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cPickleCache.c

 /*****************************************************************************

  Copyright (c) 2001, 2002 Zope Corporation and Contributors.
  All Rights Reserved.

  This software is subject to the provisions of the Zope Public License,
  Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
  THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
  WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
  FOR A PARTICULAR PURPOSE

 ****************************************************************************/

/*

Objects are stored under three different regimes:

Regime 1: Persistent Classes

Persistent Classes are part of ZClasses. They are stored in the
self->data dictionary, and are never garbage collected.

The klass_items() method returns a sequence of (oid,object) tuples for
every Persistent Class, which should make it possible to implement
garbage collection in Python if necessary.

Regime 2: Ghost Objects

There is no benefit to keeping a ghost object which has no external
references, therefore a weak reference scheme is used to ensure that
ghost objects are removed from memory as soon as possible, when the
last external reference is lost.

Ghost objects are stored in the self->data dictionary. Normally a
dictionary keeps a strong reference on its values, however this
reference count is 'stolen'.

This weak reference scheme leaves a dangling reference, in the
dictionary, when the last external reference is lost. To clean up this
dangling reference the persistent object dealloc function calls
self->cache->_oid_unreferenced(self->oid). The cache looks up the oid
in the dictionary, ensures it points to an object whose reference
count is zero, then removes it from the dictionary. Before removing
the object from the dictionary it must temporarily resurrect the
object in much the same way that class instances are resurrected
before their __del__ is called.

Since ghost objects are stored under a different regime to non-ghost
objects, an extra ghostify function in cPersistenceAPI replaces
self->state=GHOST_STATE assignments that were common in other
persistent classes (such as BTrees).

Regime 3: Non-Ghost Objects

Non-ghost objects are stored in two data structures: the dictionary
mapping oids to objects and a doubly-linked list that encodes the
order in which the objects were accessed.  The dictionary reference is
borrowed, as it is for ghosts.  The list reference is a new reference;
the list stores recently used objects, even if they are otherwise
unreferenced, to avoid loading the object from the database again.

The doubly-link-list nodes contain next and previous pointers linking
together the cache and all non-ghost persistent objects.

The node embedded in the cache is the home position. On every
attribute access a non-ghost object will relink itself just behind the
home position in the ring. Objects accessed least recently will
eventually find themselves positioned after the home position.

Occasionally other nodes are temporarily inserted in the ring as
position markers. The cache contains a ring_lock flag which must be
set and unset before and after doing so. Only if the flag is unset can
the cache assume that all nodes are either his own home node, or nodes
from persistent objects. This assumption is useful during the garbage
collection process.

The number of non-ghost objects is counted in self->non_ghost_count.
The garbage collection process consists of traversing the ring, and
deactivating (that is, turning into a ghost) every object until
self->non_ghost_count is down to the target size, or until it
reaches the home position again.

Note that objects in the sticky or changed states are still kept in
the ring, however they can not be deactivated. The garbage collection
process must skip such objects, rather than deactivating them.

*/

static char cPickleCache_doc_string[] =
"Defines the PickleCache used by ZODB Connection objects.\n"
"\n"
"$Id: cPickleCache.c 29896 2005-04-07 04:48:06Z tim_one $\n";

#define DONT_USE_CPERSISTENCECAPI
#include "cPersistence.h"
#include "structmember.h"
#include <time.h>
#include <stddef.h>
#undef Py_FindMethod

/* Python string objects to speed lookups; set by module init. */
static PyObject *py__p_changed;
static PyObject *py__p_deactivate;
static PyObject *py__p_jar;
static PyObject *py__p_oid;

static cPersistenceCAPIstruct *capi;

/* This object is the pickle cache.  The CACHE_HEAD macro guarantees
   that layout of this struct is the same as the start of
   ccobject_head in cPersistence.c */
typedef struct {
    CACHE_HEAD
    int klass_count;                     /* count of persistent classes */
    PyObject *data;                      /* oid -> object dict */
    PyObject *jar;                       /* Connection object */
    int cache_size;                      /* target number of items in cache */

