jetforce.py

#!/usr/bin/env python3
"""
Jetforce, an experimental Gemini server.

Overview
--------

GeminiServer:
    An asynchronous TCP server built on top of python's asyncio stream
    abstraction. This is a lightweight class that accepts incoming requests,
    logs them, and sends them to a configurable request handler to be processed.

GeminiRequestHandler:
    The request handler manages the life of a single gemini request. It exposes
    a simplified interface to read the request URL and write the gemini response
    status line and body to the socket. The request URL and other server
    information is stuffed into an ``environ`` dictionary that encapsulates the
    request at a low level. This dictionary, along with a callback to write the
    response data, and passed to a configurable "application" function or class.

JetforceApplication:
    This is a base class for writing jetforce server applications. It doesn't
    anything on its own, but it does provide a convenient interface to define
    custom server endpoints using route decorators. If you want to utilize
    jetforce as a library and write your own server in python, this is the class
    that you want to extend. The examples/ directory contains some examples of
    how to accomplish this.

StaticDirectoryApplication:
    This is a pre-built application that serves files from a static directory.
    It provides an "out-of-the-box" gemini server without needing to write any
    lines of code. This is what is invoked when you launch jetforce from the
    command line.
"""
from __future__ import annotations

import argparse
import asyncio
import codecs
import dataclasses
import mimetypes
import os
import pathlib
import re
import socket
import ssl
import subprocess
import sys
import tempfile
import time
import typing
import urllib.parse

if sys.version_info < (3, 7):
    sys.exit("Fatal Error: jetforce requires Python 3.7+")

__version__ = "0.2.2"
__title__ = "Jetforce Gemini Server"
__author__ = "Michael Lazar"
__license__ = "Floodgap Free Software License"
__copyright__ = "(c) 2020 Michael Lazar"

ABOUT = fr"""
You are now riding on...
_________    _____________
______  /______  /___  __/_______________________
___ _  /_  _ \  __/_  /_ _  __ \_  ___/  ___/  _ \
/ /_/ / /  __/ /_ _  __/ / /_/ /  /   / /__ /  __/
\____/  \___/\__/ /_/    \____//_/    \___/ \___/

An Experimental Gemini Server, v{__version__}
https://github.com/michael-lazar/jetforce
"""


class Status:
    """
    Gemini response status codes.
    """

    INPUT = 10

    SUCCESS = 20
    SUCCESS_END_OF_SESSION = 21

    REDIRECT_TEMPORARY = 30
    REDIRECT_PERMANENT = 31

    TEMPORARY_FAILURE = 40
    SERVER_UNAVAILABLE = 41
    CGI_ERROR = 42
    PROXY_ERROR = 43
    SLOW_DOWN = 44

    PERMANENT_FAILURE = 50
    NOT_FOUND = 51
    GONE = 52
    PROXY_REQUEST_REFUSED = 53
    BAD_REQUEST = 59

    CLIENT_CERTIFICATE_REQUIRED = 60
    TRANSIENT_CERTIFICATE_REQUESTED = 61
    AUTHORISED_CERTIFICATE_REQUIRED = 62
    CERTIFICATE_NOT_ACCEPTED = 63
    FUTURE_CERTIFICATE_REJECTED = 64
    EXPIRED_CERTIFICATE_REJECTED = 65


class Request:
    """
    Object that encapsulates information about a single gemini request.
    """

    def __init__(self, environ: dict):
        self.environ = environ
        self.url = environ["GEMINI_URL"]

        url_parts = urllib.parse.urlparse(self.url)
        if not url_parts.hostname:
            raise ValueError("URL must contain a `hostname` part")

        if not url_parts.scheme:
            # If scheme is missing, infer it to be gemini://
            self.scheme = "gemini"
        else:
            self.scheme = url_parts.scheme

        self.hostname = url_parts.hostname
        self.port = url_parts.port
        self.path = url_parts.path
        self.params = url_parts.params
        self.query = urllib.parse.unquote(url_parts.query)
        self.fragment = url_parts.fragment


@dataclasses.dataclass
class Response:
    """
    Object that encapsulates information about a single gemini response.
    """

    status: int
    meta: str
    body: typing.Union[None, bytes, str, typing.Iterator[bytes]] = None


