handlers.py

"""Base Tornado handlers for the Jupyter server."""
# Copyright (c) Jupyter Development Team.
# Distributed under the terms of the Modified BSD License.
import datetime
import functools
import ipaddress
import json
import mimetypes
import os
import re
import traceback
import types
import warnings
from http.client import responses
from http.cookies import Morsel
from urllib.parse import urlparse

import prometheus_client
from jinja2 import TemplateNotFound
from jupyter_core.paths import is_hidden
from tornado import escape, httputil, web
from tornado.log import app_log
from traitlets.config import Application

import jupyter_server
from jupyter_server._sysinfo import get_sys_info
from jupyter_server._tz import utcnow
from jupyter_server.i18n import combine_translations
from jupyter_server.services.security import csp_report_uri
from jupyter_server.utils import (
    ensure_async,
    filefind,
    url_escape,
    url_is_absolute,
    url_path_join,
    urldecode_unix_socket_path,
)

# -----------------------------------------------------------------------------
# Top-level handlers
# -----------------------------------------------------------------------------
non_alphanum = re.compile(r"[^A-Za-z0-9]")

_sys_info_cache = None


def json_sys_info():
    global _sys_info_cache
    if _sys_info_cache is None:
        _sys_info_cache = json.dumps(get_sys_info())
    return _sys_info_cache


def log():
    if Application.initialized():
        return Application.instance().log
    else:
        return app_log


class AuthenticatedHandler(web.RequestHandler):
    """A RequestHandler with an authenticated user."""

    @property
    def content_security_policy(self):
        """The default Content-Security-Policy header

        Can be overridden by defining Content-Security-Policy in settings['headers']
        """
        if "Content-Security-Policy" in self.settings.get("headers", {}):
            # user-specified, don't override
            return self.settings["headers"]["Content-Security-Policy"]

        return "; ".join(
            [
                "frame-ancestors 'self'",
                # Make sure the report-uri is relative to the base_url
                "report-uri "
                + self.settings.get("csp_report_uri", url_path_join(self.base_url, csp_report_uri)),
            ]
        )

    def set_default_headers(self):
        headers = {}
        headers["X-Content-Type-Options"] = "nosniff"
        headers.update(self.settings.get("headers", {}))

        headers["Content-Security-Policy"] = self.content_security_policy

        # Allow for overriding headers
        for header_name, value in headers.items():
            try:
                self.set_header(header_name, value)
            except Exception as e:
                # tornado raise Exception (not a subclass)
                # if method is unsupported (websocket and Access-Control-Allow-Origin
                # for example, so just ignore)
                self.log.debug(e)

    def force_clear_cookie(self, name, path="/", domain=None):
        """Deletes the cookie with the given name.

        Tornado's cookie handling currently (Jan 2018) stores cookies in a dict
        keyed by name, so it can only modify one cookie with a given name per
        response. The browser can store multiple cookies with the same name
        but different domains and/or paths. This method lets us clear multiple
        cookies with the same name.

        Due to limitations of the cookie protocol, you must pass the same
        path and domain to clear a cookie as were used when that cookie
        was set (but there is no way to find out on the server side
        which values were used for a given cookie).
        """
        name = escape.native_str(name)
        expires = datetime.datetime.utcnow() - datetime.timedelta(days=365)

        morsel: Morsel = Morsel()
        morsel.set(name, "", '""')
        morsel["expires"] = httputil.format_timestamp(expires)
        morsel["path"] = path
        if domain:
            morsel["domain"] = domain
        self.add_header("Set-Cookie", morsel.OutputString())

    def clear_login_cookie(self):
        cookie_options = self.settings.get("cookie_options", {})
        path = cookie_options.setdefault("path", self.base_url)
        self.clear_cookie(self.cookie_name, path=path)
        if path and path != "/":
            # also clear cookie on / to ensure old cookies are cleared
            # after the change in path behavior.
            # N.B. This bypasses the normal cookie handling, which can't update
            # two cookies with the same name. See the method above.
            self.force_clear_cookie(self.cookie_name)

    def get_current_user(self):
        if self.login_handler is None:
            return "anonymous"
        return self.login_handler.get_user(self)

    def skip_check_origin(self):
        """Ask my login_handler if I should skip the origin_check

