, if any

        Return False if nothing is pending.
        """
        if self._spawn_pending:
            return 'spawn'
        elif self._stop_pending:
            return 'stop'
        return False

    @property
    def ready(self):
        """Is this server ready to use?

        A server is not ready if an event is pending.
        """
        if self.pending:
            return False
        if self.server is None:
            return False
        return True

    @property
    def active(self):
        """Return True if the server is active.

        This includes fully running and ready or any pending start/stop event.
        """
        return bool(self.pending or self.ready)


    authenticator = Any()
    hub = Any()
    orm_spawner = Any()
    db = Any()

    @observe('orm_spawner')
    def _orm_spawner_changed(self, change):
        if change.new and change.new.server:
            self._server = Server(orm_server=change.new.server)
        else:
            self._server = None

    user = Any()

    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__()

        missing = []
        for attr in ('start','stop', 'poll'):
            if getattr(Spawner, attr) is getattr(cls, attr):
                missing.append(attr)

        if missing:
            raise NotImplementedError("class `{}` needs to redefine the `start`,"
                  "`stop` and `poll` methods. `{}` not redefined.".format(cls.__name__, '`, `'.join(missing)))
    
    proxy_spec = Unicode()

    @property
    def server(self):
        if hasattr(self, '_server'):
            return self._server
        if self.orm_spawner and self.orm_spawner.server:
            return Server(orm_server=self.orm_spawner.server)

    @server.setter
    def server(self, server):
        self._server = server
        if self.orm_spawner:
            if self.orm_spawner.server is not None:
                # delete the old value
                db = inspect(self.orm_spawner.server).session
                db.delete(self.orm_spawner.server)
            if server is None:
                self.orm_spawner.server = None
            else:
                self.orm_spawner.server = server.orm_server

    @property
    def name(self):
        if self.orm_spawner:
            return self.orm_spawner.name
        return ''
    admin_access = Bool(False)
    api_token = Unicode()
    oauth_client_id = Unicode()

    will_resume = Bool(False,
        help="""Whether the Spawner will resume on next start


        Default is False where each launch of the Spawner will be a new instance.
        If True, an existing Spawner will resume instead of starting anew
        (e.g. resuming a Docker container),
        and API tokens in use when the Spawner stops will not be deleted.
        """
    )

    ip = Unicode('',
        help="""
        The IP address (or hostname) the single-user server should listen on.

        The JupyterHub proxy implementation should be able to send packets to this interface.
        """
    ).tag(config=True)

    port = Integer(0,
        help="""
        The port for single-user servers to listen on.

        Defaults to `0`, which uses a randomly allocated port number each time.

        If set to a non-zero value, all Spawners will use the same port,
        which only makes sense if each server is on a different address,
        e.g. in containers.

        New in version 0.7.
        """
    ).tag(config=True)

    start_timeout = Integer(60,
        help="""
        Timeout (in seconds) before giving up on starting of single-user server.

        This is the timeout for start to return, not the timeout for the server to respond.
        Callers of spawner.start will assume that startup has failed if it takes longer than this.
        start should return when the server process is started and its location is known.
        """
    ).tag(config=True)

    http_timeout = Integer(30,
        help="""
        Timeout (in seconds) before giving up on a spawned HTTP server

        Once a server has successfully been spawned, this is the amount of time
        we wait before assuming that the server is unable to accept
        connections.
        """
    ).tag(config=True)

    poll_interval = Integer(30,
        help="""
        Interval (in seconds) on which to poll the spawner for single-user server's status.

        At every poll interval, each spawner's `.poll` method is called, which checks
        if the single-user server is still running. If it isn't running, then JupyterHub modifies
        its own state accordingly and removes appropriate routes from the configurable proxy.
        """
    ).tag(config=True)

    _callbacks = List()
    _poll_callback = Any()

    debug = Bool(False,
        help="Enable debug-logging of the single-user server"
    ).tag(config=True)

    options_form = Unicode(
        help="""
        An HTML form for options a user can specify on launching their server.

        The surrounding `<form>` element and the submit button are already provided.