@dataclasses.dataclass
class RoutePattern:
    """
    A pattern for matching URLs with a single endpoint or route.
    """

    path: str = ".*"
    scheme: str = "gemini"
    hostname: typing.Optional[str] = None

    strict_hostname: bool = True
    strict_port: bool = True
    strict_trailing_slash: bool = False

    def match(self, request: Request) -> typing.Optional[re.Match]:
        """
        Check if the given request URL matches this route pattern.
        """
        if self.hostname is None:
            server_hostname = request.environ["HOSTNAME"]
        else:
            server_hostname = self.hostname
        server_port = int(request.environ["SERVER_PORT"])

        if self.strict_hostname and request.hostname != server_hostname:
            return
        if self.strict_port and request.port is not None:
            if request.port != server_port:
                return
        if self.scheme and self.scheme != request.scheme:
            return

        if self.strict_trailing_slash:
            request_path = request.path
        else:
            request_path = request.path.rstrip("/")

        return re.fullmatch(self.path, request_path)


class JetforceApplication:
    """
    Base Jetforce application class with primitive URL routing.

    This is a base class for writing jetforce server applications. It doesn't
    anything on its own, but it does provide a convenient interface to define
    custom server endpoints using route decorators. If you want to utilize
    jetforce as a library and write your own server in python, this is the class
    that you want to extend. The examples/ directory contains some examples of
    how to accomplish this.
    """

    def __init__(self):
        self.routes: typing.List[
            typing.Tuple[RoutePattern, typing.Callable[[Request], Response]]
        ] = []

    def __call__(
        self, environ: dict, send_status: typing.Callable
    ) -> typing.Iterator[bytes]:
        try:
            request = Request(environ)
        except Exception:
            send_status(Status.BAD_REQUEST, "Unrecognized URL format")
            return

        for route_pattern, callback in self.routes[::-1]:
            if route_pattern.match(request):
                break
        else:
            callback = self.default_callback

        response = callback(request)
        send_status(response.status, response.meta)
        if isinstance(response.body, bytes):
            yield response.body
        elif isinstance(response.body, str):
            yield response.body.encode()
        elif response.body:
            yield from response.body

    def route(
        self,
        path: str = ".*",
        scheme: str = "gemini",
        hostname: typing.Optional[str] = None,
        strict_hostname: bool = True,
        strict_trailing_slash: bool = False,
    ) -> typing.Callable:
        """
        Decorator for binding a function to a route based on the URL path.

            app = JetforceApplication()

            @app.route('/my-path')
            def my_path(request):
                return Response(Status.SUCCESS, 'text/plain', 'Hello world!')
        """
        route_pattern = RoutePattern(
            path, scheme, hostname, strict_hostname, strict_trailing_slash
        )

        def wrap(func: typing.Callable) -> typing.Callable:
            self.routes.append((route_pattern, func))
            return func

        return wrap

    def default_callback(self, request: Request) -> Response:
        """
        Set the error response based on the URL type.
        """
        return Response(Status.PERMANENT_FAILURE, "Not Found")


class StaticDirectoryApplication(JetforceApplication):
    """
    Application for serving static files & CGI over gemini.

    This is a pre-built application that serves files from a static directory.
    It provides an "out-of-the-box" gemini server without needing to write any
    lines of code. This is what is invoked when you launch jetforce from the
    command line.

    If a directory contains a file with the name "index.gmi", that file will
    be returned when the directory path is requested. Otherwise, a directory
    listing will be auto-generated.
    """

    def __init__(
        self,
        root_directory: str = "/var/gemini",
        index_file: str = "index.gmi",
        cgi_directory: str = "cgi-bin",
    ):
        super().__init__()
        self.routes.append((RoutePattern(), self.serve_static_file))

        self.root = pathlib.Path(root_directory).resolve(strict=True)
        self.cgi_directory = cgi_directory.strip("/") + "/"

        self.index_file = index_file
        self.mimetypes = mimetypes.MimeTypes()
        self.mimetypes.add_type("text/gemini", ".gmi")
        self.mimetypes.add_type("text/gemini", ".gemini")

