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lint.py

# (c) 2005 Ian Bicking and contributors; written for Paste (http://pythonpaste.org)
# Licensed under the MIT license: http://www.opensource.org/licenses/mit-license.php
# Also licenced under the Apache License, 2.0: http://opensource.org/licenses/apache2.0.php
# Licensed to PSF under a Contributor Agreement
"""
Middleware to check for obedience to the WSGI specification.

Some of the things this checks:

* Signature of the application and start_response (including that
  keyword arguments are not used).

* Environment checks:

  - Environment is a dictionary (and not a subclass).

  - That all the required keys are in the environment: REQUEST_METHOD,
    SERVER_NAME, SERVER_PORT, wsgi.version, wsgi.input, wsgi.errors,
    wsgi.multithread, wsgi.multiprocess, wsgi.run_once

  - That HTTP_CONTENT_TYPE and HTTP_CONTENT_LENGTH are not in the
    environment (these headers should appear as CONTENT_LENGTH and
    CONTENT_TYPE).

  - Warns if QUERY_STRING is missing, as the cgi module acts
    unpredictably in that case.

  - That CGI-style variables (that don't contain a .) have
    (non-unicode) string values

  - That wsgi.version is a tuple

  - That wsgi.url_scheme is 'http' or 'https' (@@: is this too
    restrictive?)

  - Warns if the REQUEST_METHOD is not known (@@: probably too
    restrictive).

  - That SCRIPT_NAME and PATH_INFO are empty or start with /

  - That at least one of SCRIPT_NAME or PATH_INFO are set.

  - That CONTENT_LENGTH is a positive integer.

  - That SCRIPT_NAME is not '/' (it should be '', and PATH_INFO should
    be '/').

  - That wsgi.input has the methods read, readline, readlines, and
    __iter__

  - That wsgi.errors has the methods flush, write, writelines

* The status is a string, contains a space, starts with an integer,
  and that integer is in range (> 100).

* That the headers is a list (not a subclass, not another kind of
  sequence).

* That the items of the headers are tuples of strings.

* That there is no 'status' header (that is used in CGI, but not in
  WSGI).

* That the headers don't contain newlines or colons, end in _ or -, or
  contain characters codes below 037.

* That Content-Type is given if there is content (CGI often has a
  default content type, but WSGI does not).

* That no Content-Type is given when there is no content (@@: is this
  too restrictive?)

* That the exc_info argument to start_response is a tuple or None.

* That all calls to the writer are with strings, and no other methods
  on the writer are accessed.

* That wsgi.input is used properly:

  - .read() is called with zero or one argument

  - That it returns a string

  - That readline, readlines, and __iter__ return strings

  - That .close() is not called

  - No other methods are provided

* That wsgi.errors is used properly:

  - .write() and .writelines() is called with a string

  - That .close() is not called, and no other methods are provided.

* The response iterator:

  - That it is not a string (it should be a list of a single string; a
    string will work, but perform horribly).

  - That .next() returns a string

  - That the iterator is not iterated over until start_response has
    been called (that can signal either a server or application
    error).

  - That .close() is called (doesn't raise exception, only prints to
    sys.stderr, because we only know it isn't called when the object
    is garbage collected).
"""

import re
import sys
from types import DictType, StringType, TupleType, ListType
import warnings

header_re = re.compile(r'^[a-zA-Z][a-zA-Z0-9\-_]*$')
bad_header_value_re = re.compile(r'[\000-\037]')

00120 class WSGIWarning(Warning):
    """
    Raised in response to WSGI-spec-related warnings
    """

def middleware(application, global_conf=None):

    """
    When applied between a WSGI server and a WSGI application, this
    middleware will check for WSGI compliancy on a number of levels.
    This middleware does not modify the request or response in any
    way, but will throw an AssertionError if anything seems off
    (except for a failure to close the application iterator, which
    will be printed to stderr -- there's no way to throw an exception
    at that point).
    """
    
    def lint_app(*args, **kw):
        assert len(args) == 2, "Two arguments required"
        assert not kw, "No keyword arguments allowed"
        environ, start_response = args

        check_environ(environ)

