Object relation types
---------------------

This document describes the relations between objects that exist in GStreamer.
It will also describe the way of handling the relation wrt locking and 
refcounting.

parent-child relation
~~~~~~~~~~~~~~~~~~~~~

      +---------+    +-------+
      | parent  |    | child |
 *--->|       *----->|       |
      |       F1|<-----*    1|
      +---------+    +-------+

 - properties

   - parent has references to multiple children
   - child has reference to parent
   - reference fields protected with LOCK
   - the reference held by each child to the parent is
     NOT reflected in the refcount of the parent.
   - the parent removes the floating flag of the child when taking
     ownership.
   - the application has valid reference to parent
   - creation/destruction requires two unnested locks and 1 refcount.

 - usage in GStreamer

    GstBin -> GstElement
    GstElement -> GstRealPad

 - lifecycle
 
 a) object creation

    The application creates two object and holds a pointer
    to them. The objects are initially FLOATING with a refcount
    of 1.

         +---------+              +-------+
    *--->| parent  |         *--->| child |
         |       * |              |       |
         |       F1|              | *   F1|
         +---------+              +-------+

 b) establishing the parent-child relationship

    The application then calls a method on the parent object to take
    ownership of the child object. The parent performs the following
    actions:

      result = _set_parent (child, parent);
      if (result) {
        LOCK (parent);
        ref_pointer = child;

        .. update other data structures ..
        UNLOCK (parent);
      }
      else {
        .. child had parent ..
      }

    The _set_parent() method performs the following actions:
    
      LOCK (child);
      if (child->parent != NULL) {
        UNLOCK (child);
        return FALSE;
      }
      if (IS_FLOATING (child)) {
        UNSET (child, FLOATING);
      }
      else {
        _ref (child);
      }
      child->parent = parent;
      UNLOCK (child);
      _signal (PARENT_SET, child, parent);
      return TRUE;
      
    The function atomically checks if the child has no parent yet
    and will set the parent if not. It will also sink the child, meaning
    all floating references to the child are invalid now as it takes
    over the refcount of the object.

    Visually:

      after _set_parent() returns TRUE:

          +---------+            +-------+
    *---->| parent  |      *-//->| child |
          |       * |            |       |
          |       F1|<-------------*    1|
          +---------+            +-------+

      after parent updates ref_pointer to child.

          +---------+        +-------+
    *---->| parent  |  *-//->| child |
          |       *--------->|       |
          |       F1|<---------*    1|
          +---------+        +-------+
     
   - only one parent is able to _sink the same object because the
     _set_parent() method is atomic.
   - since only one parent is able to _set_parent() the object, only
     one will add a reference to the object.
   - since the parent can hold multiple references to children, we don't
     need to lock the parent when locking the child. Many threads can
     call _set_parent() on the children with the same parent, the parent
     can then add all those to its lists.

   Note: that the signal is emitted before the parent has added the
   element to its internal data structures. This is not a problem
   since the parent usually has his own signal to inform the app that
   the child was reffed. One possible solution would be to update the
   internal structure first and then perform a rollback if the _set_parent()
   failed. This is not a good solution as iterators might grab the
   'half-added' child too soon.
   
 c) using the parent-child relationship
   
    - since the initial floating reference to the child object became
      invalid after giving it to the parent, any reference to a child
      has at least a refcount > 1.

    - this means that unreffing a child object cannot decrease the refcount
      to 0. In fact, only the parent can destroy and dispose the child
      object.

    - given a reference to the child object, the parent pointer is only
      valid when holding the child LOCK. Indeed, after unlocking the child
      LOCK, the parent can unparent the child or the parent could even become
      disposed. To avoid the parent dispose problem, when obtaining the 
      parent pointer, if should be reffed before releasing the child LOCK.
    
    1) getting a reference to the parent.
       
       - a referece is held to the child, so it cannot be disposed.
       
       LOCK (child);
       parent = _ref (child->parent);
       UNLOCK (child);

       .. use parent ..

       _unref (parent);

    2) getting a reference to a child
       
       - a reference to a child can be obtained by reffing it before 
         adding it to the parent or by querying the parent.

       - when requesting a child from the parent, a reference is held to 
         the parent so it cannot be disposed. The parent will use its
         internal data structures to locate the child element and will
         return a reference to it with an incremented refcount. The
         requester should _unref() the child after usage.
       
   
 d) destroying the parent-child relationship

    - only the parent can actively destroy the parent-child relationship
      this typically happens when a method is called on the parent to release
      ownership of the child.

    - a child shall never remove itself from the parent.

    - since calling a method on the parent with the child as an argument
      requires the caller to obtain a valid reference to the child, the child
      refcount is at least > 1.

    - the parent will perform the folowing actions:

        LOCK (parent);
        if (ref_pointer == child) {
          ref_pointer = NULL;

          .. update other data structures ..
          UNLOCK (parent);

          _unparent (child);
        }
        else {
          UNLOCK (parent);
          .. not our child ..
        }

    The _unparent() method performs the following actions:
    
      LOCK (child);
      if (child->parent != NULL) {
        child->parent = NULL;
        UNLOCK (child);
        _signal (PARENT_UNSET, child, parent);

        _unref (child);
      }
      else {
        UNLOCK (child);
      }
      
    Since the _unparent() method unrefs the child object, it is possible that
    the child pointer is invalid after this function. If the parent wants to
    perform other actions on the child (such as signal emmision) it should
    _ref() the child first.


single-reffed relation
~~~~~~~~~~~~~~~~~~~~~~

