Bebop/bebop/browser/gemini.py

"""Gemini-related features of the browser."""

from pathlib import Path

from bebop.browser.browser import Browser
from bebop.command_line import CommandLine
from bebop.fs import get_downloads_path
from bebop.navigation import set_parameter
from bebop.page import Page
from bebop.protocol import Request, Response
from bebop.tofu import trust_fingerprint, untrust_fingerprint, WRONG_FP_ALERT


MAX_URL_LEN = 1024


def open_gemini_url(browser: Browser, url, redirects=0, use_cache=True):
    """Open a Gemini URL and set the formatted response as content.

    While the specification is not set in stone, every client takes a slightly
    different approach to enforcing TOFU. Read the `Request.connect` docs to
    find about cases where connection is aborted without asking the user. What
    interests us here is what happens when the user should decide herself? This
    happens in several cases, matching the request possible states. Here is
    what Bebop do (or want to do):

    - STATE_INVALID_CERT: the certificate has non-fatal issues; we may
      present the user the problems found and let her decide whether to trust
      temporarily the certificate or not BUT we currently do not parse the
      certificate's fields, so this state is never used.
    - STATE_UNKNOWN_CERT: the certificate is valid but has not been seen before;
      as we're doing TOFU here, we could automatically trust it or let the user
      choose. For simplicity, we always trust it permanently.

    Arguments:
    - browser: Browser object making the request.
    - url: a valid URL with Gemini scheme to open.
    - redirects: current amount of redirections done to open the initial URL.
    - use_cache: if true, look up if the page is cached before requesting it.

    Returns:
    True on success, False otherwise.
    """
    if len(url) >= MAX_URL_LEN:
        browser.set_status_error("Request URL too long.")
        return

    browser.set_status(f"Loading {url}")

    if use_cache and url in browser.cache:
        browser.load_page(browser.cache[url])
        browser.current_url = url
        browser.set_status(url)
        return True

    req = Request(url, browser.stash)
    connect_timeout = browser.config["connect_timeout"]
    connected = req.connect(connect_timeout)
    if not connected:
        if req.state == Request.STATE_ERROR_CERT:
            error = f"Certificate was missing or corrupt ({url})."
        elif req.state == Request.STATE_UNTRUSTED_CERT:
            _handle_untrusted_cert(browser, req)
            error = f"Certificate has been changed ({url})."
        elif req.state == Request.STATE_CONNECTION_FAILED:
            error_details = ": " + req.error if req.error else "."
            error = f"Connection failed ({url})" + error_details
        else:
            error = f"Connection failed ({url})."
        browser.set_status_error(error)
        return False

    if req.state == Request.STATE_INVALID_CERT:
        pass
    elif req.state == Request.STATE_UNKNOWN_CERT:
        # Certificate is valid but unknown: trust it permanently.
        hostname = req.hostname
        fingerprint = req.cert_validation["hash"]
        trust_fingerprint(
            browser.stash,
            hostname,
            "SHA-512",
            fingerprint,
            trust_always=True
        )

    data = req.proceed()
    if not data:
        browser.set_status_error(f"Server did not respond in time ({url}).")
        return False
    response = Response.parse(data)
    if not response:
        browser.set_status_error(f"Server response parsing failed ({url}).")
        return False

    return _handle_response(browser, response, url, redirects)


def _handle_untrusted_cert(browser: Browser, request: Request):
    """Handle a mismatch between known & server fingerprints.

    This function formats an alert page to explain to the user what the hell is
    going on and displays it.
    """
    remote_fp = request.cert_validation["hash"]
    local_fp = request.cert_validation["saved_hash"]
    alert_page_source = WRONG_FP_ALERT.format(
        hostname=request.hostname,
        local_fp=local_fp,
        remote_fp=remote_fp,
    )
    alert_page = Page.from_gemtext(
        alert_page_source,
        browser.config["text_width"]
    )
    browser.load_page(alert_page)


def _handle_response(browser: Browser, response: Response, url: str,
                     redirects: int):
    """Handle a response from a Gemini server.

    Returns:
    True on success, False otherwise.
    """
    if response.code == 20:
        return _handle_successful_response(browser, response, url)
    elif response.generic_code == 30 and response.meta:
        browser.open_url(response.meta, base_url=url, redirects=redirects + 1)
    elif response.generic_code in (40, 50):
        error = f"Server error: {response.meta or Response.code.name}"
        browser.set_status_error(error)
    elif response.generic_code == 10:
        _handle_input_request(browser, url, response.meta)
    else:
        error = f"Unhandled response code {response.code}"
        browser.set_status_error(error)
    return False


def _handle_successful_response(browser: Browser, response: Response, url: str):
    """Handle a successful response content from a Gemini server.

    According to the MIME type received or inferred, the response is either
    rendered by the browser, or saved to disk. If an error occurs, the browser
    displays it.

    Only text content is rendered. For Gemini, the encoding specified in the
    response is used, if available on the Python distribution. For other text
    formats, only UTF-8 is attempted.

    Arguments:
    - browser: Browser instance that made the initial request.
    - url: original URL.
    - response: a successful Response.

    Returns:
    True on success, False otherwise.
    """
    # Use appropriate response parser according to the MIME type.
    mime_type = response.get_mime_type()
    page = None
    error = None
    filepath = None
    if mime_type.main_type == "text":
        if mime_type.sub_type == "gemini":
            encoding = mime_type.charset
            try:
                text = response.content.decode(encoding, errors="replace")
            except LookupError:
                error = f"Unknown encoding {encoding}."
            else:
                page = Page.from_gemtext(text, browser.config["text_width"])
        else:
            text = response.content.decode("utf-8", errors="replace")
            page = Page.from_text(text)
    else:
        filepath = _get_download_path(url)

    # If a page has been produced, load it. Else if a file has been retrieved,
    # download it.
    if page:
        browser.load_page(page)
        browser.current_url = url
        browser.cache[url] = page
        browser.set_status(url)
        return True
    elif filepath:
        try:
            with open(filepath, "wb") as download_file:
                download_file.write(response.content)
        except OSError as exc:
            browser.set_status_error(f"Failed to save {url} ({exc})")
        else:
            browser.set_status(f"Downloaded {url} ({mime_type.short}).")
            return True
    elif error:
        browser.set_status_error(error)
    return False


def _get_download_path(url: str) -> Path:
    """Try to find the best download file path possible from this URL."""
    download_dir = get_downloads_path()
    url_parts = url.rsplit("/", maxsplit=1)
    if url_parts:
        filename = url_parts[-1]
    else:
        filename = url.split("://")[1] if "://" in url else url
        filename = filename.replace("/", "_")
    return download_dir / filename


def _handle_input_request(browser: Browser, from_url: str, message: str =None):
    """Focus command-line to pass input to the server."""
    if message:
        browser.set_status(f"Input needed: {message}")
    else:
        browser.set_status("Input needed:")
    user_input = browser.command_line.focus(CommandLine.CHAR_TEXT)
    if not user_input:
        return
    url = set_parameter(from_url, user_input)
    open_gemini_url(browser, url)


def forget_certificate(browser: Browser, hostname: str):
    """Remove the fingerprint associated to this hostname for the cert stash."""
    key = browser.prompt(f"Remove fingerprint from {hostname}? [y/N]", "ynN")
    if key != "y":
        browser.reset_status()
        return
    if untrust_fingerprint(browser.stash, hostname):
        browser.set_status(f"Known certificate for {hostname} removed.")
    else:
        browser.set_status_error(f"Known certificate for {hostname} not found.")