lib: rework management of user pointers in the northbound layer

Introduce a hash table to keep track of user pointers associated
to configuration entries. The previous strategy was to embed
the user pointers inside libyang data nodes, but this solution
incurred a substantial performance overhead. The user pointers
embedded in candidate configurations could be lost while the
configuration was being edited, so they needed to be regenerated
before the candidate could be committed.  This was done by the
nb_candidate_restore_priv_pointers() function, which was extremely
expensive for large configurations. The new hash table solves this
performance problem.

The yang_dnode_[gs]et_entry() functions were renamed and moved from
yang.[ch] to northbound.[ch], which is a more appropriate place
for them. This patch also introduces the nb_running_unset_entry()
function, the counterpart of nb_running_set_entry() (unsetting
user pointers was done automatically before, now it needs to be
done manually).

As a consequence of these changes, we shouldn't need support for
libyang private pointers anymore (-DENABLE_LYD_PRIV=ON). But it's
probably a good idea to keep requiring this feature as we might
need it in the future for other things (e.g. disable configuration
settings without removing them).

Fixes #4136.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

1 files changed, 69 insertions, 0 deletions

diff --git a/lib/northbound.h b/lib/northbound.h
index bfa28b3f65..14f27c1d41 100644
--- a/lib/northbound.h
+++ b/lib/northbound.h
@@ -808,6 +808,75 @@ extern bool nb_operation_is_valid(enum nb_operation operation,
 extern int nb_notification_send(const char *xpath, struct list *arguments);
 
 /*
+ * Associate a user pointer to a configuration node.
+ *
+ * This should be called by northbound 'create' callbacks in the NB_EV_APPLY
+ * phase only.
+ *
+ * dnode
+ *    libyang data node - only its XPath is used.
+ *
+ * entry
+ *    Arbitrary user-specified pointer.
+ */
+extern void nb_running_set_entry(const struct lyd_node *dnode, void *entry);
+
+/*
+ * Unset the user pointer associated to a configuration node.
+ *
+ * This should be called by northbound 'destroy' callbacks in the NB_EV_APPLY
+ * phase only.
+ *
+ * dnode
+ *    libyang data node - only its XPath is used.
+ *
+ * Returns:
+ *    The user pointer that was unset.
+ */
+extern void *nb_running_unset_entry(const struct lyd_node *dnode);
+
+/*
+ * Find the user pointer (if any) associated to a configuration node.
+ *
+ * The XPath associated to the configuration node can be provided directly or
+ * indirectly through a libyang data node.
+ *
+ * If an user point is not found, this function follows the parent nodes in the
+ * running configuration until an user pointer is found or until the root node
+ * is reached.
+ *
+ * dnode
+ *    libyang data node - only its XPath is used (can be NULL if 'xpath' is
+ *    provided).
+ *
+ * xpath
+ *    XPath of the configuration node (can be NULL if 'dnode' is provided).
+ *
+ * abort_if_not_found
+ *    When set to true, abort the program if no user pointer is found.
+ *
+ *    As a rule of thumb, this parameter should be set to true in the following
+ *    scenarios:
+ *    - Calling this function from any northbound configuration callback during
+ *      the NB_EV_APPLY phase.
+ *    - Calling this function from a 'delete' northbound configuration callback
+ *      during any phase.
+ *
+ *    In both the above cases, the given configuration node should contain an
+ *    user pointer except when there's a bug in the code, in which case it's
+ *    better to abort the program right away and eliminate the need for
+ *    unnecessary NULL checks.
+ *
+ *    In all other cases, this parameter should be set to false and the caller
+ *    should check if the function returned NULL or not.
+ *
+ * Returns:
+ *    User pointer if found, NULL otherwise.
+ */
+extern void *nb_running_get_entry(const struct lyd_node *dnode, const char *xpath,
+				  bool abort_if_not_found);
+
+/*
  * Return a human-readable string representing a northbound event.
  *
  * event