# Copyright 2011 Google Inc. All Rights Reserved.

"""Multi-credential file store with lock support.

This module implements a JSON credential store where multiple
credentials can be stored in one file.  That file supports locking
both in a single process and across processes.

The credential themselves are keyed off of:
* client_id
* user_agent
* scope

The format of the stored data is like so:
{
  'file_version': 1,
  'data': [
    {
      'key': {
        'clientId': '<client id>',
        'userAgent': '<user agent>',
        'scope': '<scope>'
      },
      'credential': {
        # JSON serialized Credentials.
      }
    }
  ]
}
"""

__author__ = 'jbeda@google.com (Joe Beda)'

import base64
import fcntl
import logging
import os
import pickle
import threading

try:  # pragma: no cover
  import simplejson
except ImportError:  # pragma: no cover
  try:
    # Try to import from django, should work on App Engine
    from django.utils import simplejson
  except ImportError:
    # Should work for Python2.6 and higher.
    import json as simplejson

from client import Storage as BaseStorage
from client import Credentials

logger = logging.getLogger(__name__)

# A dict from 'filename'->_MultiStore instances
_multistores = {}
_multistores_lock = threading.Lock()


class Error(Exception):
  """Base error for this module."""
  pass


class NewerCredentialStoreError(Error):
  """The credential store is a newer version that supported."""
  pass


def get_credential_storage(filename, client_id, user_agent, scope,
                           warn_on_readonly=True):
  """Get a Storage instance for a credential.

  Args:
    filename: The JSON file storing a set of credentials
    client_id: The client_id for the credential
    user_agent: The user agent for the credential
    scope: A string for the scope being requested
    warn_on_readonly: if True, log a warning if the store is readonly

  Returns:
    An object derived from client.Storage for getting/setting the
    credential.
  """
  filename = os.path.realpath(os.path.expanduser(filename))
  _multistores_lock.acquire()
  try:
    multistore = _multistores.setdefault(
        filename, _MultiStore(filename, warn_on_readonly))
  finally:
    _multistores_lock.release()
  return multistore._get_storage(client_id, user_agent, scope)


class _MultiStore(object):
  """A file backed store for multiple credentials."""

  def __init__(self, filename, warn_on_readonly=True):
    """Initialize the class.

    This will create the file if necessary.
    """
    self._filename = filename
    self._thread_lock = threading.Lock()
    self._file_handle = None
    self._read_only = False
    self._warn_on_readonly = warn_on_readonly

    self._create_file_if_needed()

    # Cache of deserialized store.  This is only valid after the
    # _MultiStore is locked or _refresh_data_cache is called.  This is
    # of the form of:
    #
    # (client_id, user_agent, scope) -> OAuth2Credential
    #
    # If this is None, then the store hasn't been read yet.
    self._data = None

  class _Storage(BaseStorage):
    """A Storage object that knows how to read/write a single credential."""

    def __init__(self, multistore, client_id, user_agent, scope):
      self._multistore = multistore
      self._client_id = client_id
      self._user_agent = user_agent
      self._scope = scope

    def acquire_lock(self):
      """Acquires any lock necessary to access this Storage.

      This lock is not reentrant.
      """
      self._multistore._lock()

    def release_lock(self):
      """Release the Storage lock.

      Trying to release a lock that isn't held will result in a
      RuntimeError.
      """
      self._multistore._unlock()

    def locked_get(self):
      """Retrieve credential.

      The Storage lock must be held when this is called.

      Returns:
        oauth2client.client.Credentials
      """
      credential = self._multistore._get_credential(
          self._client_id, self._user_agent, self._scope)
      if credential:
        credential.set_store(self)
      return credential

    def locked_put(self, credentials):
      """Write a credential.

      The Storage lock must be held when this is called.

      Args:
        credentials: Credentials, the credentials to store.
      """
      self._multistore._update_credential(credentials, self._scope)

  def _create_file_if_needed(self):
    """Create an empty file if necessary.

    This method will not initialize the file. Instead it implements a
    simple version of "touch" to ensure the file has been created.
    """
    if not os.path.exists(self._filename):
      old_umask = os.umask(0177)
      try:
        open(self._filename, 'a+').close()
      finally:
        os.umask(old_umask)

  def _lock(self):
    """Lock the entire multistore."""
    self._thread_lock.acquire()
    # Check to see if the file is writeable.
    if os.access(self._filename, os.W_OK):
      self._file_handle = open(self._filename, 'r+')
      fcntl.lockf(self._file_handle.fileno(), fcntl.LOCK_EX)
    else:
      # Cannot open in read/write mode. Open only in read mode.
      self._file_handle = open(self._filename, 'r')
      self._read_only = True
      if self._warn_on_readonly:
        logger.warn('The credentials file (%s) is not writable. Opening in '
                    'read-only mode. Any refreshed credentials will only be '
                    'valid for this run.' % self._filename)
    if os.path.getsize(self._filename) == 0:
      logger.debug('Initializing empty multistore file')
      # The multistore is empty so write out an empty file.
      self._data = {}
      self._write()
    elif not self._read_only or self._data is None:
      # Only refresh the data if we are read/write or we haven't
      # cached the data yet.  If we are readonly, we assume is isn't
      # changing out from under us and that we only have to read it
      # once.  This prevents us from whacking any new access keys that
      # we have cached in memory but were unable to write out.
      self._refresh_data_cache()