    /* Most of the time the ring contains only:
       * many nodes corresponding to persistent objects
       * one 'home' node from the cache.
    In some cases it is handy to temporarily add other types
    of node into the ring as placeholders. 'ring_lock' is a boolean
    indicating that someone has already done this. Currently this
    is only used by the garbage collection code. */

    int ring_lock;

    /* 'cache_drain_resistance' controls how quickly the cache size will drop
    when it is smaller than the configured size. A value of zero means it will
    not drop below the configured size (suitable for most caches). Otherwise,
    it will remove cache_non_ghost_count/cache_drain_resistance items from
    the cache every time (suitable for rarely used caches, such as those
    associated with Zope versions. */

    int cache_drain_resistance;

} ccobject;

static int cc_ass_sub(ccobject *self, PyObject *key, PyObject *v);

/* ---------------------------------------------------------------- */

#define OBJECT_FROM_RING(SELF, HERE) \
    ((cPersistentObject *)(((char *)here) - offsetof(cPersistentObject, ring)))

/* Insert self into the ring, following after. */
static void
insert_after(CPersistentRing *self, CPersistentRing *after)
{
    assert(self != NULL);
    assert(after != NULL);
    self->r_prev = after;
    self->r_next = after->r_next;
    after->r_next->r_prev = self;
    after->r_next = self;
}

/* Remove self from the ring. */
static void
unlink_from_ring(CPersistentRing *self)
{
    assert(self != NULL);
    self->r_prev->r_next = self->r_next;
    self->r_next->r_prev = self->r_prev;
}

static int
scan_gc_items(ccobject *self, int target)
{
    /* This function must only be called with the ring lock held,
       because it places non-object placeholders in the ring.
    */
    cPersistentObject *object;
    CPersistentRing *here;
    CPersistentRing before_original_home;
    int result = -1;   /* guilty until proved innocent */

    /* Scan the ring, from least to most recently used, deactivating
     * up-to-date objects, until we either find the ring_home again or
     * or we've ghosted enough objects to reach the target size.
     * Tricky:  __getattr__ and __del__ methods can do anything, and in
     * particular if we ghostify an object with a __del__ method, that method
     * can load the object again, putting it back into the MRU part of the
     * ring.  Waiting to find ring_home again can thus cause an infinite
     * loop (Collector #1208).  So before_original_home records the MRU
     * position we start with, and we stop the scan when we reach that.
     */
    insert_after(&before_original_home, self->ring_home.r_prev);
    here = self->ring_home.r_next;   /* least recently used object */
    while (here != &before_original_home && self->non_ghost_count > target) {
      assert(self->ring_lock);
      assert(here != &self->ring_home);

        /* At this point we know that the ring only contains nodes
         from persistent objects, plus our own home node.  We know
         this because the ring lock is held.  We can safely assume
         the current ring node is a persistent object now we know it
         is not the home */
        object = OBJECT_FROM_RING(self, here);

        if (object->state == cPersistent_UPTODATE_STATE) {
            CPersistentRing placeholder;
            PyObject *method;
            PyObject *temp;
            int error_occurred = 0;
            /* deactivate it. This is the main memory saver. */

            /* Add a placeholder, a dummy node in the ring.  We need
             to do this to mark our position in the ring.  It is
             possible that the PyObject_GetAttr() call below will
             invoke a __getattr__() hook in Python.  Also possible
             that deactivation will lead to a __del__ method call.
             So another thread might run, and mutate the ring as a side
             effect of object accesses.  There's no predicting then where
             in the ring here->next will point after that.  The
             placeholder won't move as a side effect of calling Python
             code.
          */
            insert_after(&placeholder, here);
          method = PyObject_GetAttr((PyObject *)object, py__p_deactivate);
          if (method == NULL)
              error_occurred = 1;
          else {
            temp = PyObject_CallObject(method, NULL);
                Py_DECREF(method);
              if (temp == NULL)
                  error_occurred = 1;
          }

            here = placeholder.r_next;
            unlink_from_ring(&placeholder);
            if (error_occurred)
                goto Done;
        }
        else
            here = here->r_next;
    }
    result = 0;
 Done:
    unlink_from_ring(&before_original_home);
    return result;
}

static PyObject *
lockgc(ccobject *self, int target_size)
{
    /* This is thread-safe because of the GIL, and there's nothing
     * in between checking the ring_lock and acquiring it that calls back
     * into Python.
     */
    if (self->ring_lock) {
        Py_INCREF(Py_None);
        return Py_None;
    }

    self->ring_lock = 1;
    if (scan_gc_items(self, target_size) < 0) {
        self->ring_lock = 0;
        return NULL;
    }
    self->ring_lock = 0;

