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"""Dispatch Web Socket request.
"""


import os
import re

import util


_SOURCE_PATH_PATTERN = re.compile(r'(?i)_wsh\.py$')
_SOURCE_SUFFIX = '_wsh.py'
_DO_EXTRA_HANDSHAKE_HANDLER_NAME = 'web_socket_do_extra_handshake'
_TRANSFER_DATA_HANDLER_NAME = 'web_socket_transfer_data'


class DispatchError(Exception):
    """Exception in dispatching Web Socket request."""

    pass


def _normalize_path(path):
    """Normalize path.

    Args:
        path: the path to normalize.

    Path is converted to the absolute path.
    The input path can use either '\\' or '/' as the separator.
    The normalized path always uses '/' regardless of the platform.
    """

    path = path.replace('\\', os.path.sep)
    path = os.path.realpath(path)
    path = path.replace('\\', '/')
    return path


def _path_to_resource_converter(base_dir):
    base_dir = _normalize_path(base_dir)
    base_len = len(base_dir)
    suffix_len = len(_SOURCE_SUFFIX)
    def converter(path):
        if not path.endswith(_SOURCE_SUFFIX):
            return None
        path = _normalize_path(path)
        if not path.startswith(base_dir):
            return None
        return path[base_len:-suffix_len]
    return converter


def _source_file_paths(directory):
    """Yield Web Socket Handler source file names in the given directory."""

    for root, unused_dirs, files in os.walk(directory):
        for base in files:
            path = os.path.join(root, base)
            if _SOURCE_PATH_PATTERN.search(path):
                yield path


def _source(source_str):
    """Source a handler definition string."""

    global_dic = {}
    try:
        exec source_str in global_dic
    except Exception:
        raise DispatchError('Error in sourcing handler:' +
                            util.get_stack_trace())
    return (_extract_handler(global_dic, _DO_EXTRA_HANDSHAKE_HANDLER_NAME),
            _extract_handler(global_dic, _TRANSFER_DATA_HANDLER_NAME))


def _extract_handler(dic, name):
    if name not in dic:
        raise DispatchError('%s is not defined.' % name)
    handler = dic[name]
    if not callable(handler):
        raise DispatchError('%s is not callable.' % name)
    return handler


class Dispatcher(object):
    """Dispatches Web Socket requests.

    This class maintains a map from resource name to handlers.
    """

    def __init__(self, root_dir, scan_dir=None):
        """Construct an instance.

        Args:
            root_dir: The directory where handler definition files are
                      placed.
            scan_dir: The directory where handler definition files are
                      searched. scan_dir must be a directory under root_dir,
                      including root_dir itself.  If scan_dir is None, root_dir
                      is used as scan_dir. scan_dir can be useful in saving
                      scan time when root_dir contains many subdirectories.
        """

        self._handlers = {}
        self._source_warnings = []
        if scan_dir is None:
            scan_dir = root_dir
        if not os.path.realpath(scan_dir).startswith(
                os.path.realpath(root_dir)):
            raise DispatchError('scan_dir:%s must be a directory under '
                                'root_dir:%s.' % (scan_dir, root_dir))
        self._source_files_in_dir(root_dir, scan_dir)

    def add_resource_path_alias(self,
                                alias_resource_path, existing_resource_path):
        """Add resource path alias.

        Once added, request to alias_resource_path would be handled by
        handler registered for existing_resource_path.

        Args:
            alias_resource_path: alias resource path
            existing_resource_path: existing resource path
        """
        try:
            handler = self._handlers[existing_resource_path]
            self._handlers[alias_resource_path] = handler
        except KeyError:
            raise DispatchError('No handler for: %r' % existing_resource_path)

    def source_warnings(self):
        """Return warnings in sourcing handlers."""

        return self._source_warnings

    def do_extra_handshake(self, request):
        """Do extra checking in Web Socket handshake.

        Select a handler based on request.uri and call its
        web_socket_do_extra_handshake function.

        Args:
            request: mod_python request.
        """

        do_extra_handshake_, unused_transfer_data = self._handler(request)
        try:
            do_extra_handshake_(request)
        except Exception, e:
            util.prepend_message_to_exception(
                    '%s raised exception for %s: ' % (
                            _DO_EXTRA_HANDSHAKE_HANDLER_NAME,
                            request.ws_resource),
                    e)
            raise

    def transfer_data(self, request):
        """Let a handler transfer_data with a Web Socket client.

        Select a handler based on request.ws_resource and call its
        web_socket_transfer_data function.

        Args:
            request: mod_python request.
        """

        unused_do_extra_handshake, transfer_data_ = self._handler(request)
        try:
            transfer_data_(request)
        except Exception, e:
            util.prepend_message_to_exception(
                    '%s raised exception for %s: ' % (
                            _TRANSFER_DATA_HANDLER_NAME, request.ws_resource),
                    e)
            raise

    def _handler(self, request):
        try:
            ws_resource_path = request.ws_resource.split('?', 1)[0]
            return self._handlers[ws_resource_path]
        except KeyError:
            raise DispatchError('No handler for: %r' % request.ws_resource)

    def _source_files_in_dir(self, root_dir, scan_dir):
        """Source all the handler source files in the scan_dir directory.

        The resource path is determined relative to root_dir.
        """

        to_resource = _path_to_resource_converter(root_dir)
        for path in _source_file_paths(scan_dir):
            try:
                handlers = _source(open(path).read())
            except DispatchError, e:
                self._source_warnings.append('%s: %s' % (path, e))
                continue
            self._handlers[to_resource(path)] = handlers


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