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Antoine Pitrou 2013-05-18 11:00:19 +02:00
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@ -54,9 +54,9 @@ Cyclic garbage collector (GC)
Cyclic trash (CT) Cyclic trash (CT)
A reference cycle, or former reference cycle, in which no object A reference cycle, or former reference cycle, in which no object
is referenced from outside the cycle *and* whose objects have is referenced from outside the cycle *and* whose objects have
started being cleared by the GC. Objects in cyclic trash are potential started being cleared by the GC. Objects in cyclic trash are
zombies; if they are accessed by Python code, the symptoms can vary potential zombies; if they are accessed by Python code, the symptoms
from weird AttributeErrors to crashes. can vary from weird AttributeErrors to crashes.
Zombie / broken object Zombie / broken object
An object part of cyclic trash. The term stresses that the object An object part of cyclic trash. The term stresses that the object
@ -67,13 +67,13 @@ Zombie / broken object
Finalizer Finalizer
A function or method called when an object is intended to be A function or method called when an object is intended to be
disposed of. The finalizer can access the object and release any disposed of. The finalizer can access the object and release any
resource held by the object (for example mutexes or file descriptors). resource held by the object (for example mutexes or file
An example is a ``__del__`` method. descriptors). An example is a ``__del__`` method.
Resurrection Resurrection
The process by which a finalizer creates a new reference to an The process by which a finalizer creates a new reference to an
object in a CI. This can happen as a quirky but supported side-effect object in a CI. This can happen as a quirky but supported
of ``__del__`` methods. side-effect of ``__del__`` methods.
Impact Impact
@ -123,23 +123,24 @@ Disposal of cyclic isolates
--------------------------- ---------------------------
Cyclic isolates are first detected by the garbage collector, and then Cyclic isolates are first detected by the garbage collector, and then
disposed of. The detection phase doesn't change and won't be described here. disposed of. The detection phase doesn't change and won't be described
Disposal of a CI traditionally works in the following order: here. Disposal of a CI traditionally works in the following order:
1. Weakrefs to CI objects are cleared, and their callbacks called. At this 1. Weakrefs to CI objects are cleared, and their callbacks called. At
point, the objects are still safe to use. this point, the objects are still safe to use.
2. The CI becomes a CT as the GC systematically breaks all 2. The CI becomes a CT as the GC systematically breaks all
known references inside it (using the ``tp_clear`` function). known references inside it (using the ``tp_clear`` function).
3. Nothing. All CT objects should have been disposed of in step 2 3. Nothing. All CT objects should have been disposed of in step 2
(as a side-effect of clearing references); this collection is finished. (as a side-effect of clearing references); this collection is
finished.
This PEP proposes to turn CI disposal into the following sequence (new This PEP proposes to turn CI disposal into the following sequence (new
steps are in bold): steps are in bold):
1. Weakrefs to CI objects are cleared, and their callbacks called. At this 1. Weakrefs to CI objects are cleared, and their callbacks called. At
point, the objects are still safe to use. this point, the objects are still safe to use.
2. **The finalizers of all CI objects are called.** 2. **The finalizers of all CI objects are called.**
@ -152,7 +153,8 @@ steps are in bold):
known references inside it (using the ``tp_clear`` function). known references inside it (using the ``tp_clear`` function).
5. Nothing. All CT objects should have been disposed of in step 4 5. Nothing. All CT objects should have been disposed of in step 4
(as a side-effect of clearing references); this collection is finished. (as a side-effect of clearing references); this collection is
finished.
C-level changes C-level changes
@ -172,8 +174,8 @@ for testing purposes).
On the internal side, a bit is reserved in the GC header for GC-managed On the internal side, a bit is reserved in the GC header for GC-managed
objects to signal that they were finalized. This helps avoid finalizing objects to signal that they were finalized. This helps avoid finalizing
an object twice (and, especially, finalizing a CT object after it was broken an object twice (and, especially, finalizing a CT object after it was
by the GC). broken by the GC).
Discussion Discussion