cgi.py: Implement the `HEAD' request method.

[chopwood] / cgi.py

### -*-python-*-
###
### CGI machinery
###
### (c) 2013 Mark Wooding
###

###----- Licensing notice ---------------------------------------------------
###
### This file is part of Chopwood: a password-changing service.
###
### Chopwood is free software; you can redistribute it and/or modify
### it under the terms of the GNU Affero General Public License as
### published by the Free Software Foundation; either version 3 of the
### License, or (at your option) any later version.
###
### Chopwood is distributed in the hope that it will be useful,
### but WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
### GNU Affero General Public License for more details.
###
### You should have received a copy of the GNU Affero General Public
### License along with Chopwood; if not, see
### <http://www.gnu.org/licenses/>.

from __future__ import with_statement

import contextlib as CTX
import os as OS; ENV = OS.environ
import re as RX
import sys as SYS
import time as T
import traceback as TB

from auto import HOME, PACKAGE, VERSION
import config as CONF; CFG = CONF.CFG
import format as F
import output as O; OUT = O.OUT; PRINT = O.PRINT
import subcommand as SC
import util as U

###--------------------------------------------------------------------------
### Configuration tweaks.

_script_name = ENV.get('SCRIPT_NAME', '/cgi-bin/chpwd')

CONF.DEFAULTS.update(

  ## The URL of this program, when it's run through CGI.
  SCRIPT_NAME = _script_name,

  ## A (maybe relative) URL for static content.  By default this comes from
  ## the main script, but we hope that user agents cache it.
  STATIC = _script_name + '/static')

###--------------------------------------------------------------------------
### Escaping and encoding.

## Some handy regular expressions.
R_URLESC = RX.compile('%([0-9a-fA-F]{2})')
R_URLBAD = RX.compile('[^-\\w,.!]')
R_HTMLBAD = RX.compile('[&<>\'"]')

def urldecode(s):
  """Decode a single form-url-encoded string S."""
  return R_URLESC.sub(lambda m: chr(int(m.group(1), 16)),
                      s.replace('+', ' '))
  return s

def urlencode(s):
  """Encode a single string S using form-url-encoding."""
  return R_URLBAD.sub(lambda m: '%%%02x' % ord(m.group(0)), s)

def htmlescape(s):
  """Escape a literal string S so that HTML doesn't misinterpret it."""
  return R_HTMLBAD.sub(lambda m: '&#x%02x;' % ord(m.group(0)), s)

## Some standard character sequences, and HTML entity names for prettier
## versions.
html_quotify = U.StringSubst({
  "<": '&lt;',
  ">": '&gt;',
  "&": '&amp;',
  "`": '&lsquo;',
  "'": '&rsquo;',
  '"': '&quot;',
  "``": '&ldquo;',
  "''": '&rdquo;',
  "--": '&ndash;',
  "---": '&mdash;'
})

###--------------------------------------------------------------------------
### Output machinery.

class HTTPOutput (O.FileOutput):
  """
  Output driver providing an automatic HTTP header.

  The `headerp' attribute is true if we've written a header.  The `header'
  method will print a custom header if this is wanted.
  """

  def __init__(me, *args, **kw):
    """Constructor: initialize `headerp' flag."""
    super(HTTPOutput, me).__init__(*args, **kw)
    me.headerp = False

  def write(me, msg):
    """Output protocol: print a header if we've not written one already."""
    if not me.headerp: me.header('text/plain')
    super(HTTPOutput, me).write(msg)

  def header(me, content_type = 'text/plain', **kw):
    """
    Print a header, if none has yet been printed.

    Keyword arguments can be passed to emit HTTP headers: see `http_headers'
    for the formatting rules.
    """
    if me.headerp: return
    me.headerp = True
    for h in O.http_headers(content_type = content_type, **kw):
      me.writeln(h)
    me.writeln('')
    if METHOD == 'HEAD':
      HEADER_DONE()

def cookie(name, value, **kw):
  """
  Return a HTTP `Set-Cookie' header.