    Py_INCREF(Py_None);
    return Py_None;
}

static PyObject *
cc_incrgc(ccobject *self, PyObject *args)
{
    int obsolete_arg = -999;
    int starting_size = self->non_ghost_count;
    int target_size = self->cache_size;

    if (self->cache_drain_resistance >= 1) {
        /* This cache will gradually drain down to a small size. Check
           a (small) number of objects proportional to the current size */

        int target_size_2 = (starting_size - 1
                       - starting_size / self->cache_drain_resistance);
        if (target_size_2 < target_size)
            target_size = target_size_2;
    }


    if (!PyArg_ParseTuple(args, "|i:incrgc", &obsolete_arg))
      return NULL;

    if (obsolete_arg != -999
        &&
        (PyErr_Warn(PyExc_DeprecationWarning,
                    "No argument expected")
         < 0))
        return NULL;

    return lockgc(self, target_size);
}

static PyObject *
cc_full_sweep(ccobject *self, PyObject *args)
{
    int dt = -999;

    /* TODO:  This should be deprecated;  */

    if (!PyArg_ParseTuple(args, "|i:full_sweep", &dt))
      return NULL;
    if (dt == -999)
      return lockgc(self, 0);
    else
      return cc_incrgc(self, args);
}

static PyObject *
cc_minimize(ccobject *self, PyObject *args)
{
    int ignored = -999;

    if (!PyArg_ParseTuple(args, "|i:minimize", &ignored))
      return NULL;

    if (ignored != -999
        &&
        (PyErr_Warn(PyExc_DeprecationWarning,
                    "No argument expected")
         < 0))
        return NULL;

    return lockgc(self, 0);
}

static int
_invalidate(ccobject *self, PyObject *key)
{
    static PyObject *_p_invalidate = NULL;
    PyObject *meth, *v;

    v = PyDict_GetItem(self->data, key);
    if (v == NULL)
      return 0;

    if (_p_invalidate == NULL)
      {
      _p_invalidate = PyString_InternFromString("_p_invalidate");
      if (_p_invalidate == NULL)
          {
          /* It doesn't make any sense to ignore this error, but
             the caller ignores all errors.

               TODO: and why does it do that? This should be fixed
          */
          return -1;
          }
      }

    if (v->ob_refcnt <= 1 && PyType_Check(v)) {
      /* This looks wrong, but it isn't. We use strong references to types
         because they don't have the ring members.

         The result is that we *never* remove classes unless
         they are modified.  We can fix this by using wekrefs uniformly.
      */
      self->klass_count--;
      return PyDict_DelItem(self->data, key);
    }

    meth = PyObject_GetAttr(v, _p_invalidate);
    if (meth == NULL)
      return -1;

    v = PyObject_CallObject(meth, NULL);
    Py_DECREF(meth);
    return v == NULL ? -1 : 0;
}

static PyObject *
cc_invalidate(ccobject *self, PyObject *inv)
{
  PyObject *key, *v;
  int i = 0;

  if (PyDict_Check(inv))
    {
      while (PyDict_Next(inv, &i, &key, &v))
        {
        if (_invalidate(self, key) < 0)
            return NULL;
        }
      PyDict_Clear(inv);
    }
  else {
      if (PyString_Check(inv))
        {
        if (_invalidate(self, inv) < 0)
            return NULL;
        }
      else {
        int l, r;

        l = PyObject_Length(inv);
        if (l < 0)
            return NULL;
        for (i=l; --i >= 0; ) {
            key = PySequence_GetItem(inv, i);
            if (!key)
              return NULL;
            r = _invalidate(self, key);
            Py_DECREF(key);
              if (r < 0)
                return NULL;
        }
        /* Dubious:  modifying the input may be an unexpected side effect. */
        PySequence_DelSlice(inv, 0, l);
      }
  }

