.

wcfs/zodb.go

@@ -42,6 +42,52 @@ type PyObject struct {
 type Connection struct {
 	stor	zodb.IStorage	// underlying storage
 	at	zodb.Tid	// current view of database
+
+	// {} oid -> pyobj
+	//
+	// rationale:
+	//
+	// on invalidations: we need to go oid -> pyobj and invalidate it.
+	// -> Connection need to keep {} oid -> pyobj.
+	// -> we can use that {} when loading a persistent Ref twice to get to the same object.
+	//
+	// however: if Connection keeps strong link to pyobj, just
+	// pyobj.PDeactivate will not fully release pyobj if there are not
+	// references to it from other objects:
+	//
+	//	- deactivate will release pyobj state (ok)
+	//	- but there will be still reference from connection `oid -> pyobj` map to this object.
+	//
+	// -> we can solve it by using "weak" pointers in the map.
+	//
+	// NOTE we cannot use regular map and arbitrarily manually "gc" entries
+	// there periodically: since for a pyobj we don't know whether other
+	// objects are referencing it, we can't just remove pyobj's oid from
+	// the map - if we do so and there are other live objects that
+	// reference pyobj, user code can still reach pyobj via those
+	// references. On the other hand, if another, not yet loaded, object
+	// also references pyobj and gets loaded, traversing reference from
+	// that loaded object will load second copy of pyobj, thus breaking 1
+	// object in db <-> 1 live object invariant:
+	//
+	//	A  →  B  →  C
+	//	↓           |
+	//      D <--------- - - -> D2 (wrong)
+	//
+	// - A activate
+	// - D activate
+	// - B activate
+	// - D gc, A still keeps link on D
+	// - C activate -> it needs to get to D, but D was removed from objtab
+	//   -> new D2 is wrongly created
+	//
+	// that's why we have to depend on Go's GC to know whether there are
+	// still live references left or not. And that in turn means finalizers
+	// and thus weak references.
+	//
+	// some link on the subject:
+	// https://groups.google.com/forum/#!topic/golang-nuts/PYWxjT2v6ps
+	objtab map[zodb.Oid]WeakRef
 }
 
 // Gets loads and decodes object from the database according to its current view.
@@ -98,12 +144,15 @@ func (conn *Connection) newGhost(pyclass pickle.Class, oid zodb.Oid) interface{}
 	return &pyobj
 }
 
+
+// XXX pyobj.PInvalidate()	= deactivate without checking if state != modified
+
 // PDeactivates transforms object to ghost state.
 //
 // In ghost state object data is dropped and only oid/pyclass information is left in RAM.
-//
-// XXX if state=modified PDeactivate must be noop.
 func (pyobj *PyObject) PDeactivate() {
+	// FIXME if state=modified PDeactivate must be noop.
+
 	pyobj.pystate = nil
 	pyobj.serial  = zodb.InvalidTID
 }