        For example:

        .. code:: html

            Set your key:
            <input name="key" val="default_key"></input>
            <br>
            Choose a letter:
            <select name="letter" multiple="true">
              <option value="A">The letter A</option>
              <option value="B">The letter B</option>
            </select>

        The data from this form submission will be passed on to your spawner in `self.user_options`
    """).tag(config=True)

    def options_from_form(self, form_data):
        """Interpret HTTP form data

        Form data will always arrive as a dict of lists of strings.
        Override this function to understand single-values, numbers, etc.

        This should coerce form data into the structure expected by self.user_options,
        which must be a dict.

        Instances will receive this data on self.user_options, after passing through this function,
        prior to `Spawner.start`.
        """
        return form_data

    user_options = Dict(
        help="""
        Dict of user specified options for the user's spawned instance of a single-user server.

        These user options are usually provided by the `options_form` displayed to the user when they start
        their server.
        """)

    env_keep = List([
        'PATH',
        'PYTHONPATH',
        'CONDA_ROOT',
        'CONDA_DEFAULT_ENV',
        'VIRTUAL_ENV',
        'LANG',
        'LC_ALL',
    ],
        help="""
        Whitelist of environment variables for the single-user server to inherit from the JupyterHub process.

        This whitelist is used to ensure that sensitive information in the JupyterHub process's environment
        (such as `CONFIGPROXY_AUTH_TOKEN`) is not passed to the single-user server's process.
        """
    ).tag(config=True)

    env = Dict(help="""Deprecated: use Spawner.get_env or Spawner.environment

    - extend Spawner.get_env for adding required env in Spawner subclasses
    - Spawner.environment for config-specified env
    """)

    environment = Dict(
        help="""
        Extra environment variables to set for the single-user server's process.

        Environment variables that end up in the single-user server's process come from 3 sources:
          - This `environment` configurable
          - The JupyterHub process' environment variables that are whitelisted in `env_keep`
          - Variables to establish contact between the single-user notebook and the hub (such as JUPYTERHUB_API_TOKEN)

        The `enviornment` configurable should be set by JupyterHub administrators to add
        installation specific environment variables. It is a dict where the key is the name of the environment
        variable, and the value can be a string or a callable. If it is a callable, it will be called
        with one parameter (the spawner instance), and should return a string fairly quickly (no blocking
        operations please!).

        Note that the spawner class' interface is not guaranteed to be exactly same across upgrades,
        so if you are using the callable take care to verify it continues to work after upgrades!
        """
    ).tag(config=True)

    cmd = Command(['jupyterhub-singleuser'],
        allow_none=True,
        help="""
        The command used for starting the single-user server.

        Provide either a string or a list containing the path to the startup script command. Extra arguments,
        other than this path, should be provided via `args`.

        This is usually set if you want to start the single-user server in a different python
        environment (with virtualenv/conda) than JupyterHub itself.

        Some spawners allow shell-style expansion here, allowing you to use environment variables.
        Most, including the default, do not. Consult the documentation for your spawner to verify!
        """
    ).tag(config=True)

    args = List(Unicode(),
        help="""
        Extra arguments to be passed to the single-user server.

        Some spawners allow shell-style expansion here, allowing you to use environment variables here.
        Most, including the default, do not. Consult the documentation for your spawner to verify!
        """
    ).tag(config=True)

    notebook_dir = Unicode(
        help="""
        Path to the notebook directory for the single-user server.

        The user sees a file listing of this directory when the notebook interface is started. The
        current interface does not easily allow browsing beyond the subdirectories in this directory's
        tree.

        `~` will be expanded to the home directory of the user, and {username} will be replaced
        with the name of the user.

        Note that this does *not* prevent users from accessing files outside of this path! They
        can do so with many other means.
        """
    ).tag(config=True)

    default_url = Unicode(
        help="""
        The URL the single-user server should start in.

