#
# Copyright 2009 Google Inc. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""A mechanism for library configuration.

Whenever App Engine library code needs a user-configurable value, it should use
the following protocol:
    1. Pick a prefix unique to the library module, for example: `mylib`.
    2. Call `lib_config.register(prefix, mapping)` with that prefix as the first
       argument and a dict mapping suffixes to default functions as the second.
    3. The `register()` function returns a configuration handle that is unique
       to this prefix. The configuration handle object has attributes that
       correspond to each of the suffixes given in the mapping. Call these
       functions to access the user's configuration value.  If the user didn't
       configure a function, the default function from the mapping is called
       instead.
    4. Document the function name and its signature and semantics.

Users that want to provide configuration values should create a module named
`appengine_config.py` in the top-level directory of their application and define
functions as documented by various App Engine library components in that module.
To change the configuration, edit the file and re-deploy the application. When
using the SDK, no redeployment is required; the development server will pick up
the changes the next time it handles a request.

Third party libraries can also use this mechanism. For casual use, calling the
`register()` method with a unique prefix is acceptable. For more complex
libraries, however, you should instantiate a new `LibConfigRegistry` instance
that uses a different module name.


Example `appengine_config.py` file::

    from somewhere import MyMiddleWareClass

    def mylib_add_middleware(app):
      app = MyMiddleWareClass(app)
      return app


Example library use::

    from google.appengine.api import lib_config

    config_handle = lib_config.register(
        'mylib',
        {'add_middleware': lambda app: app})

    def add_middleware(app):
      return config_handle.add_middleware(app)
"""



__all__ = ['DEFAULT_MODNAME',
           'LibConfigRegistry',
           'ConfigHandle',
           'register',
           'main',
           ]


import logging
import os
import sys
import threading


DEFAULT_MODNAME = 'appengine_config'

# N.B.(sammc): To avoid issues with lock ordering, we never hold the
# LibConfigRegistry lock when calling ConfigHandle methods.


class LibConfigRegistry(object):
  """A registry containing library configuration values."""

  def __init__(self, modname):
    """Constructor.

    Args:
      modname: The module name to be imported.

    Note: the actual import of this module is deferred until the first
    time a configuration value is requested through attribute access
    on a ConfigHandle instance.
    """
    self._modname = modname
    self._registrations = {}
    self._module = None
    self._lock = threading.RLock()

  def register(self, prefix, mapping):
    """Registers a set of configuration names.

    Args:
      prefix: A shared prefix for the configuration names being registered.
          If the prefix doesn't end in `_`, that character is appended.
      mapping: A dict that maps suffix strings to default values.

    Returns:
      A `ConfigHandle` instance.

    You can re-register the same prefix: the mappings are merged, and for
    duplicate suffixes, the most recent registration is used.
    """
    if not prefix.endswith('_'):
      prefix += '_'
    self._lock.acquire()
    try:
      handle = self._registrations.get(prefix)
      if handle is None:
        handle = ConfigHandle(prefix, self)
        self._registrations[prefix] = handle
    finally:
      self._lock.release()
    handle._update_defaults(mapping)
    return handle

  def initialize(self, import_func=__import__):
    """Tries to import the configuration module if it is not already imported.

    This function always sets `self._module` to a value that is not `None`;
    either the imported module (if it was imported successfully) or a dummy
    `object()` instance (if an `ImportError` was raised) is used. Other
    exceptions are not caught.

    When a dummy instance is used, the instance is also put in `sys.modules`.
    This usage allows us to detect when `sys.modules` was changed (as
    `dev_appserver.py` does when it notices source code changes) and retries the
    `__import__` in that case, while skipping it (for speed) if nothing has
    changed.

    Args:
      import_func: Used for dependency injection.
    """
    self._lock.acquire()
    try:
      if (self._module is not None and
          self._module is sys.modules.get(self._modname)):
        return
      try:
        import_func(self._modname)
      except ImportError, err:
        if str(err) != 'No module named %s' % self._modname:
          # Something else failed to import.  Don't swallow the failure.
          raise
        self._module = object()  # Cache negative result.
        sys.modules[self._modname] = self._module
      else:
        self._module = sys.modules[self._modname]
    finally:
      self._lock.release()

  def reset(self):
    """Drops the imported configuration module.

    If the configuration module has not been imported, no operation occurs, and
    the next operation takes place.
    """
    self._lock.acquire()
    try:
      if self._module is None:
        # There is nothing to do.
        return

      self._module = None
      handles = self._registrations.values()
    finally:
      self._lock.release()
    for handle in handles:
      handle._clear_cache()

  def _pairs(self, prefix):
    """Generates `(key, value)` pairs from the config module matching prefix.

    Args:
      prefix: A prefix string ending in `_`, for example: `mylib_`.