        For example: in the default LoginHandler, if a request is token-authenticated,
        origin checking should be skipped.
        """
        if self.request.method == "OPTIONS":
            # no origin-check on options requests, which are used to check origins!
            return True
        if self.login_handler is None or not hasattr(self.login_handler, "should_check_origin"):
            return False
        return not self.login_handler.should_check_origin(self)

    @property
    def token_authenticated(self):
        """Have I been authenticated with a token?"""
        if self.login_handler is None or not hasattr(self.login_handler, "is_token_authenticated"):
            return False
        return self.login_handler.is_token_authenticated(self)

    @property
    def cookie_name(self):
        default_cookie_name = non_alphanum.sub("-", f"username-{self.request.host}")
        return self.settings.get("cookie_name", default_cookie_name)

    @property
    def logged_in(self):
        """Is a user currently logged in?"""
        user = self.get_current_user()
        return user and not user == "anonymous"

    @property
    def login_handler(self):
        """Return the login handler for this application, if any."""
        return self.settings.get("login_handler_class", None)

    @property
    def token(self):
        """Return the login token for this application, if any."""
        return self.settings.get("token", None)

    @property
    def login_available(self):
        """May a user proceed to log in?

        This returns True if login capability is available, irrespective of
        whether the user is already logged in or not.

        """
        if self.login_handler is None:
            return False
        return bool(self.login_handler.get_login_available(self.settings))

    @property
    def authorizer(self):
        if "authorizer" not in self.settings:
            warnings.warn(
                "The Tornado web application does not have an 'authorizer' defined "
                "in its settings. In future releases of jupyter_server, this will "
                "be a required key for all subclasses of `JupyterHandler`. For an "
                "example, see the jupyter_server source code for how to "
                "add an authorizer to the tornado settings: "
                "https://github.com/jupyter-server/jupyter_server/blob/"
                "653740cbad7ce0c8a8752ce83e4d3c2c754b13cb/jupyter_server/serverapp.py"
                "#L234-L256",
            )
            from jupyter_server.auth import AllowAllAuthorizer

            self.settings["authorizer"] = AllowAllAuthorizer(
                config=self.settings.get("config", None)
            )
        return self.settings["authorizer"]


class JupyterHandler(AuthenticatedHandler):
    """Jupyter-specific extensions to authenticated handling

    Mostly property shortcuts to Jupyter-specific settings.
    """

    @property
    def config(self):
        return self.settings.get("config", None)

    @property
    def log(self):
        """use the Jupyter log by default, falling back on tornado's logger"""
        return log()

    @property
    def jinja_template_vars(self):
        """User-supplied values to supply to jinja templates."""
        return self.settings.get("jinja_template_vars", {})

    @property
    def serverapp(self):
        return self.settings["serverapp"]

    # ---------------------------------------------------------------
    # URLs
    # ---------------------------------------------------------------

    @property
    def version_hash(self):
        """The version hash to use for cache hints for static files"""
        return self.settings.get("version_hash", "")

    @property
    def mathjax_url(self):
        url = self.settings.get("mathjax_url", "")
        if not url or url_is_absolute(url):
            return url
        return url_path_join(self.base_url, url)

    @property
    def mathjax_config(self):
        return self.settings.get("mathjax_config", "TeX-AMS-MML_HTMLorMML-full,Safe")

    @property
    def base_url(self) -> str:
        return self.settings.get("base_url", "/")

    @property
    def default_url(self):
        return self.settings.get("default_url", "")

    @property
    def ws_url(self):
        return self.settings.get("websocket_url", "")

    @property
    def contents_js_source(self):
        self.log.debug(
            "Using contents: %s",
            self.settings.get("contents_js_source", "services/contents"),
        )
        return self.settings.get("contents_js_source", "services/contents")

    # ---------------------------------------------------------------
    # Manager objects
    # ---------------------------------------------------------------

    @property
    def kernel_manager(self):
        return self.settings["kernel_manager"]

    @property
    def contents_manager(self):
        return self.settings["contents_manager"]

    @property
    def session_manager(self):
        return self.settings["session_manager"]

    @property
    def terminal_manager(self):
        return self.settings["terminal_manager"]