  The NAME and VALUE give the name and value of the cookie; both are
  form-url-encoded to prevent misinterpretation (fortunately, `cgiparse'
  knows to undo this transformation).  The KW are other attributes to
  declare: the names are forced to lower-case and underscores `_' are
  replaced by hyphens `-'; a `True' value is assumed to indicate that the
  attribute is boolean, and omitted.
  """
  attr = {}
  for k, v in kw.iteritems():
    k = '-'.join(i.lower() for i in k.split('_'))
    attr[k] = v
  try: maxage = int(attr['max-age'])
  except KeyError: pass
  else:
    attr['expires'] = T.strftime('%a, %d %b %Y %H:%M:%S GMT',
                                 T.gmtime(U.NOW + maxage))
  return '; '.join(['%s=%s' % (urlencode(name), urlencode(value))] +
                   [v is not True and '%s=%s' % (k, v) or k
                    for k, v in attr.iteritems() if v])

def action(*v, **kw):
  """
  Build a URL invoking this script.

  The positional arguments V are used to construct a path which is appended
  to the (deduced or configured) script name (and presumably will be read
  back as `PATH_INFO').  The keyword arguments are (form-url-encoded and)
  appended as a query string, if present.
  """
  url = '/'.join([CFG.SCRIPT_NAME] + list(v))
  if kw:
    url += '?' + ';'.join('%s=%s' % (urlencode(k), urlencode(kw[k]))
                          for k in sorted(kw))
  return htmlescape(url)

def static(name):
  """Build a URL for the static file NAME."""
  return htmlescape(CFG.STATIC + '/' + name)

def redirect(where, **kw):
  """
  Write a complete redirection to some other URL.
  """
  OUT.header(content_type = 'text/html',
             status = 302, location = where,
             **kw)
  PRINT("""\
<html>
<head><title>No, sorry, it's moved again.</title></head>
<body><p>I'm <a href="%s">over here</a> now.<body>
</html>""" % htmlescape(where))

###--------------------------------------------------------------------------
### Templates.

## Where we find our templates.
TMPLDIR = HOME

## Keyword arguments for templates.
STATE = U.Fluid()
STATE.kw = {}

## Set some basic keyword arguments.
@CONF.hook
def set_template_keywords():
  STATE.kw.update(
    package = PACKAGE,
    version = VERSION,
    script = CFG.SCRIPT_NAME,
    static = CFG.STATIC,
    allowop = CFG.ALLOWOP)

class TemplateFinder (object):
  """
  A magical fake dictionary whose keys are templates.
  """
  def __init__(me, dir):
    me._cache = {}
    me._dir = dir
  def __getitem__(me, key):
    try: return me._cache[key]
    except KeyError: pass
    with open(OS.path.join(me._dir, key)) as f: tmpl = f.read()
    me._cache[key] = tmpl
    return tmpl
STATE.kw['TMPL'] = TMPL = TemplateFinder(TMPLDIR)

@CTX.contextmanager
def tmplkw(**kw):
  """
  Context manager: execute the body with additional keyword arguments
  """
  d = dict()
  d.update(STATE.kw)
  d.update(kw)
  with STATE.bind(kw = d): yield

FORMATOPS = {}

class FormatHTML (F.SimpleFormatOperation):
  """
  ~H: escape output suitable for inclusion in HTML.

  With `:', additionally apply quotification.
  """
  def _convert(me, arg):
    if me.colonp: return html_quotify(arg)
    else: return htmlescape(arg)
FORMATOPS['H'] = FormatHTML

class FormatWrap (F.BaseFormatOperation):
  """
  ~<...~@>: wrap enclosed material in another formatting control string.

  The argument is a formatting control.  The enclosed material is split into
  pieces separated by `~;' markers.  The formatting control is performed, and
  passed the list of pieces (as compiled formatting operations) in the
  keyword argument `wrapped'.
  """
  def __init__(me, *args):
    super(FormatWrap, me).__init__(*args)
    pieces = []
    while True:
      piece, delim = F.collect_subformat('>;')
      pieces.append(piece)
      if delim.char == '>': break
    me.pieces = pieces
  def _format(me, atp, colonp):
    op = F.compile(me.getarg.get())
    with F.FORMAT.bind(argmap = dict(F.FORMAT.argmap, wrapped = me.pieces)):
      op.format()
FORMATOPS['<'] = FormatWrap

def format_tmpl(control, **kw):
  with F.COMPILE.bind(opmaps = [FORMATOPS, F.BASEOPS]):
    with tmplkw(**kw):
      F.format(OUT, control, **STATE.kw)

def page(template, header = {}, title = 'Chopwood', **kw):
  header = dict(header, content_type = 'text/html')
  OUT.header(**header)
  format_tmpl(TMPL['wrapper.fhtml'],
              title = title, payload = TMPL[template], **kw)

###--------------------------------------------------------------------------
### Error reporting.