    def serve_static_file(self, request: Request) -> Response:
        """
        Convert a URL into a filesystem path, and attempt to serve the file
        or directory that is represented at that path.
        """
        url_path = pathlib.Path(request.path.strip("/"))

        filename = pathlib.Path(os.path.normpath(str(url_path)))
        if filename.is_absolute() or str(filename.name).startswith(".."):
            # Guard against breaking out of the directory
            return Response(Status.NOT_FOUND, "Not Found")

        filesystem_path = self.root / filename

        try:
            if not os.access(filesystem_path, os.R_OK):
                # File not readable
                return Response(Status.NOT_FOUND, "Not Found")
        except OSError:
            # Filename too large, etc.
            return Response(Status.NOT_FOUND, "Not Found")

        if filesystem_path.is_file():
            is_cgi = str(filename).startswith(self.cgi_directory)
            is_exe = os.access(filesystem_path, os.X_OK)
            if is_cgi and is_exe:
                return self.run_cgi_script(filesystem_path, request.environ)

            mimetype = self.guess_mimetype(filesystem_path.name)
            generator = self.load_file(filesystem_path)
            return Response(Status.SUCCESS, mimetype, generator)

        elif filesystem_path.is_dir():
            if not request.path.endswith("/"):
                url_parts = urllib.parse.urlparse(request.url)
                url_parts = url_parts._replace(path=request.path + "/")
                return Response(Status.REDIRECT_PERMANENT, url_parts.geturl())

            index_file = filesystem_path / self.index_file
            if index_file.exists():
                generator = self.load_file(index_file)
                return Response(Status.SUCCESS, "text/gemini", generator)

            generator = self.list_directory(url_path, filesystem_path)
            return Response(Status.SUCCESS, "text/gemini", generator)

        else:
            return Response(Status.NOT_FOUND, "Not Found")

    def run_cgi_script(self, filesystem_path: pathlib.Path, environ: dict) -> Response:
        """
        Execute the given file as a CGI script and return the script's stdout
        stream to the client.
        """
        script_name = str(filesystem_path)
        cgi_env = environ.copy()
        cgi_env["GATEWAY_INTERFACE"] = "GCI/1.1"
        cgi_env["SCRIPT_NAME"] = script_name

        # Decode the stream as unicode so we can parse the status line
        # Use surrogateescape to preserve any non-UTF8 byte sequences.
        out = subprocess.Popen(
            [script_name],
            stdout=subprocess.PIPE,
            env=cgi_env,
            bufsize=1,
            universal_newlines=True,
            errors="surrogateescape",
        )

        status_line = out.stdout.readline().strip()
        status_parts = status_line.split(maxsplit=1)
        if len(status_parts) != 2 or not status_parts[0].isdecimal():
            return Response(Status.CGI_ERROR, "Unexpected Error")

        status, meta = status_parts

        # Re-encode the rest of the body as bytes
        body = codecs.iterencode(out.stdout, encoding="utf-8", errors="surrogateescape")
        return Response(int(status), meta, body)

    def load_file(self, filesystem_path: pathlib.Path) -> typing.Iterator[bytes]:
        """
        Load a file using a generator to allow streaming data to the TCP socket.
        """
        with filesystem_path.open("rb") as fp:
            data = fp.read(1024)
            while data:
                yield data
                data = fp.read(1024)

    def list_directory(
        self, url_path: pathlib.Path, filesystem_path: pathlib.Path
    ) -> typing.Iterator[bytes]:
        """
        Auto-generate a text/gemini document based on the contents of the file system.
        """
        yield f"Directory: /{url_path}\r\n".encode()
        if url_path.parent != url_path:
            yield f"=>/{url_path.parent}\t..\r\n".encode()

        for file in sorted(filesystem_path.iterdir()):
            if file.name.startswith("."):
                # Skip hidden directories/files that may contain sensitive info
                continue
            elif file.is_dir():
                yield f"=>/{url_path / file.name}/\t{file.name}/\r\n".encode()
            else:
                yield f"=>/{url_path / file.name}\t{file.name}\r\n".encode()

    def guess_mimetype(self, filename: str) -> str:
        """
        Guess the mimetype of a file based on the file extension.
        """
        mime, encoding = self.mimetypes.guess_type(filename)
        if encoding:
            return f"{mime}; charset={encoding}"
        else:
            return mime or "text/plain"