    @property
    def kernel_spec_manager(self):
        return self.settings["kernel_spec_manager"]

    @property
    def config_manager(self):
        return self.settings["config_manager"]

    # ---------------------------------------------------------------
    # CORS
    # ---------------------------------------------------------------

    @property
    def allow_origin(self):
        """Normal Access-Control-Allow-Origin"""
        return self.settings.get("allow_origin", "")

    @property
    def allow_origin_pat(self):
        """Regular expression version of allow_origin"""
        return self.settings.get("allow_origin_pat", None)

    @property
    def allow_credentials(self):
        """Whether to set Access-Control-Allow-Credentials"""
        return self.settings.get("allow_credentials", False)

    def set_default_headers(self):
        """Add CORS headers, if defined"""
        super().set_default_headers()
        if self.allow_origin:
            self.set_header("Access-Control-Allow-Origin", self.allow_origin)
        elif self.allow_origin_pat:
            origin = self.get_origin()
            if origin and re.match(self.allow_origin_pat, origin):
                self.set_header("Access-Control-Allow-Origin", origin)
        elif self.token_authenticated and "Access-Control-Allow-Origin" not in self.settings.get(
            "headers", {}
        ):
            # allow token-authenticated requests cross-origin by default.
            # only apply this exception if allow-origin has not been specified.
            self.set_header("Access-Control-Allow-Origin", self.request.headers.get("Origin", ""))

        if self.allow_credentials:
            self.set_header("Access-Control-Allow-Credentials", "true")

    def set_attachment_header(self, filename):
        """Set Content-Disposition: attachment header

        As a method to ensure handling of filename encoding
        """
        escaped_filename = url_escape(filename)
        self.set_header(
            "Content-Disposition",
            "attachment;"
            " filename*=utf-8''{utf8}".format(
                utf8=escaped_filename,
            ),
        )

    def get_origin(self):
        # Handle WebSocket Origin naming convention differences
        # The difference between version 8 and 13 is that in 8 the
        # client sends a "Sec-Websocket-Origin" header and in 13 it's
        # simply "Origin".
        if "Origin" in self.request.headers:
            origin = self.request.headers.get("Origin")
        else:
            origin = self.request.headers.get("Sec-Websocket-Origin", None)
        return origin

    # origin_to_satisfy_tornado is present because tornado requires
    # check_origin to take an origin argument, but we don't use it
    def check_origin(self, origin_to_satisfy_tornado=""):
        """Check Origin for cross-site API requests, including websockets

        Copied from WebSocket with changes:

        - allow unspecified host/origin (e.g. scripts)
        - allow token-authenticated requests
        """
        if self.allow_origin == "*" or self.skip_check_origin():
            return True

        host = self.request.headers.get("Host")
        origin = self.request.headers.get("Origin")

        # If no header is provided, let the request through.
        # Origin can be None for:
        # - same-origin (IE, Firefox)
        # - Cross-site POST form (IE, Firefox)
        # - Scripts
        # The cross-site POST (XSRF) case is handled by tornado's xsrf_token
        if origin is None or host is None:
            return True

        origin = origin.lower()
        origin_host = urlparse(origin).netloc

        # OK if origin matches host
        if origin_host == host:
            return True

        # Check CORS headers
        if self.allow_origin:
            allow = self.allow_origin == origin
        elif self.allow_origin_pat:
            allow = bool(re.match(self.allow_origin_pat, origin))
        else:
            # No CORS headers deny the request
            allow = False
        if not allow:
            self.log.warning(
                "Blocking Cross Origin API request for %s.  Origin: %s, Host: %s",
                self.request.path,
                origin,
                host,
            )
        return allow

    def check_referer(self):
        """Check Referer for cross-site requests.
        Disables requests to certain endpoints with
        external or missing Referer.
        If set, allow_origin settings are applied to the Referer
        to whitelist specific cross-origin sites.
        Used on GET for api endpoints and /files/
        to block cross-site inclusion (XSSI).
        """
        if self.allow_origin == "*" or self.skip_check_origin():
            return True

        host = self.request.headers.get("Host")
        referer = self.request.headers.get("Referer")

        if not host:
            self.log.warning("Blocking request with no host")
            return False
        if not referer:
            self.log.warning("Blocking request with no referer")
            return False

        referer_url = urlparse(referer)
        referer_host = referer_url.netloc
        if referer_host == host:
            return True