  Py_INCREF(Py_None);
  return Py_None;
}

static PyObject *
cc_get(ccobject *self, PyObject *args)
{
    PyObject *r, *key, *d = NULL;

    if (!PyArg_ParseTuple(args, "O|O:get", &key, &d))
      return NULL;

    r = PyDict_GetItem(self->data, key);
    if (!r) {
      if (d)
          r = d;
      else
          r = Py_None;
    }
    Py_INCREF(r);
    return r;
}

static PyObject *
cc_items(ccobject *self)
{
    return PyObject_CallMethod(self->data, "items", "");
}

static PyObject *
cc_klass_items(ccobject *self)
{
    PyObject *l,*k,*v;
    int p = 0;

    l = PyList_New(0);
    if (l == NULL)
      return NULL;

    while (PyDict_Next(self->data, &p, &k, &v)) {
        if(PyType_Check(v)) {
          v = Py_BuildValue("OO", k, v);
          if (v == NULL) {
            Py_DECREF(l);
            return NULL;
          }
          if (PyList_Append(l, v) < 0) {
            Py_DECREF(v);
            Py_DECREF(l);
            return NULL;
          }
          Py_DECREF(v);
        }
    }

    return l;
}

static PyObject *
cc_debug_info(ccobject *self)
{
    PyObject *l,*k,*v;
    int p = 0;

    l = PyList_New(0);
    if (l == NULL)
      return NULL;

    while (PyDict_Next(self->data, &p, &k, &v))
      {
        if (v->ob_refcnt <= 0)
          v = Py_BuildValue("Oi", k, v->ob_refcnt);

        else if (! PyType_Check(v) &&
                 (v->ob_type->tp_basicsize >= sizeof(cPersistentObject))
                 )
          v = Py_BuildValue("Oisi",
                            k, v->ob_refcnt, v->ob_type->tp_name,
                            ((cPersistentObject*)v)->state);
        else
          v = Py_BuildValue("Ois", k, v->ob_refcnt, v->ob_type->tp_name);

        if (v == NULL)
          goto err;

        if (PyList_Append(l, v) < 0)
          goto err;
      }

    return l;

 err:
    Py_DECREF(l);
    return NULL;

}

static PyObject *
cc_lru_items(ccobject *self)
{
    PyObject *l;
    CPersistentRing *here;

    if (self->ring_lock) {
      /* When the ring lock is held, we have no way of know which
         ring nodes belong to persistent objects, and which a
         placeholders. */
        PyErr_SetString(PyExc_ValueError,
            ".lru_items() is unavailable during garbage collection");
        return NULL;
    }

    l = PyList_New(0);
    if (l == NULL)
      return NULL;

    here = self->ring_home.r_next;
    while (here != &self->ring_home) {
        PyObject *v;
        cPersistentObject *object = OBJECT_FROM_RING(self, here);

        if (object == NULL) {
            Py_DECREF(l);
            return NULL;
        }
      v = Py_BuildValue("OO", object->oid, object);
      if (v == NULL) {
            Py_DECREF(l);
            return NULL;
      }
      if (PyList_Append(l, v) < 0) {
          Py_DECREF(v);
            Py_DECREF(l);
            return NULL;
      }
        Py_DECREF(v);
        here = here->r_next;
    }

    return l;
}

static void
cc_oid_unreferenced(ccobject *self, PyObject *oid)
{
    /* This is called by the persistent object deallocation function
       when the reference count on a persistent object reaches
       zero. We need to fix up our dictionary; its reference is now
       dangling because we stole its reference count. Be careful to
       not release the global interpreter lock until this is
       complete. */

    PyObject *v;

    /* If the cache has been cleared by GC, data will be NULL. */
    if (!self->data)
      return;

    v = PyDict_GetItem(self->data, oid);
    assert(v);
    assert(v->ob_refcnt == 0);
    /* Need to be very hairy here because a dictionary is about
       to decref an already deleted object.
    */