    def default_callback(self, request: Request) -> Response:
        """
        Since the StaticDirectoryApplication only serves gemini URLs, return
        a proxy request refused for suspicious URLs.
        """
        if request.scheme != "gemini":
            return Response(
                Status.PROXY_REQUEST_REFUSED,
                "This server does not allow proxy requests",
            )
        elif request.hostname != request.environ["HOSTNAME"]:
            return Response(
                Status.PROXY_REQUEST_REFUSED,
                "This server does not allow proxy requests",
            )
        elif request.port and request.port != request.environ["SERVER_PORT"]:
            return Response(
                Status.PROXY_REQUEST_REFUSED,
                "This server does not allow proxy requests",
            )
        else:
            return Response(Status.NOT_FOUND, "Not Found")


class GeminiRequestHandler:
    """
    Handle a single Gemini Protocol TCP request.

    The request handler manages the life of a single gemini request. It exposes
    a simplified interface to read the request URL and write the gemini response
    status line and body to the socket. The request URL and other server
    information is stuffed into an ``environ`` dictionary that encapsulates the
    request at a low level. This dictionary, along with a callback to write the
    response data, and passed to a configurable "application" function or class.

    This design borrows heavily from the standard library's HTTP request
    handler (http.server.BaseHTTPRequestHandler). However, I did not make any
    attempts to directly emulate the existing conventions, because Gemini is an
    inherently simpler protocol than HTTP and much of the boilerplate could be
    removed.
    """

    TIMESTAMP_FORMAT = "%d/%b/%Y:%H:%M:%S %z"

    reader: asyncio.StreamReader
    writer: asyncio.StreamWriter
    received_timestamp: time.struct_time
    remote_addr: str
    client_cert: dict
    url: str
    status: int
    meta: str
    response_buffer: str
    response_size: int

    def __init__(self, server: GeminiServer, app: typing.Callable) -> None:
        self.server = server
        self.app = app
        self.response_size = 0

    async def handle(
        self, reader: asyncio.StreamReader, writer: asyncio.StreamWriter
    ) -> None:
        """
        Main method for the request handler, performs the following:

            1. Read the request bytes from the reader stream
            2. Parse the request and generate response data
            3. Write the response bytes to the writer stream
        """
        self.reader = reader
        self.writer = writer
        self.remote_addr = writer.get_extra_info("peername")[0]
        self.client_cert = writer.get_extra_info("peercert")
        self.received_timestamp = time.localtime()

        try:
            await self.parse_header()
        except Exception:
            # Malformed request, throw it away and exit immediately
            self.write_status(Status.BAD_REQUEST, "Malformed request")
            return await self.close_connection()

        try:
            environ = self.build_environ()
            app = self.app(environ, self.write_status)
            for data in app:
                await self.write_body(data)
        except Exception:
            self.write_status(Status.CGI_ERROR, "An unexpected error occurred")
            raise
        finally:
            await self.close_connection()

    def build_environ(self) -> typing.Dict[str, typing.Any]:
        """
        Construct a dictionary that will be passed to the application handler.

        Variable names conform to the CGI spec defined in RFC 3875.
        """
        url_parts = urllib.parse.urlparse(self.url)
        environ = {
            "GEMINI_URL": self.url,
            "HOSTNAME": self.server.hostname,
            "PATH_INFO": url_parts.path,
            "QUERY_STRING": url_parts.query,
            "REMOTE_ADDR": self.remote_addr,
            "REMOTE_HOST": self.remote_addr,
            "SERVER_NAME": self.server.hostname,
            "SERVER_PORT": str(self.server.port),
            "SERVER_PROTOCOL": "GEMINI",
            "SERVER_SOFTWARE": f"jetforce/{__version__}",
        }

        if self.client_cert:
            subject = dict(x[0] for x in self.client_cert["subject"])
            environ.update(
                {
                    "AUTH_TYPE": "CERTIFICATE",
                    "REMOTE_USER": subject.get("commonName", ""),
                    "TLS_CLIENT_NOT_BEFORE": self.client_cert["notBefore"],
                    "TLS_CLIENT_NOT_AFTER": self.client_cert["notAfter"],
                    "TLS_CLIENT_SERIAL_NUMBER": self.client_cert["serialNumber"],
                }
            )

        return environ

    async def parse_header(self) -> None:
        """
        Parse the gemini header line.