        # apply cross-origin checks to Referer:
        origin = f"{referer_url.scheme}://{referer_url.netloc}"
        if self.allow_origin:
            allow = self.allow_origin == origin
        elif self.allow_origin_pat:
            allow = bool(re.match(self.allow_origin_pat, origin))
        else:
            # No CORS settings, deny the request
            allow = False

        if not allow:
            self.log.warning(
                "Blocking Cross Origin request for %s.  Referer: %s, Host: %s",
                self.request.path,
                origin,
                host,
            )
        return allow

    def check_xsrf_cookie(self):
        """Bypass xsrf cookie checks when token-authenticated"""
        if self.token_authenticated or self.settings.get("disable_check_xsrf", False):
            # Token-authenticated requests do not need additional XSRF-check
            # Servers without authentication are vulnerable to XSRF
            return
        try:
            return super().check_xsrf_cookie()
        except web.HTTPError:
            if self.request.method in {"GET", "HEAD"}:
                # Consider Referer a sufficient cross-origin check for GET requests
                if not self.check_referer():
                    referer = self.request.headers.get("Referer")
                    if referer:
                        msg = f"Blocking Cross Origin request from {referer}."
                    else:
                        msg = "Blocking request from unknown origin"
                    raise web.HTTPError(403, msg)
            else:
                raise

    def check_host(self):
        """Check the host header if remote access disallowed.

        Returns True if the request should continue, False otherwise.
        """
        if self.settings.get("allow_remote_access", False):
            return True

        # Remove port (e.g. ':8888') from host
        match = re.match(r"^(.*?)(:\d+)?$", self.request.host)
        assert match is not None
        host = match.group(1)

        # Browsers format IPv6 addresses like [::1]; we need to remove the []
        if host.startswith("[") and host.endswith("]"):
            host = host[1:-1]

        # UNIX socket handling
        check_host = urldecode_unix_socket_path(host)
        if check_host.startswith("/") and os.path.exists(check_host):
            allow = True
        else:
            try:
                addr = ipaddress.ip_address(host)
            except ValueError:
                # Not an IP address: check against hostnames
                allow = host in self.settings.get("local_hostnames", ["localhost"])
            else:
                allow = addr.is_loopback

        if not allow:
            self.log.warning(
                (
                    "Blocking request with non-local 'Host' %s (%s). "
                    "If the server should be accessible at that name, "
                    "set ServerApp.allow_remote_access to disable the check."
                ),
                host,
                self.request.host,
            )
        return allow

    def prepare(self):
        if not self.check_host():
            raise web.HTTPError(403)
        return super().prepare()

    # ---------------------------------------------------------------
    # template rendering
    # ---------------------------------------------------------------

    def get_template(self, name):
        """Return the jinja template object for a given name"""
        return self.settings["jinja2_env"].get_template(name)

    def render_template(self, name, **ns):
        ns.update(self.template_namespace)
        template = self.get_template(name)
        return template.render(**ns)

    @property
    def template_namespace(self):
        return dict(
            base_url=self.base_url,
            default_url=self.default_url,
            ws_url=self.ws_url,
            logged_in=self.logged_in,
            allow_password_change=self.settings.get("allow_password_change"),
            login_available=self.login_available,
            token_available=bool(self.token),
            static_url=self.static_url,
            sys_info=json_sys_info(),
            contents_js_source=self.contents_js_source,
            version_hash=self.version_hash,
            xsrf_form_html=self.xsrf_form_html,
            token=self.token,
            xsrf_token=self.xsrf_token.decode("utf8"),
            nbjs_translations=json.dumps(
                combine_translations(self.request.headers.get("Accept-Language", ""))
            ),
            **self.jinja_template_vars,
        )

    def get_json_body(self):
        """Return the body of the request as JSON data."""
        if not self.request.body:
            return None
        # Do we need to call body.decode('utf-8') here?
        body = self.request.body.strip().decode("utf-8")
        try:
            model = json.loads(body)
        except Exception as e:
            self.log.debug("Bad JSON: %r", body)
            self.log.error("Couldn't parse JSON", exc_info=True)
            raise web.HTTPError(400, "Invalid JSON in body of request") from e
        return model

    def write_error(self, status_code, **kwargs):
        """render custom error pages"""
        exc_info = kwargs.get("exc_info")
        message = ""
        status_message = responses.get(status_code, "Unknown HTTP Error")

        if exc_info:
            exception = exc_info[1]
            # get the custom message, if defined
            try:
                message = exception.log_message % exception.args
            except Exception:
                pass