#ifdef Py_TRACE_REFS
    /* This is called from the deallocation function after the
       interpreter has untracked the reference.  Track it again.
     */
    _Py_NewReference(v);
    /* Don't increment total refcount as a result of the
       shenanigans played in this function.  The _Py_NewReference()
       call above creates artificial references to v.
    */
    _Py_RefTotal--;
    assert(v->ob_type);
#else
    Py_INCREF(v);
#endif
    assert(v->ob_refcnt == 1);
    /* Incremement the refcount again, because delitem is going to
       DECREF it.  If it's refcount reached zero again, we'd call back to
       the dealloc function that called us.
    */
    Py_INCREF(v);

    /* TODO:  Should we call _Py_ForgetReference() on error exit? */
    if (PyDict_DelItem(self->data, oid) < 0)
      return;
    Py_DECREF((ccobject *)((cPersistentObject *)v)->cache);
    ((cPersistentObject *)v)->cache = NULL;

    assert(v->ob_refcnt == 1);

    /* Undo the temporary resurrection.
       Don't DECREF the object, because this function is called from
       the object's dealloc function. If the refcnt reaches zero, it
       will all be invoked recursively.
     */
    _Py_ForgetReference(v);
}

static PyObject *
cc_ringlen(ccobject *self)
{
    CPersistentRing *here;
    int c = 0;

    for (here = self->ring_home.r_next; here != &self->ring_home;
       here = here->r_next)
      c++;
    return PyInt_FromLong(c);
}

static struct PyMethodDef cc_methods[] = {
    {"items", (PyCFunction)cc_items, METH_NOARGS,
     "Return list of oid, object pairs for all items in cache."},
    {"lru_items", (PyCFunction)cc_lru_items, METH_NOARGS,
     "List (oid, object) pairs from the lru list, as 2-tuples."},
    {"klass_items", (PyCFunction)cc_klass_items, METH_NOARGS,
     "List (oid, object) pairs of cached persistent classes."},
    {"full_sweep", (PyCFunction)cc_full_sweep, METH_VARARGS,
     "full_sweep() -- Perform a full sweep of the cache."},
    {"minimize",  (PyCFunction)cc_minimize, METH_VARARGS,
     "minimize([ignored]) -- Remove as many objects as possible\n\n"
     "Ghostify all objects that are not modified.  Takes an optional\n"
     "argument, but ignores it."},
    {"incrgc", (PyCFunction)cc_incrgc, METH_VARARGS,
     "incrgc() -- Perform incremental garbage collection\n\n"
     "This method had been depricated!"
     "Some other implementations support an optional parameter 'n' which\n"
     "indicates a repetition count; this value is ignored."},
    {"invalidate", (PyCFunction)cc_invalidate, METH_O,
     "invalidate(oids) -- invalidate one, many, or all ids"},
    {"get", (PyCFunction)cc_get, METH_VARARGS,
     "get(key [, default]) -- get an item, or a default"},
    {"ringlen", (PyCFunction)cc_ringlen, METH_NOARGS,
     "ringlen() -- Returns number of non-ghost items in cache."},
    {"debug_info", (PyCFunction)cc_debug_info, METH_NOARGS,
     "debug_info() -- Returns debugging data about objects in the cache."},
    {NULL, NULL}        /* sentinel */
};

static int
cc_init(ccobject *self, PyObject *args, PyObject *kwds)
{
    int cache_size = 100;
    PyObject *jar;

    if (!PyArg_ParseTuple(args, "O|i", &jar, &cache_size))
      return -1;

    self->jar = NULL;
    self->data = PyDict_New();
    if (self->data == NULL) {
      Py_DECREF(self);
      return -1;
    }
    /* Untrack the dict mapping oids to objects.

    The dict contains uncounted references to ghost objects, so it
    isn't safe for GC to visit it.  If GC finds an object with more
    referents that refcounts, it will die with an assertion failure.

    When the cache participates in GC, it will need to traverse the
    objects in the doubly-linked list, which will account for all the
    non-ghost objects.
    */
    PyObject_GC_UnTrack((void *)self->data);
    self->jar = jar;
    Py_INCREF(jar);
    self->cache_size = cache_size;
    self->non_ghost_count = 0;
    self->klass_count = 0;
    self->cache_drain_resistance = 0;
    self->ring_lock = 0;
    self->ring_home.r_next = &self->ring_home;
    self->ring_home.r_prev = &self->ring_home;
    return 0;
}

static void
cc_dealloc(ccobject *self)
{
    Py_XDECREF(self->data);
    Py_XDECREF(self->jar);
    PyObject_GC_Del(self);
}

static int
cc_clear(ccobject *self)
{
    int pos = 0;
    PyObject *k, *v;
    /* Clearing the cache is delicate.