        The request is a single UTF-8 line formatted as: <URL>\r\n
        """
        data = await self.reader.readuntil(b"\r\n")
        data = data[:-2]  # strip the line ending
        if len(data) > 1024:
            raise ValueError("URL exceeds max length of 1024 bytes")

        self.url = data.decode()

    def write_status(self, status: int, meta: str) -> None:
        """
        Write the gemini status line to an internal buffer.

        The status line is a single UTF-8 line formatted as:
            <code>\t<meta>\r\n

        If the response status is 2, the meta field will contain the mimetype
        of the response data sent. If the status is something else, the meta
        will contain a descriptive message.

        The status is not written immediately, it's added to an internal buffer
        that must be flushed. This is done so that the status can be updated as
        long as no other data has been written to the stream yet.
        """
        self.status = status
        self.meta = meta
        self.response_buffer = f"{status}\t{meta}\r\n"

    async def write_body(self, data: bytes) -> None:
        """
        Write bytes to the gemini response body.
        """
        await self.flush_status()
        self.response_size += len(data)
        self.writer.write(data)
        await self.writer.drain()

    async def flush_status(self) -> None:
        """
        Flush the status line from the internal buffer to the socket stream.
        """
        if self.response_buffer and not self.response_size:
            data = self.response_buffer.encode()
            self.response_size += len(data)
            self.writer.write(data)
            await self.writer.drain()
        self.response_buffer = ""

    async def close_connection(self) -> None:
        """
        Flush any remaining bytes and close the stream.
        """
        await self.flush_status()
        self.log_request()
        await self.writer.drain()

    def log_request(self) -> None:
        """
        Log a gemini request using a format derived from the Common Log Format.
        """
        try:
            self.server.log_message(
                f"{self.remote_addr} "
                f"[{time.strftime(self.TIMESTAMP_FORMAT, self.received_timestamp)}] "
                f'"{self.url}" '
                f"{self.status} "
                f'"{self.meta}" '
                f"{self.response_size}"
            )
        except AttributeError:
            # Malformed request or dropped connection
            pass


class GeminiServer:
    """
    An asynchronous TCP server that uses the asyncio stream abstraction.

    This is a lightweight class that accepts incoming requests, logs them, and
    sends them to a configurable request handler to be processed.
    """

    request_handler_class = GeminiRequestHandler

    def __init__(
        self,
        app: typing.Callable,
        host: str = "127.0.0.1",
        port: int = 1965,
        ssl_context: ssl.SSLContext = None,
        hostname: str = "localhost",
    ) -> None:

        self.host = host
        self.port = port
        self.hostname = hostname
        self.app = app
        self.ssl_context = ssl_context

    async def run(self) -> None:
        """
        The main asynchronous server loop.
        """
        self.log_message(ABOUT)
        server = await asyncio.start_server(
            self.accept_connection, self.host, self.port, ssl=self.ssl_context
        )

        self.log_message(f"Server hostname is {self.hostname}")
        for sock in server.sockets:
            sock_ip, sock_port, *_ = sock.getsockname()
            if sock.family == socket.AF_INET:
                self.log_message(f"Listening on {sock_ip}:{sock_port}")
            else:
                self.log_message(f"Listening on [{sock_ip}]:{sock_port}")

        async with server:
            await server.serve_forever()

    async def accept_connection(
        self, reader: asyncio.StreamReader, writer: asyncio.StreamWriter
    ) -> None:
        """
        Hook called by the socket server when a new connection is accepted.
        """
        request_handler = self.request_handler_class(self, self.app)
        try:
            await request_handler.handle(reader, writer)
        finally:
            writer.close()

    def log_message(self, message: str) -> None:
        """
        Log a diagnostic server message.
        """
        print(message, file=sys.stderr)