        `{username}` will be expanded to the user's username

        Example uses:

        - You can set `notebook_dir` to `/` and `default_url` to `/tree/home/{username}` to allow people to
          navigate the whole filesystem from their notebook server, but still start in their home directory.
        - Start with `/notebooks` instead of `/tree` if `default_url` points to a notebook instead of a directory.
        - You can set this to `/lab` to have JupyterLab start by default, rather than Jupyter Notebook.
        """
    ).tag(config=True)

    @validate('notebook_dir', 'default_url')
    def _deprecate_percent_u(self, proposal):
        v = proposal['value']
        if '%U' in v:
            self.log.warning("%%U for username in %s is deprecated in JupyterHub 0.7, use {username}",
                proposal['trait'].name,
            )
            v = v.replace('%U', '{username}')
            self.log.warning("Converting %r to %r", proposal['value'], v)
        return v

    disable_user_config = Bool(False,
        help="""
        Disable per-user configuration of single-user servers.

        When starting the user's single-user server, any config file found in the user's $HOME directory
        will be ignored.

        Note: a user could circumvent this if the user modifies their Python environment, such as when
        they have their own conda environments / virtualenvs / containers.
        """
    ).tag(config=True)

    mem_limit = ByteSpecification(None,
        help="""
        Maximum number of bytes a single-user notebook server is allowed to use.

        Allows the following suffixes:
          - K -> Kilobytes
          - M -> Megabytes
          - G -> Gigabytes
          - T -> Terabytes

        If the single user server tries to allocate more memory than this,
        it will fail. There is no guarantee that the single-user notebook server
        will be able to allocate this much memory - only that it can not
        allocate more than this.

        This needs to be supported by your spawner for it to work.
        """
    ).tag(config=True)

    cpu_limit = Float(None,
        allow_none=True,
        help="""
        Maximum number of cpu-cores a single-user notebook server is allowed to use.

        If this value is set to 0.5, allows use of 50% of one CPU.
        If this value is set to 2, allows use of up to 2 CPUs.

        The single-user notebook server will never be scheduled by the kernel to
        use more cpu-cores than this. There is no guarantee that it can
        access this many cpu-cores.

        This needs to be supported by your spawner for it to work.
        """
    ).tag(config=True)

    mem_guarantee = ByteSpecification(None,
        help="""
        Minimum number of bytes a single-user notebook server is guaranteed to have available.

        Allows the following suffixes:
          - K -> Kilobytes
          - M -> Megabytes
          - G -> Gigabytes
          - T -> Terabytes

        This needs to be supported by your spawner for it to work.
        """
    ).tag(config=True)

    cpu_guarantee = Float(None,
        allow_none=True,
        help="""
        Minimum number of cpu-cores a single-user notebook server is guaranteed to have available.

        If this value is set to 0.5, allows use of 50% of one CPU.
        If this value is set to 2, allows use of up to 2 CPUs.

        Note that this needs to be supported by your spawner for it to work.
        """
    ).tag(config=True)

    pre_spawn_hook = Any(
        help="""
        An optional hook function that you can implement to do some bootstrapping work before
        the spawner starts. For example, create a directory for your user or load initial content.

        This can be set independent of any concrete spawner implementation.

        Example::

            from subprocess import check_call
            def my_hook(spawner):
                username = spawner.user.name
                check_call(['./examples/bootstrap-script/bootstrap.sh', username])

            c.Spawner.pre_spawn_hook = my_hook

        """
    ).tag(config=True)

    def load_state(self, state):
        """Restore state of spawner from database.

        Called for each user's spawner after the hub process restarts.

        `state` is a dict that'll contain the value returned by `get_state` of
        the spawner, or {} if the spawner hasn't persisted any state yet.

        Override in subclasses to restore any extra state that is needed to track
        the single-user server for that user. Subclasses should call super().
        """
        pass

    def get_state(self):
        """Save state of spawner into database.

        A black box of extra state for custom spawners. The returned value of this is
        passed to `load_state`.

        Subclasses should call `super().get_state()`, augment the state returned from
        there, and return that state.

        Returns
        -------
        state: dict
             a JSONable dict of state
        """
        state = {}
        return state

    def clear_state(self):
        """Clear any state that should be cleared when the single-user server stops.

        State that should be preserved across single-user server instances should not be cleared.

        Subclasses should call super, to ensure that state is properly cleared.
        """
        self.api_token = ''

    def get_env(self):
        """Return the environment dict to use for the Spawner.

        This applies things like `env_keep`, anything defined in `Spawner.environment`,
        and adds the API token to the env.