            # construct the custom reason, if defined
            reason = getattr(exception, "reason", "")
            if reason:
                status_message = reason
        else:
            exception = "(unknown)"

        # build template namespace
        ns = dict(
            status_code=status_code,
            status_message=status_message,
            message=message,
            exception=exception,
        )

        self.set_header("Content-Type", "text/html")
        # render the template
        try:
            html = self.render_template("%s.html" % status_code, **ns)
        except TemplateNotFound:
            html = self.render_template("error.html", **ns)

        self.write(html)


class IPythonHandler(JupyterHandler):
    """The IPythonHandler ensure backwards compatibility
    for the notebook extensions running on nbclassic.
    """

    pass


class APIHandler(JupyterHandler):
    """Base class for API handlers"""

    _user_cache: dict

    def prepare(self):
        if not self.check_origin():
            raise web.HTTPError(404)
        return super().prepare()

    def write_error(self, status_code, **kwargs):
        """APIHandler errors are JSON, not human pages"""
        self.set_header("Content-Type", "application/json")
        message = responses.get(status_code, "Unknown HTTP Error")
        reply: dict = {
            "message": message,
        }
        exc_info = kwargs.get("exc_info")
        if exc_info:
            e = exc_info[1]
            if isinstance(e, HTTPError):
                reply["message"] = e.log_message or message
                reply["reason"] = e.reason
            else:
                reply["message"] = "Unhandled error"
                reply["reason"] = None
                reply["traceback"] = "".join(traceback.format_exception(*exc_info))
        self.log.warning(reply["message"])
        self.finish(json.dumps(reply))

    def get_current_user(self):
        """Raise 403 on API handlers instead of redirecting to human login page"""
        # preserve _user_cache so we don't raise more than once
        if hasattr(self, "_user_cache"):
            return self._user_cache
        self._user_cache = user = super().get_current_user()
        return user

    def get_login_url(self):
        # if get_login_url is invoked in an API handler,
        # that means @web.authenticated is trying to trigger a redirect.
        # instead of redirecting, raise 403 instead.
        if not self.current_user:
            raise web.HTTPError(403)
        return super().get_login_url()

    @property
    def content_security_policy(self):
        csp = "; ".join(
            [
                super().content_security_policy,
                "default-src 'none'",
            ]
        )
        return csp

    # set _track_activity = False on API handlers that shouldn't track activity
    _track_activity = True

    def update_api_activity(self):
        """Update last_activity of API requests"""
        # record activity of authenticated requests
        if (
            self._track_activity
            and getattr(self, "_user_cache", None)
            and self.get_argument("no_track_activity", None) is None
        ):
            self.settings["api_last_activity"] = utcnow()

    def finish(self, *args, **kwargs):
        self.update_api_activity()
        self.set_header("Content-Type", "application/json")
        return super().finish(*args, **kwargs)

    def options(self, *args, **kwargs):
        if "Access-Control-Allow-Headers" in self.settings.get("headers", {}):
            self.set_header(
                "Access-Control-Allow-Headers",
                self.settings["headers"]["Access-Control-Allow-Headers"],
            )
        else:
            self.set_header(
                "Access-Control-Allow-Headers",
                "accept, content-type, authorization, x-xsrftoken",
            )
        self.set_header("Access-Control-Allow-Methods", "GET, PUT, POST, PATCH, DELETE, OPTIONS")