  def _unlock(self):
    """Release the lock on the multistore."""
    if not self._read_only:
      fcntl.lockf(self._file_handle.fileno(), fcntl.LOCK_UN)
    self._file_handle.close()
    self._thread_lock.release()

  def _locked_json_read(self):
    """Get the raw content of the multistore file.

    The multistore must be locked when this is called.

    Returns:
      The contents of the multistore decoded as JSON.
    """
    assert self._thread_lock.locked()
    self._file_handle.seek(0)
    return simplejson.load(self._file_handle)

  def _locked_json_write(self, data):
    """Write a JSON serializable data structure to the multistore.

    The multistore must be locked when this is called.

    Args:
      data: The data to be serialized and written.
    """
    assert self._thread_lock.locked()
    if self._read_only:
      return
    self._file_handle.seek(0)
    simplejson.dump(data, self._file_handle, sort_keys=True, indent=2)
    self._file_handle.truncate()

  def _refresh_data_cache(self):
    """Refresh the contents of the multistore.

    The multistore must be locked when this is called.

    Raises:
      NewerCredentialStoreError: Raised when a newer client has written the
        store.
    """
    self._data = {}
    try:
      raw_data = self._locked_json_read()
    except Exception:
      logger.warn('Credential data store could not be loaded. '
                  'Will ignore and overwrite.')
      return

    version = 0
    try:
      version = raw_data['file_version']
    except Exception:
      logger.warn('Missing version for credential data store. It may be '
                  'corrupt or an old version. Overwriting.')
    if version > 1:
      raise NewerCredentialStoreError(
          'Credential file has file_version of %d. '
          'Only file_version of 1 is supported.' % version)

    credentials = []
    try:
      credentials = raw_data['data']
    except (TypeError, KeyError):
      pass

    for cred_entry in credentials:
      try:
        (key, credential) = self._decode_credential_from_json(cred_entry)
        self._data[key] = credential
      except:
        # If something goes wrong loading a credential, just ignore it
        logger.info('Error decoding credential, skipping', exc_info=True)

  def _decode_credential_from_json(self, cred_entry):
    """Load a credential from our JSON serialization.

    Args:
      cred_entry: A dict entry from the data member of our format

    Returns:
      (key, cred) where the key is the key tuple and the cred is the
        OAuth2Credential object.
    """
    raw_key = cred_entry['key']
    client_id = raw_key['clientId']
    user_agent = raw_key['userAgent']
    scope = raw_key['scope']
    key = (client_id, user_agent, scope)
    credential = None
    credential = Credentials.new_from_json(simplejson.dumps(cred_entry['credential']))
    return (key, credential)

  def _write(self):
    """Write the cached data back out.

    The multistore must be locked.
    """
    raw_data = {'file_version': 1}
    raw_creds = []
    raw_data['data'] = raw_creds
    for (cred_key, cred) in self._data.items():
      raw_key = {
          'clientId': cred_key[0],
          'userAgent': cred_key[1],
          'scope': cred_key[2]
          }
      raw_cred = simplejson.loads(cred.to_json())
      raw_creds.append({'key': raw_key, 'credential': raw_cred})
    self._locked_json_write(raw_data)

  def _get_credential(self, client_id, user_agent, scope):
    """Get a credential from the multistore.

    The multistore must be locked.

    Args:
      client_id: The client_id for the credential
      user_agent: The user agent for the credential
      scope: A string for the scope being requested

    Returns:
      The credential specified or None if not present
    """
    key = (client_id, user_agent, scope)

    return self._data.get(key, None)

  def _update_credential(self, cred, scope):
    """Update a credential and write the multistore.

    This must be called when the multistore is locked.

    Args:
      cred: The OAuth2Credential to update/set
      scope: The scope that this credential covers
    """
    key = (cred.client_id, cred.user_agent, scope)
    self._data[key] = cred
    self._write()

  def _get_storage(self, client_id, user_agent, scope):
    """Get a Storage object to get/set a credential.

    This Storage is a 'view' into the multistore.

    Args:
      client_id: The client_id for the credential
      user_agent: The user agent for the credential
      scope: A string for the scope being requested

    Returns:
      A Storage object that can be used to get/set this cred
    """
    return self._Storage(self, client_id, user_agent, scope)