        When overriding in subclasses, subclasses must call `super().get_env()`, extend the
        returned dict and return it.

        Use this to access the env in Spawner.start to allow extension in subclasses.
        """
        env = {}
        if self.env:
            warnings.warn("Spawner.env is deprecated, found %s" % self.env, DeprecationWarning)
            env.update(self.env)

        for key in self.env_keep:
            if key in os.environ:
                env[key] = os.environ[key]

        # config overrides. If the value is a callable, it will be called with
        # one parameter - the current spawner instance - and the return value
        # will be assigned to the environment variable. This will be called at
        # spawn time.
        for key, value in self.environment.items():
            if callable(value):
                env[key] = value(self)
            else:
                env[key] = value

        env['JUPYTERHUB_API_TOKEN'] = self.api_token
        # deprecated (as of 0.7.2), for old versions of singleuser
        env['JPY_API_TOKEN'] = self.api_token
        if self.admin_access:
            env['JUPYTERHUB_ADMIN_ACCESS'] = '1'
        # OAuth settings
        env['JUPYTERHUB_CLIENT_ID'] = self.oauth_client_id
        env['JUPYTERHUB_HOST'] = self.hub.public_host
        env['JUPYTERHUB_OAUTH_CALLBACK_URL'] = \
            url_path_join(self.user.url, self.name, 'oauth_callback')

        # Info previously passed on args
        env['JUPYTERHUB_USER'] = self.user.name
        env['JUPYTERHUB_API_URL'] = self.hub.api_url
        env['JUPYTERHUB_BASE_URL'] = self.hub.base_url[:-4]
        if self.server:
            env['JUPYTERHUB_SERVICE_PREFIX'] = self.server.base_url

        # Put in limit and guarantee info if they exist.
        # Note that this is for use by the humans / notebook extensions in the
        # single-user notebook server, and not for direct usage by the spawners
        # themselves. Spawners should just use the traitlets directly.
        if self.mem_limit:
            env['MEM_LIMIT'] = str(self.mem_limit)
        if self.mem_guarantee:
            env['MEM_GUARANTEE'] = str(self.mem_guarantee)
        if self.cpu_limit:
            env['CPU_LIMIT'] = str(self.cpu_limit)
        if self.cpu_guarantee:
            env['CPU_GUARANTEE'] = str(self.cpu_guarantee)

        return env

    def template_namespace(self):
        """Return the template namespace for format-string formatting.

        Currently used on default_url and notebook_dir.

        Subclasses may add items to the available namespace.

        The default implementation includes::

            {
              'username': user.name,
              'base_url': users_base_url,
            }

        Returns:

            ns (dict): namespace for string formatting.
        """
        d = {'username': self.user.name}
        if self.server:
            d['base_url'] = self.server.base_url
        return d

    def format_string(self, s):
        """Render a Python format string

        Uses :meth:`Spawner.template_namespace` to populate format namespace.

        Args:

            s (str): Python format-string to be formatted.

        Returns:

            str: Formatted string, rendered
        """
        return s.format(**self.template_namespace())

    def get_args(self):
        """Return the arguments to be passed after self.cmd

        Doesn't expect shell expansion to happen.
        """
        args = []

        if self.ip:
            args.append('--ip="%s"' % self.ip)

        if self.port:
            args.append('--port=%i' % self.port)
        elif self.server.port:
            self.log.warning("Setting port from user.server is deprecated as of JupyterHub 0.7.")
            args.append('--port=%i' % self.server.port)

        if self.notebook_dir:
            notebook_dir = self.format_string(self.notebook_dir)
            args.append('--notebook-dir="%s"' % notebook_dir)
        if self.default_url:
            default_url = self.format_string(self.default_url)
            args.append('--NotebookApp.default_url="%s"' % default_url)

        if self.debug:
            args.append('--debug')
        if self.disable_user_config:
            args.append('--disable-user-config')
        args.extend(self.args)
        return args

    def run_pre_spawn_hook(self):
        """Run the pre_spawn_hook if defined"""
        if self.pre_spawn_hook:
            return self.pre_spawn_hook(self)

    @gen.coroutine
    def start(self):
        """Start the single-user server

        Returns:
          (str, int): the (ip, port) where the Hub can connect to the server.