      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |       *--------->|         |
      |        1|        |        2|
      +---------+        +---------+

 - properties

   - one object has a reference to another
   - reference field protected with LOCK
   - the reference held by the object is reflected in the
     refcount of the other object.
   - typically the other object can be shared among multiple
     other objects where each ref is counted for in the
     refcount.
   - no object has ownership of the other. 
   - either shared state or copy-on-write.
   - creation/destruction requires one lock and one refcount.

 - usage

    GstRealPad -> GstCaps
    GstBuffer -> GstCaps
    GstEvent -> GstCaps
    GstEvent -> GstObject
    GstMessage -> GstCaps
    GstMessage -> GstObject

 - lifecycle

 a) Two objects exist unlinked.
   
      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |      *  |        |         |
      |        1|        |        1|
      +---------+        +---------+

 b) establishing the single-reffed relationship

   The second object is attached to the first one using a method
   on the first object. The second object is reffed and a pointer
   is updated in the first object using the following algorithm:

     LOCK (object1);
     if (object1->pointer)
       _unref (object1->pointer);
     object1->pointer = _ref (object2);
     UNLOCK (object1);

   After releasing the lock on the first object is is not sure that
   object2 is still reffed from object1.

      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |       *--------->|         |
      |        1|        |        2|
      +---------+        +---------+

 c) using the single-reffed relationship

   The only way to access object2 is by holding a ref to it or by
   getting the reference from object1.
   Reading the object pointed to by object1 can be done like this:

     LOCK (object1);
     object2 = object1->pointer;
     _ref (object2);
     UNLOCK (object1);

     .. use object2 ...
     _unref (object2);

   Depending on the type of the object, modifications can be done either
   with copy-on-write or directly into the object.

   Copy on write can practically only be done like this:

     LOCK (object1);
     object2 = object1->pointer;
     object2 = _copy_on_write (object2);
     ... make modifications to object2 ...
     UNLOCK (object1);

   Releasing the lock has only a very small window where the copy_on_write
   actually does not perform a copy:

     LOCK (object1);
     object2 = object1->pointer;
     _ref (object2);
     UNLOCK (object1);

     .. object2 now has at least 2 refcounts making the next
        copy-on-write make a real copy, unless some other thread
        writes another object2 to object1 here ...

     object2 = _copy_on_write (object2);

     .. make modifications to object2 ...

     LOCK (object1);
     if (object1->pointer != object2) {
       if (object1->pointer)
         _unref (object1->pointer);
       object1->pointer = gst_object_ref (object2);
     }
     UNLOCK (object1);

 d) destroying the single-reffed relationship
  
   The folowing algorithm removes the single-reffed link between
   object1 and object2.