def generate_ad_hoc_certificate(hostname: str) -> typing.Tuple[str, str]:
    """
    Utility function to generate a self-signed SSL certificate key pair if
    one isn't provided. Results may vary depending on your version of OpenSSL.
    """
    certfile = pathlib.Path(tempfile.gettempdir()) / f"{hostname}.crt"
    keyfile = pathlib.Path(tempfile.gettempdir()) / f"{hostname}.key"
    if not certfile.exists() or not keyfile.exists():
        print(f"Writing ad hoc TLS certificate to {certfile}")
        subprocess.run(
            [
                f"openssl req -newkey rsa:2048 -nodes -keyout {keyfile}"
                f' -nodes -x509 -out {certfile} -subj "/CN={hostname}"'
            ],
            shell=True,
            check=True,
        )
    return str(certfile), str(keyfile)


def make_ssl_context(
    hostname: str = "localhost",
    certfile: typing.Optional[str] = None,
    keyfile: typing.Optional[str] = None,
    cafile: typing.Optional[str] = None,
    capath: typing.Optional[str] = None,
) -> ssl.SSLContext:
    """
    Generate a sane default SSL context for a Gemini server.

    For more information on what these variables mean and what values they can
    contain, see the python standard library documentation:

        https://docs.python.org/3/library/ssl.html#ssl-contexts

    verify_mode: ssl.CERT_OPTIONAL
        A client certificate request is sent to the client. The client may
        either ignore the request or send a certificate in order perform TLS
        client cert authentication. If the client chooses to send a certificate,
        it is verified. Any verification error immediately aborts the TLS
        handshake.
    """
    if certfile is None:
        certfile, keyfile = generate_ad_hoc_certificate(hostname)

    context = ssl.SSLContext()
    context.verify_mode = ssl.CERT_OPTIONAL
    context.load_cert_chain(certfile, keyfile)

    if not cafile and not capath:
        # Load from the system's default client CA directory
        context.load_default_certs(purpose=ssl.Purpose.CLIENT_AUTH)
    else:
        # Use a custom CA for validating client certificates
        context.load_verify_locations(cafile, capath)

    return context


def command_line_parser() -> argparse.ArgumentParser:
    """
    Construct the default argument parser when launching the server from
    the command line. These are meant to be application-agnostic arguments
    that could apply to any subclass of the JetforceApplication.
    """
    parser = argparse.ArgumentParser(
        prog="jetforce",
        description="An Experimental Gemini Protocol Server",
        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
    )
    parser.add_argument(
        "-V", "--version", action="version", version="jetforce " + __version__
    )
    parser.add_argument("--host", help="Server address to bind to", default="127.0.0.1")
    parser.add_argument("--port", help="Server port to bind to", type=int, default=1965)
    parser.add_argument("--hostname", help="Server hostname", default="localhost")
    parser.add_argument(
        "--tls-certfile",
        dest="certfile",
        help="Server TLS certificate file",
        metavar="FILE",
    )
    parser.add_argument(
        "--tls-keyfile",
        dest="keyfile",
        help="Server TLS private key file",
        metavar="FILE",
    )
    parser.add_argument(
        "--tls-cafile",
        dest="cafile",
        help="A CA file to use for validating clients",
        metavar="FILE",
    )
    parser.add_argument(
        "--tls-capath",
        dest="capath",
        help="A directory containing CA files for validating clients",
        metavar="DIR",
    )
    return parser


def run_server() -> None:
    """
    Entry point for running the static directory server.
    """
    parser = command_line_parser()
    parser.add_argument(
        "--dir",
        help="Root directory on the filesystem to serve",
        default="/var/gemini",
        metavar="DIR",
    )
    parser.add_argument(
        "--cgi-dir",
        help="CGI script directory, relative to the server's root directory",
        default="cgi-bin",
        metavar="DIR",
    )
    parser.add_argument(
        "--index-file",
        help="If a directory contains a file with this name, that file will be "
        "served instead of auto-generating an index page",
        default="index.gmi",
        metavar="FILE",
    )
    args = parser.parse_args()

    app = StaticDirectoryApplication(args.dir, args.index_file, args.cgi_dir)
    ssl_context = make_ssl_context(
        args.hostname, args.certfile, args.keyfile, args.cafile, args.capath
    )
    server = GeminiServer(
        host=args.host,
        port=args.port,
        ssl_context=ssl_context,
        hostname=args.hostname,
        app=app,
    )
    asyncio.run(server.run())


if __name__ == "__main__":
    run_server()