        .. versionchanged:: 0.7
            Return ip, port instead of setting on self.user.server directly.
        """
        raise NotImplementedError("Override in subclass. Must be a Tornado gen.coroutine.")

    @gen.coroutine
    def stop(self, now=False):
        """Stop the single-user server

        If `now` is False (default), shutdown the server as gracefully as possible,
        e.g. starting with SIGINT, then SIGTERM, then SIGKILL.
        If `now` is True, terminate the server immediately.

        The coroutine should return when the single-user server process is no longer running.

        Must be a coroutine.
        """
        raise NotImplementedError("Override in subclass. Must be a Tornado gen.coroutine.")

    @gen.coroutine
    def poll(self):
        """Check if the single-user process is running

        Returns:
          None if single-user process is running.
          Integer exit status (0 if unknown), if it is not running.

        State transitions, behavior, and return response:

        - If the Spawner has not been initialized (neither loaded state, nor called start),
          it should behave as if it is not running (status=0).
        - If the Spawner has not finished starting,
          it should behave as if it is running (status=None).

        Design assumptions about when `poll` may be called:

        - On Hub launch: `poll` may be called before `start` when state is loaded on Hub launch.
          `poll` should return exit status 0 (unknown) if the Spawner has not been initialized via
          `load_state` or `start`.
        - If `.start()` is async: `poll` may be called during any yielded portions of the `start`
          process. `poll` should return None when `start` is yielded, indicating that the `start`
          process has not yet completed.

        """
        raise NotImplementedError("Override in subclass. Must be a Tornado gen.coroutine.")

    def add_poll_callback(self, callback, *args, **kwargs):
        """Add a callback to fire when the single-user server stops"""
        if args or kwargs:
            cb = callback
            callback = lambda: cb(*args, **kwargs)
        self._callbacks.append(callback)

    def stop_polling(self):
        """Stop polling for single-user server's running state"""
        if self._poll_callback:
            self._poll_callback.stop()
            self._poll_callback = None

    def start_polling(self):
        """Start polling periodically for single-user server's running state.

        Callbacks registered via `add_poll_callback` will fire if/when the server stops.
        Explicit termination via the stop method will not trigger the callbacks.
        """
        if self.poll_interval <= 0:
            self.log.debug("Not polling subprocess")
            return
        else:
            self.log.debug("Polling subprocess every %is", self.poll_interval)

        self.stop_polling()

        self._poll_callback = PeriodicCallback(
            self.poll_and_notify,
            1e3 * self.poll_interval
        )
        self._poll_callback.start()

    @gen.coroutine
    def poll_and_notify(self):
        """Used as a callback to periodically poll the process and notify any watchers"""
        status = yield self.poll()
        if status is None:
            # still running, nothing to do here
            return

        self.stop_polling()

        # clear callbacks list
        self._callbacks, callbacks = ([], self._callbacks)

        for callback in callbacks:
            try:
                yield gen.maybe_future(callback())
            except Exception:
                self.log.exception("Unhandled error in poll callback for %s", self)
        return status

    death_interval = Float(0.1)
    @gen.coroutine
    def wait_for_death(self, timeout=10):
        """Wait for the single-user server to die, up to timeout seconds"""
        @gen.coroutine
        def _wait_for_death():
            status = yield self.poll()
            return status is not None

        try:
            r = yield exponential_backoff(
                _wait_for_death,
                'Process did not die in {timeout} seconds'.format(timeout=timeout),
                start_wait=self.death_interval,
                timeout=timeout,
            )
            return r
        except TimeoutError:
            return False


def _try_setcwd(path):
    """Try to set CWD to path, walking up until a valid directory is found.

    If no valid directory is found, a temp directory is created and cwd is set to that.
    """
    while path != '/':
        try:
            os.chdir(path)
        except OSError as e:
            exc = e  # break exception instance out of except scope
            print("Couldn't set CWD to %s (%s)" % (path, e), file=sys.stderr)
            path, _ = os.path.split(path)
        else:
            return
    print("Couldn't set CWD at all (%s), using temp dir" % exc, file=sys.stderr)
    td = mkdtemp()
    os.chdir(td)


def set_user_setuid(username, chdir=True):
    """Return a preexec_fn for spawning a single-user server as a particular user.