     LOCK (object1);
     _unref (object1->pointer);
     object1->pointer = NULL;
     UNLOCK (object1);
     
   Which yields the following initial state again:

      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |      *  |        |         |
      |        1|        |        1|
      +---------+        +---------+


unreffed relation
~~~~~~~~~~~~~~~~~
 
      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |       *--------->|         |
      |        1|<---------*      1|
      +---------+        +---------+
      
 - properties

   - two objects have references to each other
   - both objects can only have 1 reference to another object.
   - reference fields protected with LOCK
   - the references held by each object are NOT reflected in the
     refcount of the other object.
   - no object has ownership of the other.
   - typically each object is owned by a different parent.
   - creation/destruction requires two nested locks and no refcounts.

 - usage

   - This type of link is used when the link is less important than 
     the existance of the objects, If one of the objects is disposed, so 
     is the link.
   
   GstRealPad <-> GstRealPad (srcpad lock taken first)

 - lifecycle

 a) Two objects exist unlinked.

      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |       * |        |         |
      |        1|        | *      1|
      +---------+        +---------+

 b) establishing the unreffed relationship
 
    Since we need to take two locks, the order in which these locks are
    taken is very important or we might cause deadlocks. This lock order
    must be defined for all unreffed relations. In these examples we always
    lock object1 first and then object2.

      LOCK (object1);
      LOCK (object2);
      object2->refpointer = object1;
      object1->refpointer = object2;
      UNLOCK (object2);
      UNLOCK (object1);

 c) using the unreffed relationship

    Reading requires taking one of the locks and reading the corresponing
    object. Again we need to ref the object before releasing the lock.

      LOCK (object1);
      object2 = _ref (object1->refpointer);
      UNLOCK (object1);

      .. use object2 ..
      _unref (object2);
      
 d) destroying the unreffed relationship
 
    Because of the lock order we need to be careful when destroying this
    Relation. 
    
    When only a reference to object1 is held:

      LOCK (object1);
      LOCK (object2);
      object1->refpointer->refpointer = NULL;
      object1->refpointer = NULL;
      UNLOCK (object2);
      UNLOCK (object1);

    When only a reference to object2 is held we need to get a handle to the
    other object fist so that we can lock it first. There is a window where 
    we need to release all locks and the relation could be invalid. To solve
    this we check the relation after grabbing both locks and retry if the
    relation changed.

    retry:
      LOCK (object2);
      object1 = _ref (object2->refpointer);
      UNLOCK (object2);
      .. things can change here ..
      LOCK (object1);
      LOCK (object2);
      if (object1 == object2->refpointer) {
        /* relation unchanged */
        object1->refpointer->refpointer = NULL;
        object1->refpointer = NULL;
      }
      else {
        /* relation changed.. retry */
        UNLOCK (object2);
        UNLOCK (object1);
        _unref (object1);
        goto retry;
      }
      UNLOCK (object2);
      UNLOCK (object1);
      _unref (object1);

    When references are held to both objects. Note that it is not possible to
    get references to both objects with the locks released since when the 
    references are taken and the locks are released, a concurrent update might
    have changed the link, making the references not point to linked objects.

      LOCK (object1);
      LOCK (object2);
      if (object1->refpointer == object2) {
        object2->refpointer = NULL;
        object1->refpointer = NULL;
      }
      else {
        .. objects are not linked ..
      }
      UNLOCK (object2);
      UNLOCK (object1);


double-reffed relation
~~~~~~~~~~~~~~~~~~~~~~

      +---------+        +---------+
 *--->| object1 |   *--->| object2 |
      |       *--------->|         |
      |        2|<---------*      2|
      +---------+        +---------+

 - properties

   - two objects have references to each other
   - reference fields protected with LOCK
   - the references held by each object are reflected in the
     refcount of the other object.
   - no object has ownership of the other.
   - typically each object is owned by a different parent.
   - creation/destruction requires two locks and two refcounts.

 - usage
   
   Not used in GStreamer.

 - lifecycle