    A non-ghost object will show up in the ring and in the dict.  If
    we deallocating the dict before clearing the ring, the GC will
    decref each object in the dict.  Since the dict references are
    uncounted, this will lead to objects having negative refcounts.

    Freeing the non-ghost objects should eliminate many objects from
    the cache, but there may still be ghost objects left.  It's
    not safe to decref the dict until it's empty, so we need to manually
    clear those out of the dict, too.  We accomplish that by replacing
    all the ghost objects with None.
    */

    /* We don't need to lock the ring, because the cache is unreachable.
    It should be impossible for anyone to be modifying the cache.
    */
    assert(! self->ring_lock);

    while (self->ring_home.r_next != &self->ring_home) {
      CPersistentRing *here = self->ring_home.r_next;
      cPersistentObject *o = OBJECT_FROM_RING(self, here);

      if (o->cache) {
          Py_INCREF(o); /* account for uncounted reference */
          if (PyDict_DelItem(self->data, o->oid) < 0)
            return -1;
      }
      o->cache = NULL;
      Py_DECREF(self);
      self->ring_home.r_next = here->r_next;
      o->ring.r_prev = NULL;
      o->ring.r_next = NULL;
      Py_DECREF(o);
      here = here->r_next;
    }

    Py_XDECREF(self->jar);

    while (PyDict_Next(self->data, &pos, &k, &v)) {
      Py_INCREF(v);
      if (PyDict_SetItem(self->data, k, Py_None) < 0)
          return -1;
    }
    Py_XDECREF(self->data);
    self->data = NULL;
    self->jar = NULL;
    return 0;
}

static int
cc_traverse(ccobject *self, visitproc visit, void *arg)
{
    int err;
    CPersistentRing *here;

    /* If we're in the midst of cleaning up old objects, the ring contains
     * assorted junk we must not pass on to the visit() callback.  This
     * should be rare (our cleanup code would need to have called back
     * into Python, which in turn triggered Python's gc).  When it happens,
     * simply don't chase any pointers.  The cache will appear to be a
     * source of external references then, and at worst we miss cleaning
     * up a dead cycle until the next time Python's gc runs.
     */
    if (self->ring_lock)
      return 0;

#define VISIT(SLOT) \
    if (SLOT) { \
      err = visit((PyObject *)(SLOT), arg); \
      if (err) \
                 return err; \
    }

    VISIT(self->jar);

    here = self->ring_home.r_next;

    /* It is possible that an object is traversed after it is cleared.
       In that case, there is no ring.
    */
    if (!here)
      return 0;

    while (here != &self->ring_home) {
      cPersistentObject *o = OBJECT_FROM_RING(self, here);
      VISIT(o);
      here = here->r_next;
    }
#undef VISIT

    return 0;
}

static int
cc_length(ccobject *self)
{
    return PyObject_Length(self->data);
}

static PyObject *
cc_subscript(ccobject *self, PyObject *key)
{
    PyObject *r;

    r = PyDict_GetItem(self->data, key);
    if (r == NULL) {
      PyErr_SetObject(PyExc_KeyError, key);
      return NULL;
    }
    Py_INCREF(r);

    return r;
}

static int
cc_add_item(ccobject *self, PyObject *key, PyObject *v)
{
    int result;
    PyObject *oid, *object_again, *jar;
    cPersistentObject *p;

    /* Sanity check the value given to make sure it is allowed in the cache */
    if (PyType_Check(v)) {
        /* Its a persistent class, such as a ZClass. Thats ok. */
    }
    else if (v->ob_type->tp_basicsize < sizeof(cPersistentObject)) {
        /* If it's not an instance of a persistent class, (ie Python
         classes that derive from persistent.Persistent, BTrees,
         etc), report an error.