    Returned preexec_fn will set uid/gid, and attempt to chdir to the target user's
    home directory.
    """
    import grp
    import pwd
    user = pwd.getpwnam(username)
    uid = user.pw_uid
    gid = user.pw_gid
    home = user.pw_dir
    gids = [ g.gr_gid for g in grp.getgrall() if username in g.gr_mem ]

    def preexec():
        """Set uid/gid of current process

        Executed after fork but before exec by python.

        Also try to chdir to the user's home directory.
        """
        os.setgid(gid)
        try:
            os.setgroups(gids)
        except Exception as e:
            print('Failed to set groups %s' % e, file=sys.stderr)
        os.setuid(uid)

        # start in the user's home dir
        if chdir:
            _try_setcwd(home)

    return preexec


class LocalProcessSpawner(Spawner):
    """
    A Spawner that uses `subprocess.Popen` to start single-user servers as local processes.

    Requires local UNIX users matching the authenticated users to exist.
    Does not work on Windows.

    This is the default spawner for JupyterHub.
    """

    interrupt_timeout = Integer(10,
        help="""
        Seconds to wait for single-user server process to halt after SIGINT.

        If the process has not exited cleanly after this many seconds, a SIGTERM is sent.
        """
    ).tag(config=True)

    term_timeout = Integer(5,
        help="""
        Seconds to wait for single-user server process to halt after SIGTERM.

        If the process does not exit cleanly after this many seconds of SIGTERM, a SIGKILL is sent.
        """
    ).tag(config=True)

    kill_timeout = Integer(5,
        help="""
        Seconds to wait for process to halt after SIGKILL before giving up.

        If the process does not exit cleanly after this many seconds of SIGKILL, it becomes a zombie
        process. The hub process will log a warning and then give up.
        """
    ).tag(config=True)

    popen_kwargs = Dict(
        help="""Extra keyword arguments to pass to Popen

        when spawning single-user servers.

        For example::

            popen_kwargs = dict(shell=True)

        """
    ).tag(config=True)
    shell_cmd = Command(minlen=0,
        help="""Specify a shell command to launch.

        The single-user command will be appended to this list,
        so it sould end with `-c` (for bash) or equivalent.

        For example::

            c.LocalProcessSpawner.shell_cmd = ['bash', '-l', '-c']

        to launch with a bash login shell, which would set up the user's own complete environment.

        .. warning::

            Using shell_cmd gives users control over PATH, etc.,
            which could change what the jupyterhub-singleuser launch command does.
            Only use this for trusted users.
        """
    )

    proc = Instance(Popen,
        allow_none=True,
        help="""
        The process representing the single-user server process spawned for current user.

        Is None if no process has been spawned yet.
        """)
    pid = Integer(0,
        help="""
        The process id (pid) of the single-user server process spawned for current user.
        """
    )

    def make_preexec_fn(self, name):
        """
        Return a function that can be used to set the user id of the spawned process to user with name `name`

        This function can be safely passed to `preexec_fn` of `Popen`
        """
        return set_user_setuid(name)

    def load_state(self, state):
        """Restore state about spawned single-user server after a hub restart.

        Local processes only need the process id.
        """
        super(LocalProcessSpawner, self).load_state(state)
        if 'pid' in state:
            self.pid = state['pid']

    def get_state(self):
        """Save state that is needed to restore this spawner instance after a hub restore.

        Local processes only need the process id.
        """
        state = super(LocalProcessSpawner, self).get_state()
        if self.pid:
            state['pid'] = self.pid
        return state

    def clear_state(self):
        """Clear stored state about this spawner (pid)"""
        super(LocalProcessSpawner, self).clear_state()
        self.pid = 0

    def user_env(self, env):
        """Augment environment of spawned process with user specific env variables."""
        import pwd
        env['USER'] = self.user.name
        home = pwd.getpwnam(self.user.name).pw_dir
        shell = pwd.getpwnam(self.user.name).pw_shell
        # These will be empty if undefined,
        # in which case don't set the env:
        if home:
            env['HOME'] = home
        if shell:
            env['SHELL'] = shell
        return env

    def get_env(self):
        """Get the complete set of environment variables to be set in the spawned process."""
        env = super().get_env()
        env = self.user_env(env)
        return env