        # We use this to check if the application returns without
        # calling start_response:
        start_response_started = []

        def start_response_wrapper(*args, **kw):
            assert len(args) == 2 or len(args) == 3, (
                "Invalid number of arguments: %s" % args)
            assert not kw, "No keyword arguments allowed"
            status = args[0]
            headers = args[1]
            if len(args) == 3:
                exc_info = args[2]
            else:
                exc_info = None

            check_status(status)
            check_headers(headers)
            check_content_type(status, headers)
            check_exc_info(exc_info)

            start_response_started.append(None)
            return WriteWrapper(start_response(*args))

        environ['wsgi.input'] = InputWrapper(environ['wsgi.input'])
        environ['wsgi.errors'] = ErrorWrapper(environ['wsgi.errors'])

        iterator = application(environ, start_response_wrapper)
        assert iterator is not None and iterator != False, (
            "The application must return an iterator, if only an empty list")

        check_iterator(iterator)

        return IteratorWrapper(iterator, start_response_started)

    return lint_app

class InputWrapper:

    def __init__(self, wsgi_input):
        self.input = wsgi_input

    def read(self, *args):
        assert len(args) <= 1
        v = self.input.read(*args)
        assert type(v) is type("")
        return v

    def readline(self):
        v = self.input.readline()
        assert type(v) is type("")
        return v

    def readlines(self, *args):
        assert len(args) <= 1
        lines = self.input.readlines(*args)
        assert type(lines) is type([])
        for line in lines:
            assert type(line) is type("")
        return lines
    
    def __iter__(self):
        while 1:
            line = self.readline()
            if not line:
                return
            yield line

    def close(self):
        assert 0, "input.close() must not be called"

class ErrorWrapper:

    def __init__(self, wsgi_errors):
        self.errors = wsgi_errors

    def write(self, s):
        assert type(s) is type("")
        self.errors.write(s)

    def flush(self):
        self.errors.flush()

    def writelines(self, seq):
        for line in seq:
            self.write(line)

    def close(self):
        assert 0, "errors.close() must not be called"

class WriteWrapper:

    def __init__(self, wsgi_writer):
        self.writer = wsgi_writer

    def __call__(self, s):
        assert type(s) is type("")
        self.writer(s)

class PartialIteratorWrapper:

    def __init__(self, wsgi_iterator):
        self.iterator = wsgi_iterator

    def __iter__(self):
        # We want to make sure __iter__ is called
        return IteratorWrapper(self.iterator)

class IteratorWrapper:

    def __init__(self, wsgi_iterator, check_start_response):
        self.original_iterator = wsgi_iterator
        self.iterator = iter(wsgi_iterator)
        self.closed = False
        self.check_start_response = check_start_response

    def __iter__(self):
        return self

    def next(self):
        assert not self.closed, (
            "Iterator read after closed")
        v = self.iterator.next()
        if self.check_start_response is not None:
            assert self.check_start_response, (
                "The application returns and we started iterating over its body, but start_response has not yet been called")
            self.check_start_response = None
        return v
        
    def close(self):
        self.closed = True
        if hasattr(self.original_iterator, 'close'):
            self.original_iterator.close()

    def __del__(self):
        if not self.closed:
            sys.stderr.write(
                "Iterator garbage collected without being closed")
        assert self.closed, (
            "Iterator garbage collected without being closed")

def check_environ(environ):
    assert type(environ) is DictType, (
        "Environment is not of the right type: %r (environment: %r)"
        % (type(environ), environ))
    
    for key in ['REQUEST_METHOD', 'SERVER_NAME', 'SERVER_PORT',
                'wsgi.version', 'wsgi.input', 'wsgi.errors',
                'wsgi.multithread', 'wsgi.multiprocess',
                'wsgi.run_once']:
        assert key in environ, (
            "Environment missing required key: %r" % key)

    for key in ['HTTP_CONTENT_TYPE', 'HTTP_CONTENT_LENGTH']:
        assert key not in environ, (
            "Environment should not have the key: %s "
            "(use %s instead)" % (key, key[5:]))

    if 'QUERY_STRING' not in environ:
        warnings.warn(
            'QUERY_STRING is not in the WSGI environment; the cgi '
            'module will use sys.argv when this variable is missing, '
            'so application errors are more likely',
            WSGIWarning)

    for key in environ.keys():
        if '.' in key:
            # Extension, we don't care about its type
            continue
        assert type(environ[key]) is StringType, (
            "Environmental variable %s is not a string: %r (value: %r)"
            % (key, type(environ[key]), environ[key]))
        
    assert type(environ['wsgi.version']) is TupleType, (
        "wsgi.version should be a tuple (%r)" % environ['wsgi.version'])
    assert environ['wsgi.url_scheme'] in ('http', 'https'), (
        "wsgi.url_scheme unknown: %r" % environ['wsgi.url_scheme'])

    check_input(environ['wsgi.input'])
    check_errors(environ['wsgi.errors'])

