Part of DjangoSpecifications

Threading improvements in Django

According to tickets #5632, #6950 and discussions ​http://groups.google.com/group/django-users/browse_frm/thread/a7d42475b66530bd, ​http://groups.google.com/group/django-developers/browse_thread/thread/fbcfa88c997d1bb3, at least the following components of Django are not entirely thread-safe:

• django.template.loader
• django.db.models
• django.contrib.sessions

This specification outlines the changes that need to be implemented to solve these issues.

A related task is to identify other components not mentioned here that have threading issues.

See #1442 and ​http://groups.google.com/group/django-developers/browse_thread/thread/905f79e350525c95 for threading discussions.

Globals

There are three types of globals:

1. read-only globals that are never assigned to (THREAD-SAFE),
2. globals assigned to in a function by using the global keyword (NOT THREAD-SAFE),
3. global mutable data structures (lists and dictionaries, also instances) that are assigned to at module level but whose elements are modified in functions and that are accessed without using the global keyword (NOT THREAD-SAFE unless never modified).

"Not thread-safe" has two broad subcategories:

• inefficiencies due to calls meant to occur only once occurring more than once (the general if not foo: initialize foo, including memoize decorator),
• errors due to incomplete initialization.

Incomplete initialization errors can generally be avoided by using full assignment instead of elementwise modification (that's how it is done in the source mostly):

BAD

foo = []
def bar():
   ...
   if not foo:
      for x in y:
	 foo.append(x)

BETTER

foo = []
def bar():
   ...
   global foo
   if not foo:
      tmp = []
      for x in y:
	 tmp.append(x)
      foo = tmp # assignment is "atomic"

Modules' use of globals

See below for raw grep results.

Module	Globals	Incomplete init	Inefficiencies
settings and global_settings	?	not reviewed
utils/_decimal.py	lots, including code	not reviewed
django/contrib/sites/models.py	SITE_CACHE	OK	one db hit intended, more than one possible
django/template/context.py	_standard_context_processors	OK	duplicated module loading with __import__ possible
django/template/__init__.py	invalid_var_format_string, libraries, builtins	OK	duplicated module loading with __import__ possible
django/template/loader.py	template_source_loaders	PROBLEM, see #6950, patch attached	duplicated module loading with __import__ possible
django/template/loaders/app_directories.py	app_template_dirs	PROBABLY OK, appending to list at module level is not thread-safe, but the module is most likely cached before threads get access to it
django/utils/translation/trans_real.py	_accepted, _active, _default, _translations	continue review here	foo
django/core/urlresolvers.py	_callable_cache, _resolver_cache	memoize decorator
django/core/serializers/__init__.py	_serializers
django/db/models/fields/related.py	pending_lookups
django/db/transaction.py	dirty, state
django/dispatch/dispatcher.py	`connections, senders, sendersBack

Raw grep results

Globals accessed with the global keyword

$ grep -r '^[[:space:]]*global ' django/ | egrep -v '(\.svn|\.html|\.css|\.pyc|_doctest\.py)' | sort | uniq

yields the following results

django/contrib/sites/models.py:        global SITE_CACHE
django/core/management/__init__.py:    global _commands
django/template/context.py:    global _standard_context_processors
django/template/__init__.py:                    global invalid_var_format_string
django/template/loader.py:    global template_source_loaders
django/utils/translation/trans_real.py:    global _accepted
django/utils/translation/trans_real.py:    global _active
django/utils/translation/trans_real.py:    global _default, _active
django/utils/translation/trans_real.py:        global _translations
django/utils/translation/trans_real.py:    global _translations

Out of these, django.core.management is not used in multi-threading context.

Global dictionaries

$ grep -r '^[[:alnum:]_]\+ *= *{' django | egrep -v '(\.svn|_doctest\.py)' | sort