    @gen.coroutine
    def start(self):
        """Start the single-user server."""
        self.port = random_port()
        cmd = []
        env = self.get_env()

        cmd.extend(self.cmd)
        cmd.extend(self.get_args())

        if self.shell_cmd:
            # using shell_cmd (e.g. bash -c),
            # add our cmd list as the last (single) argument:
            cmd = self.shell_cmd + [' '.join(pipes.quote(s) for s in cmd)]

        self.log.info("Spawning %s", ' '.join(pipes.quote(s) for s in cmd))

        popen_kwargs = dict(
            preexec_fn=self.make_preexec_fn(self.user.name),
            start_new_session=True,  # don't forward signals
        )
        popen_kwargs.update(self.popen_kwargs)
        # don't let user config override env
        popen_kwargs['env'] = env
        try:
            self.proc = Popen(cmd, **popen_kwargs)
        except PermissionError:
            # use which to get abspath
            script = shutil.which(cmd[0]) or cmd[0]
            self.log.error("Permission denied trying to run %r. Does %s have access to this file?",
                script, self.user.name,
            )
            raise

        self.pid = self.proc.pid

        if self.__class__ is not LocalProcessSpawner:
            # subclasses may not pass through return value of super().start,
            # relying on deprecated 0.6 way of setting ip, port,
            # so keep a redundant copy here for now.
            # A deprecation warning will be shown if the subclass
            # does not return ip, port.
            if self.ip:
                self.server.ip = self.ip
            self.server.port = self.port
        return (self.ip or '127.0.0.1', self.port)

    @gen.coroutine
    def poll(self):
        """Poll the spawned process to see if it is still running.

        If the process is still running, we return None. If it is not running,
        we return the exit code of the process if we have access to it, or 0 otherwise.
        """
        # if we started the process, poll with Popen
        if self.proc is not None:
            status = self.proc.poll()
            if status is not None:
                # clear state if the process is done
                self.clear_state()
            return status

        # if we resumed from stored state,
        # we don't have the Popen handle anymore, so rely on self.pid
        if not self.pid:
            # no pid, not running
            self.clear_state()
            return 0

        # send signal 0 to check if PID exists
        # this doesn't work on Windows, but that's okay because we don't support Windows.
        alive = yield self._signal(0)
        if not alive:
            self.clear_state()
            return 0
        else:
            return None

    @gen.coroutine
    def _signal(self, sig):
        """Send given signal to a single-user server's process.

        Returns True if the process still exists, False otherwise.

        The hub process is assumed to have enough privileges to do this (e.g. root).
        """
        try:
            os.kill(self.pid, sig)
        except OSError as e:
            if e.errno == errno.ESRCH:
                return False  # process is gone
            else:
                raise
        return True  # process exists

    @gen.coroutine
    def stop(self, now=False):
        """Stop the single-user server process for the current user.

        If `now` is False (default), shutdown the server as gracefully as possible,
        e.g. starting with SIGINT, then SIGTERM, then SIGKILL.
        If `now` is True, terminate the server immediately.

        The coroutine should return when the process is no longer running.
        """
        if not now:
            status = yield self.poll()
            if status is not None:
                return
            self.log.debug("Interrupting %i", self.pid)
            yield self._signal(signal.SIGINT)
            yield self.wait_for_death(self.interrupt_timeout)

        # clean shutdown failed, use TERM
        status = yield self.poll()
        if status is not None:
            return
        self.log.debug("Terminating %i", self.pid)
        yield self._signal(signal.SIGTERM)
        yield self.wait_for_death(self.term_timeout)

        # TERM failed, use KILL
        status = yield self.poll()
        if status is not None:
            return
        self.log.debug("Killing %i", self.pid)
        yield self._signal(signal.SIGKILL)
        yield self.wait_for_death(self.kill_timeout)

        status = yield self.poll()
        if status is None:
            # it all failed, zombie process
            self.log.warning("Process %i never died", self.pid)