    Yields:
      `(key, value)` pairs, where `key` is the configuration name with the
      prefix removed, and `value` is the corresponding value.
    """
    self._lock.acquire()
    try:
      mapping = getattr(self._module, '__dict__', None)
      if not mapping:
        return
      items = mapping.items()
    finally:
      self._lock.release()
    nskip = len(prefix)
    for key, value in items:
      if key.startswith(prefix):
        yield key[nskip:], value

  def _dump(self):
    """Prints information about all registrations to stdout."""
    self.initialize()
    handles = []
    self._lock.acquire()
    try:
      if not hasattr(self._module, '__dict__'):
        print 'Module %s.py does not exist.' % self._modname
      elif not self._registrations:
        print 'No registrations for %s.py.' % self._modname
      else:
        print 'Registrations in %s.py:' % self._modname
        print '-'*40
        handles = self._registrations.items()
    finally:
      self._lock.release()
    for _, handle in sorted(handles):
      handle._dump()


class ConfigHandle(object):
  """A set of configuration for a single library module or package.

  Public attributes of instances of this class are configuration values.
  Attributes are dynamically computed (in `__getattr__()`) and cached as regular
  instance attributes.
  """

  _initialized = False  # Set to True by _update_configs()

  def __init__(self, prefix, registry):
    """Constructor.

    Args:
      prefix: A shared prefix for the configuration names being registered. It
          must end in `_`. This requirement is enforced by
          `LibConfigRegistry`.
      registry: A `LibConfigRegistry` instance.
    """
    assert prefix.endswith('_')
    self._prefix = prefix
    self._defaults = {}
    self._overrides = {}
    self._registry = registry
    self._lock = threading.RLock()

  def _update_defaults(self, mapping):
    """Updates the default mappings.

    Args:
      mapping: A dict mapping suffix strings to default values.
    """
    self._lock.acquire()
    try:
      for key, value in mapping.iteritems():
        if key.startswith('__') and key.endswith('__'):
          continue
        self._defaults[key] = value
      if self._initialized:
        self._update_configs()
    finally:
      self._lock.release()

  def _update_configs(self):
    """Updates the configuration values.

    This clears the cached values, initializes the registry, and loads
    the configuration values from the config module.
    """
    self._lock.acquire()
    try:
      if self._initialized:
        self._clear_cache()
      self._registry.initialize()
      for key, value in self._registry._pairs(self._prefix):
        if key not in self._defaults:
          logging.warn('Configuration "%s" not recognized', self._prefix + key)
        else:
          self._overrides[key] = value
      self._initialized = True
    finally:
      self._lock.release()

  def _clear_cache(self):
    """Clears the cached values."""
    self._lock.acquire()
    try:
      self._initialized = False
      for key in self._defaults:
        self._overrides.pop(key, None)
        try:
          delattr(self, key)
        except AttributeError:
          pass
    finally:
      self._lock.release()

  def _dump(self):
    """Prints information about this set of registrations to stdout."""
    self._lock.acquire()
    try:
      print 'Prefix %s:' % self._prefix
      if self._overrides:
        print '  Overrides:'
        for key in sorted(self._overrides):
          print '    %s = %r' % (key, self._overrides[key])
      else:
        print '  No overrides'
      if self._defaults:
        print '  Defaults:'
        for key in sorted(self._defaults):
          print '    %s = %r' % (key, self._defaults[key])
      else:
        print '  No defaults'
      print '-'*40
    finally:
      self._lock.release()

  def __getattr__(self, suffix):
    """Dynamic attribute access.

    Args:
      suffix: The attribute name.

    Returns:
      A configuration values.

    Raises:
      AttributeError: If the suffix is not a registered suffix.

    The first time an attribute is referenced, this method is invoked. The value
    returned is taken either from the config module or from the registered
    default.
    """
    self._lock.acquire()
    try:
      if not self._initialized:
        self._update_configs()
      if suffix in self._overrides:
        value = self._overrides[suffix]
      elif suffix in self._defaults:
        value = self._defaults[suffix]
      else:
        raise AttributeError(suffix)
      ## # Cache the outcome on the instance.
      setattr(self, suffix, value)
      return value
    finally:
      self._lock.release()


# Default registry for the register() function to use.
_default_registry = LibConfigRegistry(DEFAULT_MODNAME)


def register(prefix, mapping):
  """Register a set of configurations with the default config module.

  Args:
    prefix: A shared prefix for the configuration names being registered.
        If the prefix doesn't end in `_`, that character is appended.
    mapping: A dict mapping suffix strings to default values.

  Returns:
    A `ConfigHandle` instance.
  """
  return _default_registry.register(prefix, mapping)


def main():
  """Dumps the configuration, using a CGI-style request handler.

  Put this in your `app.yaml` file to enable (you can pick any URL)::

      - url: /lib_config
        script: $PYTHON_LIB/google/appengine/api/lib_config.py


  Note:
      Unless you are using the SDK, you must be an administrator to use this
      function.
  """
  if not os.getenv('SERVER_SOFTWARE', '').startswith('Dev'):
    from googlecloudsdk.third_party.appengine.api import users
    if not users.is_current_user_admin():
      if users.get_current_user() is None:
        print 'Status: 302'
        print 'Location:', users.create_login_url(os.getenv('PATH_INFO', ''))
      else:
        print 'Status: 403'
        print
        print 'Forbidden'
      return

  print 'Content-type: text/plain'
  print
  _default_registry._dump()


if __name__ == '__main__':
  main()
