Merge revs 27788 and 27795 from Zope trunk.

See Collector #1350.

ghostify() and unghostify() detect one form of thread insanity
now.  In a debug build, they abort the process if it happens.
In a release build, unghostify() raises SystemError, and
ghostify() ignores it (ghostify() can't raise an exception).

NEWS.txt

+What's new in ZODB3 3.3.1?
+==========================
+Release date: xx-xxx-2004
+
+persistent
+----------
+
+Collector #1350:  ZODB has a default one-thread-per-connection model, and
+two threads should never do operations on a single connection
+simultaneously.  However, ZODB can't detect violations, and this happened
+in an early stage of Zope 2.8 development.  The low-level ``ghostify()``
+and ``unghostify()`` routines in ``cPerisistence.c`` were changed to give
+some help in detecting this when it happens.  In a debug build, both abort
+the process if thread interference is detected.  This is extreme, but
+impossible to overlook.  In a release build, ``unghostify()`` raises
+``SystemError`` if thread damage is detected; ``ghostify()`` ignores the
+problem in a release build (``ghostify()`` is supposed to be so simple that
+it "can't fail").
+
 What's new in ZODB3 3.3?
 ========================
 Release date: 06-Oct-2004

src/persistent/cPersistence.c

@@ -58,6 +58,24 @@ init_strings(void)
     return 0;
 }
 
+#ifdef Py_DEBUG
+static void
+fatal_1350(cPersistentObject *self, const char *caller, const char *detail)
+{
+	char buf[1000];
+
+	PyOS_snprintf(buf, sizeof(buf),
+	    "cPersistence.c %s(): object at %p with type %.200s\n"
+	    "%s.\n"
+	    "The only known cause is multiple threads trying to ghost and\n"
+	    "unghost the object simultaneously.\n"
+	    "That's not legal, but ZODB can't stop it.\n"
+	    "See Collector #1350.\n",
+	    caller, self, self->ob_type->tp_name, detail);
+	Py_FatalError(buf);
+}
+#endif
+
 static void ghostify(cPersistentObject*);
 
 /* Load the state of the object, unghostifying it.  Upon success, return 1.
@@ -88,6 +106,18 @@ unghostify(cPersistentObject *self)
         }
         self->state = cPersistent_UPTODATE_STATE;
         Py_DECREF(r);
+        if (self->cache && self->ring.r_next == NULL) {
+#ifdef Py_DEBUG
+        	fatal_1350(self, "unghostify",
+		    		 "is not in the cache despite that we just "
+		      		 "unghostified it");
+#else
+		PyErr_Format(PyExc_SystemError, "object at %p with type "
+			     "%.200s not in the cache despite that we just "
+			     "unghostified it", self, self->ob_type->tp_name);
+		return -1;
+#endif
+	}
     }
     return 1;
 }
@@ -134,13 +164,19 @@ ghostify(cPersistentObject *self)
         return;
     }
 
-    /* If the cache is still active, we must unlink the object. */
-    if (self->ring.r_next) {
-	/* if we're ghostifying an object, we better have some non-ghosts */
+    if (self->ring.r_next == NULL) {
+	/* There's no way to raise an error in this routine. */
+#ifdef Py_DEBUG
+	fatal_1350(self, "ghostify", "claims to be in a cache but isn't");
+#else
+	return;
+#endif
+    }
+
+    /* If we're ghostifying an object, we better have some non-ghosts. */
     assert(self->cache->non_ghost_count > 0);
     self->cache->non_ghost_count--;
     ring_del(&self->ring);
-    }
     self->state = cPersistent_GHOST_STATE;
     dictptr = _PyObject_GetDictPtr((PyObject *)self);
     if (dictptr && *dictptr) {