        # if authorization header is requested,
        # that means the request is token-authenticated.
        # avoid browser-side rejection of the preflight request.
        # only allow this exception if allow_origin has not been specified
        # and Jupyter server authentication is enabled.
        # If the token is not valid, the 'real' request will still be rejected.
        requested_headers = self.request.headers.get("Access-Control-Request-Headers", "").split(
            ","
        )
        if (
            requested_headers
            and any(h.strip().lower() == "authorization" for h in requested_headers)
            and (
                # FIXME: it would be even better to check specifically for token-auth,
                # but there is currently no API for this.
                self.login_available
            )
            and (
                self.allow_origin
                or self.allow_origin_pat
                or "Access-Control-Allow-Origin" in self.settings.get("headers", {})
            )
        ):
            self.set_header("Access-Control-Allow-Origin", self.request.headers.get("Origin", ""))


class Template404(JupyterHandler):
    """Render our 404 template"""

    def prepare(self):
        raise web.HTTPError(404)


class AuthenticatedFileHandler(JupyterHandler, web.StaticFileHandler):
    """static files should only be accessible when logged in"""

    @property
    def content_security_policy(self):
        # In case we're serving HTML/SVG, confine any Javascript to a unique
        # origin so it can't interact with the Jupyter server.
        return super().content_security_policy + "; sandbox allow-scripts"

    @web.authenticated
    def head(self, path):
        self.check_xsrf_cookie()
        return super().head(path)

    @web.authenticated
    def get(self, path):
        if os.path.splitext(path)[1] == ".ipynb" or self.get_argument("download", None):
            name = path.rsplit("/", 1)[-1]
            self.set_attachment_header(name)

        return web.StaticFileHandler.get(self, path)

    def get_content_type(self):
        assert self.absolute_path is not None
        path = self.absolute_path.strip("/")
        if "/" in path:
            _, name = path.rsplit("/", 1)
        else:
            name = path
        if name.endswith(".ipynb"):
            return "application/x-ipynb+json"
        else:
            cur_mime = mimetypes.guess_type(name)[0]
            if cur_mime == "text/plain":
                return "text/plain; charset=UTF-8"
            else:
                return super().get_content_type()

    def set_headers(self):
        super().set_headers()
        # disable browser caching, rely on 304 replies for savings
        if "v" not in self.request.arguments:
            self.add_header("Cache-Control", "no-cache")

    def compute_etag(self):
        return None

    def validate_absolute_path(self, root, absolute_path):
        """Validate and return the absolute path.

        Requires tornado 3.1

        Adding to tornado's own handling, forbids the serving of hidden files.
        """
        abs_path = super().validate_absolute_path(root, absolute_path)
        abs_root = os.path.abspath(root)
        if is_hidden(abs_path, abs_root) and not self.contents_manager.allow_hidden:
            self.log.info(
                "Refusing to serve hidden file, via 404 Error, use flag 'ContentsManager.allow_hidden' to enable"
            )
            raise web.HTTPError(404)
        return abs_path


def json_errors(method):
    """Decorate methods with this to return GitHub style JSON errors.

    This should be used on any JSON API on any handler method that can raise HTTPErrors.

    This will grab the latest HTTPError exception using sys.exc_info
    and then:

    1. Set the HTTP status code based on the HTTPError
    2. Create and return a JSON body with a message field describing
       the error in a human readable form.
    """
    warnings.warn(
        "@json_errors is deprecated in notebook 5.2.0. Subclass APIHandler instead.",
        DeprecationWarning,
        stacklevel=2,
    )

    @functools.wraps(method)
    def wrapper(self, *args, **kwargs):
        self.write_error = types.MethodType(APIHandler.write_error, self)
        return method(self, *args, **kwargs)

    return wrapper


# -----------------------------------------------------------------------------
# File handler
# -----------------------------------------------------------------------------

# to minimize subclass changes:
HTTPError = web.HTTPError


class FileFindHandler(JupyterHandler, web.StaticFileHandler):
    """subclass of StaticFileHandler for serving files from a search path"""