         TODO:  checking sizeof() seems a poor test.
      */
      PyErr_SetString(PyExc_TypeError,
                  "Cache values must be persistent objects.");
      return -1;
    }

    /* Can't access v->oid directly because the object might be a
     *  persistent class.
     */
    oid = PyObject_GetAttr(v, py__p_oid);
    if (oid == NULL)
      return -1;
    if (! PyString_Check(oid)) {
        PyErr_Format(PyExc_TypeError,
                     "Cached object oid must be a string, not a %s",
                 oid->ob_type->tp_name);
      return -1;
    }

    /*  we know they are both strings.
     *  now check if they are the same string.
     */
    result = PyObject_Compare(key, oid);
    if (PyErr_Occurred()) {
      Py_DECREF(oid);
      return -1;
    }
    Py_DECREF(oid);
    if (result) {
      PyErr_SetString(PyExc_ValueError, "Cache key does not match oid");
      return -1;
    }

    /* useful sanity check, but not strictly an invariant of this class */
    jar = PyObject_GetAttr(v, py__p_jar);
    if (jar == NULL)
        return -1;
    if (jar==Py_None) {
        Py_DECREF(jar);
        PyErr_SetString(PyExc_ValueError,
                        "Cached object jar missing");
      return -1;
    }
    Py_DECREF(jar);

    object_again = PyDict_GetItem(self->data, key);
    if (object_again) {
      if (object_again != v) {
          PyErr_SetString(PyExc_ValueError,
                "A different object already has the same oid");
          return -1;
      } else {
          /* re-register under the same oid - no work needed */
          return 0;
      }
    }

    if (PyType_Check(v)) {
      if (PyDict_SetItem(self->data, key, v) < 0)
          return -1;
      self->klass_count++;
      return 0;
    } else {
      PerCache *cache = ((cPersistentObject *)v)->cache;
      if (cache) {
          if (cache != (PerCache *)self)
            /* This object is already in a different cache. */
            PyErr_SetString(PyExc_ValueError,
                        "Cache values may only be in one cache.");
          return -1;
      }
      /* else:

         This object is already one of ours, which is ok.  It
         would be very strange if someone was trying to register
         the same object under a different key.
      */
    }

    if (PyDict_SetItem(self->data, key, v) < 0)
      return -1;
    /* the dict should have a borrowed reference */
    Py_DECREF(v);

    p = (cPersistentObject *)v;
    Py_INCREF(self);
    p->cache = (PerCache *)self;
    if (p->state >= 0) {
      /* insert this non-ghost object into the ring just
         behind the home position. */
      self->non_ghost_count++;
      ring_add(&self->ring_home, &p->ring);
      /* this list should have a new reference to the object */
      Py_INCREF(v);
    }
    return 0;
}

static int
cc_del_item(ccobject *self, PyObject *key)
{
    PyObject *v;
    cPersistentObject *p;

    /* unlink this item from the ring */
    v = PyDict_GetItem(self->data, key);
    if (v == NULL) {
      PyErr_SetObject(PyExc_KeyError, key);
      return -1;
    }

    if (PyType_Check(v)) {
      self->klass_count--;
    } else {
      p = (cPersistentObject *)v;
      if (p->state >= 0) {
          self->non_ghost_count--;
          ring_del(&p->ring);
          /* The DelItem below will account for the reference
             held by the list. */
      } else {
          /* This is a ghost object, so we haven't kept a reference
             count on it.  For it have stayed alive this long
             someone else must be keeping a reference to
             it. Therefore we need to temporarily give it back a
             reference count before calling DelItem below */
          Py_INCREF(v);
      }

      Py_DECREF((PyObject *)p->cache);
      p->cache = NULL;
    }

    if (PyDict_DelItem(self->data, key) < 0) {
      PyErr_SetString(PyExc_RuntimeError,
                  "unexpectedly couldn't remove key in cc_ass_sub");
      return -1;
    }

    return 0;
}

static int
cc_ass_sub(ccobject *self, PyObject *key, PyObject *v)
{
    if (!PyString_Check(key)) {
      PyErr_Format(PyExc_TypeError,
                     "cPickleCache key must be a string, not a %s",
                 key->ob_type->tp_name);
      return -1;
    }
    if (v)
      return cc_add_item(self, key, v);
    else
      return cc_del_item(self, key);
}