@CTX.contextmanager
def cgi_errors(hook = None):
  """
  Context manager: report errors in the body as useful HTML.

  If HOOK is given, then call it before reporting errors.  It may have set up
  useful stuff.
  """
  try:
    yield None
  except Exception, e:
    if hook: hook()
    if isinstance(e, U.ExpectedError) and not OUT.headerp:
      page('error.fhtml',
           header = dict(status = e.code),
           title = 'Chopwood: error', error = e)
    else:
      exty, exval, extb = SYS.exc_info()
      with tmplkw(exception = TB.format_exception_only(exty, exval),
                  traceback = TB.extract_tb(extb),
                  PARAM = sorted(PARAM),
                  COOKIE = sorted(COOKIE.items()),
                  PATH = PATH,
                  ENV = sorted(ENV.items())):
        if OUT.headerp:
          format_tmpl(TMPL['exception.fhtml'], toplevel = False)
        else:
          page('exception.fhtml',
               header = dict(status = 500),
               title = 'Chopwood: internal error',
               toplevel = True)

###--------------------------------------------------------------------------
### CGI input.

## Lots of global variables to be filled in by `cgiparse'.
METHOD = None
COOKIE = {}
SPECIAL = {}
PARAM = []
PARAMDICT = {}
PATH = []
SSLP = False
HEADER_DONE = lambda: None

## Regular expressions for splitting apart query and cookie strings.
R_QSPLIT = RX.compile('[;&]')
R_CSPLIT = RX.compile(';')

def split_keyvalue(string, delim, default):
  """
  Split a STRING, and generate the resulting KEY=VALUE pairs.

  The string is split at DELIM; the components are parsed into KEY[=VALUE]
  pairs.  The KEYs and VALUEs are stripped of leading and trailing
  whitespace, and form-url-decoded.  If the VALUE is omitted, then the
  DEFAULT is used unless the DEFAULT is `None' in which case the component is
  simply ignored.
  """
  for kv in delim.split(string):
    try:
      k, v = kv.split('=', 1)
    except ValueError:
      if default is None: continue
      else: k, v = kv, default
    k, v = k.strip(), v.strip()
    if not k: continue
    k, v = urldecode(k), urldecode(v)
    yield k, v

def cgiparse():
  """
  Process all of the various exciting CGI environment variables.

  We read environment variables and populate some tables left in global
  variables: it's all rather old-school.  Variables set are as follows.

  `COOKIE'
        A dictionary mapping cookie names to the values provided by the user
        agent.

  `SPECIAL'
        A dictionary holding some special query parameters which are of
        interest at a global level, and should not be passed to a subcommand
        handler.  No new entries will be added to this dictionary, though
        values will be modified to reflect the query parameters discovered.
        Conventionally, such parameters have names beginning with `%'.

  `PARAM'
        The query parameters as a list of (KEY, VALUE) pairs.  Special
        parameters are omitted.

  `PARAMDICT'
        The query parameters as a dictionary.  Special parameters, and
        parameters which appear more than once, are omitted.

  `PATH'
        The trailing `PATH_INFO' path, split at `/' markers, with any
        trailing empty component removed.

  `SSLP'
        True if the client connection is carried over SSL or TLS.
  """

  global METHOD, SSLP

  def getenv(var):
    try: return ENV[var]
    except KeyError: raise U.ExpectedError, (500, "No `%s' supplied" % var)

  ## Yes, we want the request method.
  METHOD = getenv('REQUEST_METHOD')

  ## Acquire the query string.
  if METHOD in ['GET', 'HEAD']:
    q = getenv('QUERY_STRING')

  elif METHOD == 'POST':

    ## We must read the query string from stdin.
    n = getenv('CONTENT_LENGTH')
    if not n.isdigit():
      raise U.ExpectedError, (500, "Invalid CONTENT_LENGTH")
    n = int(n, 10)
    if getenv('CONTENT_TYPE') != 'application/x-www-form-urlencoded':
      raise U.ExpectedError, (500, "Unexpected content type `%s'" % ct)
    q = SYS.stdin.read(n)
    if len(q) != n:
      raise U.ExpectedError, (500, "Failed to read correct length")

  else:
    raise U.ExpectedError, (500, "Unexpected request method `%s'" % METHOD)

