Add some more documentation on iterating ao2 containers. The documentation

implies that is possible to miss an object or see an object twice while
iterating.  After looking through the code and talking with mmichelson, I have
documented the exact conditions under which this can happen (which are rare and
harmless in most cases).


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@80426 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/astobj2.h

@@ -433,39 +433,51 @@ void *ao2_callback(ao2_container *c,
 
 /*!
  *
-
+ *
-When we need to walk through a container, we use
+ * When we need to walk through a container, we use
-ao2_iterator to keep track of the current position.
+ * ao2_iterator to keep track of the current position.
-
+ * 
-Because the navigation is typically done without holding the
+ * Because the navigation is typically done without holding the
-lock on the container across the loop,
+ * lock on the container across the loop,
-objects can be inserted or deleted or moved
+ * objects can be inserted or deleted or moved
-while we work. As a consequence, there is no guarantee that
+ * while we work. As a consequence, there is no guarantee that
-the we manage to touch all the elements on the list, or it
+ * the we manage to touch all the elements on the list, or it
-is possible that we touch the same object multiple times.
+ * is possible that we touch the same object multiple times.
-
+ * However, within the current hash table container, the following is true:
-An iterator must be first initialized with ao2_iterator_init(),
+ *  - It is not possible to miss an object in the container while iterating
-then we can use o = ao2_iterator_next() to move from one
+ *    unless it gets added after the iteration begins and is added to a bucket
-element to the next. Remember that the object returned by
+ *    that is before the one the current object is in.  In this case, even if
-ao2_iterator_next() has its refcount incremented,
+ *    you locked the container around the entire iteration loop, you still would
-and the reference must be explicitly released when done with it.
+ *    not see this object, because it would still be waiting on the container
-Example:
+ *    lock so that it can be added.
-
+ *  - It would be extremely rare to see an object twice.  The only way this can
-    \code
+ *    happen is if an object got unlinked from the container and added again 
-
+ *    during the same iteration.  Furthermore, when the object gets added back,
-    ao2_container *c = ... // the container we want to iterate on
+ *    it has to be in the current or later bucket for it to be seen again.
-    ao2_iterator i;
+ *
-    struct my_obj *o;
+ * An iterator must be first initialized with ao2_iterator_init(),
-
+ * then we can use o = ao2_iterator_next() to move from one
-    i = ao2_iterator_init(c, flags);
+ * element to the next. Remember that the object returned by
- 
+ * ao2_iterator_next() has its refcount incremented,
-    while ( (o = ao2_iterator_next(&i)) ) {
+ * and the reference must be explicitly released when done with it.
-	... do something on o ...
+ *
-	ao2_ref(o, -1);
+ * Example:
-    }
+ *
-
+ *  \code
-    \endcode
+ *
-
+ *  ao2_container *c = ... // the container we want to iterate on
+ *  ao2_iterator i;
+ *  struct my_obj *o;
+ *
+ *  i = ao2_iterator_init(c, flags);
+ *
+ *  while ( (o = ao2_iterator_next(&i)) ) {
+ *     ... do something on o ...
+ *     ao2_ref(o, -1);
+ *  }
+ *
+ *  \endcode
+ *
  */
 
 /*!