static PyMappingMethods cc_as_mapping = {
  (inquiry)cc_length,         /*mp_length*/
  (binaryfunc)cc_subscript,   /*mp_subscript*/
  (objobjargproc)cc_ass_sub,  /*mp_ass_subscript*/
};

static PyObject *
cc_cache_data(ccobject *self, void *context)
{
    return PyDict_Copy(self->data);
}

static PyGetSetDef cc_getsets[] = {
    {"cache_data", (getter)cc_cache_data},
    {NULL}
};


static PyMemberDef cc_members[] = {
    {"cache_size", T_INT, offsetof(ccobject, cache_size)},
    {"cache_drain_resistance", T_INT,
     offsetof(ccobject, cache_drain_resistance)},
    {"cache_non_ghost_count", T_INT, offsetof(ccobject, non_ghost_count), RO},
    {"cache_klass_count", T_INT, offsetof(ccobject, klass_count), RO},
    {NULL}
};

/* This module is compiled as a shared library.  Some compilers don't
   allow addresses of Python objects defined in other libraries to be
   used in static initializers here.  The DEFERRED_ADDRESS macro is
   used to tag the slots where such addresses appear; the module init
   function must fill in the tagged slots at runtime.  The argument is
   for documentation -- the macro ignores it.
*/
#define DEFERRED_ADDRESS(ADDR) 0

static PyTypeObject Cctype = {
    PyObject_HEAD_INIT(DEFERRED_ADDRESS(&PyType_Type))
    0,                              /* ob_size */
    "persistent.PickleCache",       /* tp_name */
    sizeof(ccobject),               /* tp_basicsize */
    0,                              /* tp_itemsize */
    (destructor)cc_dealloc,         /* tp_dealloc */
    0,                              /* tp_print */
    0,                              /* tp_getattr */
    0,                              /* tp_setattr */
    0,                              /* tp_compare */
    0,                              /* tp_repr */
    0,                              /* tp_as_number */
    0,                              /* tp_as_sequence */
    &cc_as_mapping,                 /* tp_as_mapping */
    0,                              /* tp_hash */
    0,                              /* tp_call */
    0,                              /* tp_str */
    0,                              /* tp_getattro */
    0,                              /* tp_setattro */
    0,                              /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
                              /* tp_flags */
    0,                              /* tp_doc */
    (traverseproc)cc_traverse,            /* tp_traverse */
    (inquiry)cc_clear,              /* tp_clear */
    0,                              /* tp_richcompare */
    0,                              /* tp_weaklistoffset */
    0,                              /* tp_iter */
    0,                              /* tp_iternext */
    cc_methods,                     /* tp_methods */
    cc_members,                     /* tp_members */
    cc_getsets,                     /* tp_getset */
    0,                              /* tp_base */
    0,                              /* tp_dict */
    0,                              /* tp_descr_get */
    0,                              /* tp_descr_set */
    0,                              /* tp_dictoffset */
    (initproc)cc_init,              /* tp_init */
};

void
initcPickleCache(void)
{
    PyObject *m;

    Cctype.ob_type = &PyType_Type;
    Cctype.tp_new = &PyType_GenericNew;
    if (PyType_Ready(&Cctype) < 0) {
      return;
    }

    m = Py_InitModule3("cPickleCache", NULL, cPickleCache_doc_string);

    capi = (cPersistenceCAPIstruct *)PyCObject_Import(
      "persistent.cPersistence", "CAPI");
    if (!capi)
      return;
    capi->percachedel = (percachedelfunc)cc_oid_unreferenced;

    py__p_changed = PyString_InternFromString("_p_changed");
    if (!py__p_changed)
        return;
    py__p_deactivate = PyString_InternFromString("_p_deactivate");
    if (!py__p_deactivate)
        return;
    py__p_jar = PyString_InternFromString("_p_jar");
    if (!py__p_jar)
        return;
    py__p_oid = PyString_InternFromString("_p_oid");
    if (!py__p_oid)
        return;

    if (PyModule_AddStringConstant(m, "cache_variant", "stiff/c") < 0)
      return;

    /* This leaks a reference to Cctype, but it doesn't matter. */
    if (PyModule_AddObject(m, "PickleCache", (PyObject *)&Cctype) < 0)
      return;
}

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