  ## Populate the `SPECIAL', `PARAM' and `PARAMDICT' tables.
  seen = set()
  for k, v in split_keyvalue(q, R_QSPLIT, 't'):
    if k in SPECIAL:
      SPECIAL[k] = v
    else:
      PARAM.append((k, v))
      if k in seen:
        del PARAMDICT[k]
      else:
        PARAMDICT[k] = v
        seen.add(k)

  ## Parse out the cookies, if any.
  try: c = ENV['HTTP_COOKIE']
  except KeyError: pass
  else:
    for k, v in split_keyvalue(c, R_CSPLIT, None): COOKIE[k] = v

  ## Set up the `PATH'.
  try: p = ENV['PATH_INFO']
  except KeyError: pass
  else:
    pp = p.lstrip('/').split('/')
    if pp and not pp[-1]: pp.pop()
    PATH[:] = pp

  ## Check the crypto for the connection.
  if ENV.get('SSL_PROTOCOL'):
    SSLP = True

###--------------------------------------------------------------------------
### CGI subcommands.

class Subcommand (SC.Subcommand):
  """
  A CGI subcommand object.

  As for `subcommand.Subcommand', but with additional protocol for processing
  CGI parameters.
  """

  def cgi(me, param, path):
    """
    Invoke the subcommand given a collection of CGI parameters.

    PARAM is a list of (KEY, VALUE) pairs from the CGI query.  The CGI query
    parameters are checked against the subcommand's parameters (making sure
    that mandatory parameters are supplied, that any switches are given
    boolean values, and that only the `rest' parameter, if any, is
    duplicated).

    PATH is a list of trailing path components.  They are used to satisfy the
    `rest' parameter if there is one and there are no query parameters which
    satisfy the `rest' parameter; otherwise, an `ExpectedError' is raised if
    the list of path elements is non-empty.
    """

    global HEADER_DONE

    ## We're going to make a pass over the supplied parameters, and we'll
    ## check them off against the formal parameters as we go; so we'll need
    ## to be able to look them up.  We'll also keep track of the ones we've
    ## seen so that we can make sure that all of the mandatory parameters
    ## were actually supplied.
    ##
    ## To that end: `want' is a dictionary mapping parameter names to
    ## functions which will do something useful with the value; `seen' is a
    ## set of the parameters which have been assigned; and `kw' is going to
    ## be the keyword-argument dictionary we pass to the handler function.
    want = {}
    kw = {}

    def set_value(k, v):
      """Set a simple value: we shouldn't see multiple values."""
      if k in kw:
        raise U.ExpectedError, (400, "Repeated parameter `%s'" % k)
      kw[k] = v
    def set_bool(k, v):
      """Set a simple boolean value: for switches."""
      set_value(k, v.lower() in ['true', 't', 'yes', 'y'])
    def set_list(k, v):
      """Append the value to a list: for the `rest' parameter."""
      kw.setdefault(k, []).append(v)

    ## Set up the `want' map.
    for o in me.opts:
      if o.argname: want[o.name] = set_value
      else: want[o.name] = set_bool
    for p in me.params: want[p.name] = set_value
    for p in me.oparams: want[p.name] = set_value
    if me.rparam: want[me.rparam.name] = set_list

    ## Work through the list of supplied parameters.
    for k, v in param:
      try:
        f = want[k]
      except KeyError:
        if v:
          raise U.ExpectedError, (400, "Unexpected parameter `%s'" % k)
      else:
        f(k, v)

    ## Deal with a path, if there is one.
    if path:
      if me.rparam and me.rparam.name not in kw:
        kw[me.rparam.name] = path
      else:
        raise U.ExpectedError, (404, "Superfluous path elements")

    ## Make sure we saw all of the mandatory parameters.
    for p in me.params:
      if p.name not in kw:
        raise U.ExpectedError, (400, "Missing parameter `%s'" % p.name)

    ## Invoke the subcommand.
    me.func(**kw)

def subcommand(name, contexts, desc, cls = Subcommand, *args, **kw):
  """Decorator for defining CGI subcommands."""
  return SC.subcommand(name, contexts, desc, cls = cls, *args, **kw)

###----- That's all, folks --------------------------------------------------