track embedded python distribution

python/gevent/builtins.py

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+# Copyright (c) 2015 gevent contributors. See LICENSE for details.
+"""gevent friendly implementations of builtin functions."""
+from __future__ import absolute_import
+
+import imp # deprecated since 3.4; issues PendingDeprecationWarning in 3.5
+import sys
+import weakref
+from gevent.lock import RLock
+
+# Normally we'd have the "expected" case inside the try
+# (Python 3, because Python 3 is the way forward). But
+# under Python 2, the popular `future` library *also* provides
+# a `builtins` module---which lacks the __import__ attribute.
+# So we test for the old, deprecated version first
+
+try: # Py2
+    import __builtin__ as builtins
+    _allowed_module_name_types = (basestring,) # pylint:disable=undefined-variable
+    __target__ = '__builtin__'
+except ImportError:
+    import builtins # pylint: disable=import-error
+    _allowed_module_name_types = (str,)
+    __target__ = 'builtins'
+
+_import = builtins.__import__
+
+# We need to protect imports both across threads and across greenlets.
+# And the order matters. Note that under 3.4, the global import lock
+# and imp module are deprecated. It seems that in all Py3 versions, a
+# module lock is used such that this fix is not necessary.
+
+# We emulate the per-module locking system under Python 2 in order to
+# avoid issues acquiring locks in multiple-level-deep imports
+# that attempt to use the gevent blocking API at runtime; using one lock
+# could lead to a LoopExit error as a greenlet attempts to block on it while
+# it's already held by the main greenlet (issue #798).
+
+# We base this approach on a simplification of what `importlib._bootstrap`
+# does; notably, we don't check for deadlocks
+
+_g_import_locks = {} # name -> wref of RLock
+
+__lock_imports = True
+
+
+def __module_lock(name):
+    # Return the lock for the given module, creating it if necessary.
+    # It will be removed when no longer needed.
+    # Nothing in this function yields, so we're multi-greenlet safe
+    # (But not multi-threading safe.)
+    # XXX: What about on PyPy, where the GC is asynchronous (not ref-counting)?
+    # (Does it stop-the-world first?)
+    lock = None
+    try:
+        lock = _g_import_locks[name]()
+    except KeyError:
+        pass
+
+    if lock is None:
+        lock = RLock()
+
+        def cb(_):
+            # We've seen a KeyError on PyPy on RPi2
+            _g_import_locks.pop(name, None)
+        _g_import_locks[name] = weakref.ref(lock, cb)
+    return lock
+
+
+def __import__(*args, **kwargs):
+    """
+    __import__(name, globals=None, locals=None, fromlist=(), level=0) -> object
+
+    Normally python protects imports against concurrency by doing some locking
+    at the C level (at least, it does that in CPython).  This function just
+    wraps the normal __import__ functionality in a recursive lock, ensuring that
+    we're protected against greenlet import concurrency as well.
+    """
+    if args and not issubclass(type(args[0]), _allowed_module_name_types):
+        # if a builtin has been acquired as a bound instance method,
+        # python knows not to pass 'self' when the method is called.
+        # No such protection exists for monkey-patched builtins,
+        # however, so this is necessary.
+        args = args[1:]
+
+    if not __lock_imports:
+        return _import(*args, **kwargs)
+
+    module_lock = __module_lock(args[0]) # Get a lock for the module name
+    imp.acquire_lock()
+    try:
+        module_lock.acquire()
+        try:
+            result = _import(*args, **kwargs)
+        finally:
+            module_lock.release()
+    finally:
+        imp.release_lock()
+    return result
+
+
+def _unlock_imports():
+    """
+    Internal function, called when gevent needs to perform imports
+    lazily, but does not know the state of the system. It may be impossible
+    to take the import lock because there are no other running greenlets, for
+    example. This causes a monkey-patched __import__ to avoid taking any locks.
+    until the corresponding call to lock_imports. This should only be done for limited
+    amounts of time and when the set of imports is statically known to be "safe".
+    """
+    global __lock_imports
+    # This could easily become a list that we push/pop from or an integer
+    # we increment if we need to do this recursively, but we shouldn't get
+    # that complex.
+    __lock_imports = False
+
+
+def _lock_imports():
+    global __lock_imports
+    __lock_imports = True
+
+if sys.version_info[:2] >= (3, 3):
+    __implements__ = []
+else:
+    __implements__ = ['__import__']
+__all__ = __implements__