    # cache search results, don't search for files more than once
    _static_paths: dict = {}
    root: tuple

    def set_headers(self):
        super().set_headers()
        # disable browser caching, rely on 304 replies for savings
        if "v" not in self.request.arguments or any(
            self.request.path.startswith(path) for path in self.no_cache_paths
        ):
            self.set_header("Cache-Control", "no-cache")

    def initialize(self, path, default_filename=None, no_cache_paths=None):
        self.no_cache_paths = no_cache_paths or []

        if isinstance(path, str):
            path = [path]

        self.root = tuple(os.path.abspath(os.path.expanduser(p)) + os.sep for p in path)
        self.default_filename = default_filename

    def compute_etag(self):
        return None

    @classmethod
    def get_absolute_path(cls, roots, path):
        """locate a file to serve on our static file search path"""
        with cls._lock:
            if path in cls._static_paths:
                return cls._static_paths[path]
            try:
                abspath = os.path.abspath(filefind(path, roots))
            except OSError:
                # IOError means not found
                return ""

            cls._static_paths[path] = abspath

            log().debug(f"Path {path} served from {abspath}")
            return abspath

    def validate_absolute_path(self, root, absolute_path):
        """check if the file should be served (raises 404, 403, etc.)"""
        if absolute_path == "":
            raise web.HTTPError(404)

        for root in self.root:
            if (absolute_path + os.sep).startswith(root):
                break

        return super().validate_absolute_path(root, absolute_path)


class APIVersionHandler(APIHandler):
    def get(self):
        # not authenticated, so give as few info as possible
        self.finish(json.dumps({"version": jupyter_server.__version__}))


class TrailingSlashHandler(web.RequestHandler):
    """Simple redirect handler that strips trailing slashes

    This should be the first, highest priority handler.
    """

    def get(self):
        assert self.request.uri is not None
        path, *rest = self.request.uri.partition("?")
        # trim trailing *and* leading /
        # to avoid misinterpreting repeated '//'
        path = "/" + path.strip("/")
        new_uri = "".join([path, *rest])
        self.redirect(new_uri)

    post = put = get


class MainHandler(JupyterHandler):
    """Simple handler for base_url."""

    def get(self):
        html = self.render_template("main.html")
        self.write(html)

    post = put = get


class FilesRedirectHandler(JupyterHandler):
    """Handler for redirecting relative URLs to the /files/ handler"""

    @staticmethod
    async def redirect_to_files(self, path):
        """make redirect logic a reusable static method

        so it can be called from other handlers.
        """
        cm = self.contents_manager
        if await ensure_async(cm.dir_exists(path)):
            # it's a *directory*, redirect to /tree
            url = url_path_join(self.base_url, "tree", url_escape(path))
        else:
            orig_path = path
            # otherwise, redirect to /files
            parts = path.split("/")

            if not await ensure_async(cm.file_exists(path=path)) and "files" in parts:
                # redirect without files/ iff it would 404
                # this preserves pre-2.0-style 'files/' links
                self.log.warning("Deprecated files/ URL: %s", orig_path)
                parts.remove("files")
                path = "/".join(parts)

            if not await ensure_async(cm.file_exists(path=path)):
                raise web.HTTPError(404)

            url = url_path_join(self.base_url, "files", url_escape(path))
        self.log.debug("Redirecting %s to %s", self.request.path, url)
        self.redirect(url)

    def get(self, path=""):
        return self.redirect_to_files(self, path)


class RedirectWithParams(web.RequestHandler):
    """Sam as web.RedirectHandler, but preserves URL parameters"""

    def initialize(self, url, permanent=True):
        self._url = url
        self._permanent = permanent

    def get(self):
        sep = "&" if "?" in self._url else "?"
        url = sep.join([self._url, self.request.query])
        self.redirect(url, permanent=self._permanent)


class PrometheusMetricsHandler(JupyterHandler):
    """
    Return prometheus metrics for this notebook server
    """

    def get(self):
        if self.settings["authenticate_prometheus"] and not self.logged_in:
            raise web.HTTPError(403)

        self.set_header("Content-Type", prometheus_client.CONTENT_TYPE_LATEST)
        self.write(prometheus_client.generate_latest(prometheus_client.REGISTRY))


# -----------------------------------------------------------------------------
# URL pattern fragments for re-use
# -----------------------------------------------------------------------------

# path matches any number of `/foo[/bar...]` or just `/` or ''
path_regex = r"(?P<path>(?:(?:/[^/]+)+|/?))"

# -----------------------------------------------------------------------------
# URL to handler mappings
# -----------------------------------------------------------------------------


default_handlers = [
    (r".*/", TrailingSlashHandler),
    (r"api", APIVersionHandler),
    (r"/(robots\.txt|favicon\.ico)", web.StaticFileHandler),
    (r"/metrics", PrometheusMetricsHandler),
]