yields the following results

django/conf/global_settings.py:ABSOLUTE_URL_OVERRIDES = {}
django/conf/global_settings.py:DATABASE_OPTIONS = {}          # Set to empty dictionary for default.
django/contrib/admin/utils.py:ROLES = {
django/contrib/admin/views/doc.py:DATA_TYPE_MAPPING = {
django/contrib/formtools/tests.py:test_data = {'field1': u'foo',
django/contrib/localflavor/ca/ca_provinces.py:PROVINCES_NORMALIZED = {
django/contrib/localflavor/in_/in_states.py:STATES_NORMALIZED = {
django/contrib/localflavor/us/us_states.py:STATES_NORMALIZED = {
django/contrib/sites/models.py:SITE_CACHE = {}
django/core/cache/__init__.py:BACKENDS = {
django/core/cache/__init__.py:DEPRECATED_BACKENDS = {
django/core/handlers/wsgi.py:STATUS_CODE_TEXT = {
django/core/serializers/__init__.py:BUILTIN_SERIALIZERS = {
django/core/serializers/__init__.py:_serializers = {}
django/core/servers/basehttp.py:_hop_headers = {
django/core/servers/fastcgi.py:FASTCGI_OPTIONS = {
django/core/urlresolvers.py:_callable_cache = {} # Maps view and url pattern names to their view functions.
django/core/urlresolvers.py:_resolver_cache = {} # Maps urlconf modules to RegexURLResolver instances.
django/db/backends/dummy/creation.py:DATA_TYPES = {}
django/db/backends/dummy/introspection.py:DATA_TYPES_REVERSE = {}
django/db/backends/mysql/creation.py:DATA_TYPES = {
django/db/backends/mysql/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/mysql_old/creation.py:DATA_TYPES = {
django/db/backends/mysql_old/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/oracle/creation.py:DATA_TYPES = {
django/db/backends/oracle/creation.py:REMEMBER = {}
django/db/backends/oracle/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/postgresql/creation.py:DATA_TYPES = {
django/db/backends/postgresql/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/postgresql_psycopg2/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/sqlite3/creation.py:DATA_TYPES = {
django/db/backends/sqlite3/introspection.py:BASE_DATA_TYPES_REVERSE = {
django/db/models/fields/related.py:pending_lookups = {}
django/db/models/query.py:LEGACY_ORDERING_MAPPING = {'ASC': '_', 'DESC': '-_', 'RANDOM': '?'}
django/db/transaction.py:dirty = {}
django/db/transaction.py:state = {}
django/dispatch/dispatcher.py:connections = {}
django/dispatch/dispatcher.py:senders = {}
django/dispatch/dispatcher.py:sendersBack = {}
django/template/__init__.py:libraries = {}
django/utils/dates.py:MONTHS = {
django/utils/dates.py:MONTHS_3 = {
django/utils/dates.py:MONTHS_3_REV = {
django/utils/dates.py:MONTHS_AP = { # month names in Associated Press style
django/utils/dates.py:WEEKDAYS = {
django/utils/dates.py:WEEKDAYS_ABBR = {
django/utils/dates.py:WEEKDAYS_REV = {
django/utils/_decimal.py:_condition_map = {ConversionSyntax:InvalidOperation,
django/utils/_decimal.py:_infinity_map = {
django/utils/simplejson/decoder.py:BACKSLASH = {
django/utils/simplejson/decoder.py:_CONSTANTS = {
django/utils/simplejson/encoder.py:ESCAPE_DCT = {
django/utils/termcolors.py:opt_dict = {'bold': '1', 'underscore': '4', 'blink': '5', 'reverse': '7', 'conceal': '8'}
django/utils/translation/trans_null.py:TECHNICAL_ID_MAP = {
django/utils/translation/trans_real.py:_accepted = {}
django/utils/translation/trans_real.py:_active = {}
django/utils/translation/trans_real.py:_translations = {}

Out of these, the following are read-only (i.e. not changed anywhere in code) or otherwise irrelevant: contrib/admin, formtools tests, localflavor mappings, core/cache, core/handlers, core/serializers/__init__.py:BUILTIN_SERIALIZERS, core/servers, db/backends, db/models/query.py, utils/dates.py, utils/_decimal.py, utils/simplejson, utils/termcolors.py, utils/translation/trans_null.py.

SITE_CACHE and everything in django.utils.translation.trans_real has already been listed under globals above.

_callable_cache and _resolver_cache in django/core/urlresolvers.py are used within the memoize decorator, result = func(*args) may be called more than once in utils/functional.py, but this should generally be a non-issue.

That leaves the following relevant global dicts not listed before:

django/core/serializers/__init__.py:_serializers = {}
django/db/models/fields/related.py:pending_lookups = {}
django/db/transaction.py:dirty = {}
django/db/transaction.py:state = {}
django/dispatch/dispatcher.py:connections = {}
django/dispatch/dispatcher.py:senders = {}
django/dispatch/dispatcher.py:sendersBack = {}
django/template/__init__.py:libraries = {}

Global lists

$ grep -r '^[[:alnum:]_]\+ *= *\[' django | egrep -v '(\.svn|_doctest\.py|__all__)' | sort

yields the following results

django/db/models/fields/__init__.py:BLANK_CHOICE_DASH = [("", "---------")]
django/db/models/fields/__init__.py:BLANK_CHOICE_NONE = [("", "None")]
django/template/__init__.py:builtins = []
django/template/loaders/app_directories.py:app_template_dirs = []
django/utils/_decimal.py:rounding_functions = [name for name in Decimal.__dict__.keys() if name.startswith('_round_')]
django/utils/_decimal.py:_signals = [Clamped, DivisionByZero, Inexact, Overflow, Rounded,
django/utils/html.py:DOTS = ['·', '*', '\xe2\x80\xa2', '•', '•', '•']
django/utils/html.py:LEADING_PUNCTUATION  = ['(', '<', '&lt;']
django/utils/html.py:TRAILING_PUNCTUATION = ['.', ',', ')', '>', '\n', '&gt;']
django/utils/simplejson/decoder.py:ANYTHING = [

Leaving out the irrelevant read-only ones, the following remain:

django/template/__init__.py:builtins = []
django/template/loaders/app_directories.py:app_template_dirs = []

As a matter of style, the read-only ones should really be tuples, not lists -- the 'say what you mean' idiom: if it shouldn't be modified, don't let it be by making it a tuple. Tuples are also marginally more efficient speed- and space-wise. There is a slight semantic distinction between lists and tuples though ​http://jtauber.com/blog/2006/04/15/python_tuples_are_not_just_constant_lists/ . But as there are no constant lists in Python, tuples are the only way to be const-correct.