    # @@: these need filling out:
    if environ['REQUEST_METHOD'] not in (
        'GET', 'HEAD', 'POST', 'OPTIONS','PUT','DELETE','TRACE'):
        warnings.warn(
            "Unknown REQUEST_METHOD: %r" % environ['REQUEST_METHOD'],
            WSGIWarning)

    assert (not environ.get('SCRIPT_NAME')
            or environ['SCRIPT_NAME'].startswith('/')), (
        "SCRIPT_NAME doesn't start with /: %r" % environ['SCRIPT_NAME'])
    assert (not environ.get('PATH_INFO')
            or environ['PATH_INFO'].startswith('/')), (
        "PATH_INFO doesn't start with /: %r" % environ['PATH_INFO'])
    if environ.get('CONTENT_LENGTH'):
        assert int(environ['CONTENT_LENGTH']) >= 0, (
            "Invalid CONTENT_LENGTH: %r" % environ['CONTENT_LENGTH'])

    if not environ.get('SCRIPT_NAME'):
        assert environ.has_key('PATH_INFO'), (
            "One of SCRIPT_NAME or PATH_INFO are required (PATH_INFO "
            "should at least be '/' if SCRIPT_NAME is empty)")
    assert environ.get('SCRIPT_NAME') != '/', (
        "SCRIPT_NAME cannot be '/'; it should instead be '', and "
        "PATH_INFO should be '/'")

def check_input(wsgi_input):
    for attr in ['read', 'readline', 'readlines', '__iter__']:
        assert hasattr(wsgi_input, attr), (
            "wsgi.input (%r) doesn't have the attribute %s"
            % (wsgi_input, attr))

def check_errors(wsgi_errors):
    for attr in ['flush', 'write', 'writelines']:
        assert hasattr(wsgi_errors, attr), (
            "wsgi.errors (%r) doesn't have the attribute %s"
            % (wsgi_errors, attr))

def check_status(status):
    assert type(status) is StringType, (
        "Status must be a string (not %r)" % status)
    # Implicitly check that we can turn it into an integer:
    status_code = status.split(None, 1)[0]
    assert len(status_code) == 3, (
        "Status codes must be three characters: %r" % status_code)
    status_int = int(status_code)
    assert status_int >= 100, "Status code is invalid: %r" % status_int
    if len(status) < 4 or status[3] != ' ':
        warnings.warn(
            "The status string (%r) should be a three-digit integer "
            "followed by a single space and a status explanation"
            % status, WSGIWarning)

def check_headers(headers):
    assert type(headers) is ListType, (
        "Headers (%r) must be of type list: %r"
        % (headers, type(headers)))
    header_names = {}
    for item in headers:
        assert type(item) is TupleType, (
            "Individual headers (%r) must be of type tuple: %r"
            % (item, type(item)))
        assert len(item) == 2
        name, value = item
        assert name.lower() != 'status', (
            "The Status header cannot be used; it conflicts with CGI "
            "script, and HTTP status is not given through headers "
            "(value: %r)." % value)
        header_names[name.lower()] = None
        assert '\n' not in name and ':' not in name, (
            "Header names may not contain ':' or '\\n': %r" % name)
        assert header_re.search(name), "Bad header name: %r" % name
        assert not name.endswith('-') and not name.endswith('_'), (
            "Names may not end in '-' or '_': %r" % name)
        assert not bad_header_value_re.search(value), (
            "Bad header value: %r (bad char: %r)"
            % (value, bad_header_value_re.search(value).group(0)))

def check_content_type(status, headers):
    code = int(status.split(None, 1)[0])
    # @@: need one more person to verify this interpretation of RFC 2616
    #     http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
    NO_MESSAGE_BODY = (204, 304)
    for name, value in headers:
        if name.lower() == 'content-type':
            if code not in NO_MESSAGE_BODY:
                return
            assert 0, (("Content-Type header found in a %s response, "
                        "which must not return content.") % code)
    if code not in NO_MESSAGE_BODY:
        assert 0, "No Content-Type header found in headers (%s)" % headers

def check_exc_info(exc_info):
    assert exc_info is None or type(exc_info) is type(()), (
        "exc_info (%r) is not a tuple: %r" % (exc_info, type(exc_info)))
    # More exc_info checks?

def check_iterator(iterator):
    # Technically a string is legal, which is why it's a really bad
    # idea, because it may cause the response to be returned
    # character-by-character
    assert not isinstance(iterator, str), (
        "You should not return a string as your application iterator, "
        "instead return a single-item list containing that string.")

def make_middleware(application, global_conf):
    # @@: global_conf should be taken out of the middleware function,
    # and isolated here
    return middleware(application)

make_middleware.__doc__ = __doc__

__all__ = ['middleware', 'make_middleware']

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