[datastore] Fix #3743

This change adds support for key == NULL to the datastore plugins
and replaces the offset argument with a next_uid and random arguments to
increase performance in the key == NULL case.

With the offset argument a datastore plugin would have to count all
matching keys before fetching the key at the right offset, which would
iterate over the entire database in the case of key == NULL.

The offset argument was used in two ways: to iterate over a set of
matching values and to start iteration at a random matching value. The new API
seperates these into two arguments: if random is true it will return a
random matching value, otherwise next_uid can be set to uid + 1 to return the
next matching value.

The random argument was not added to get_zero_anonymity. This function
is used to periodically insert zero anonymity values into the DHT. I
don't think it's necessary to randomize this.

1 files changed, 8 insertions, 9 deletions

diff --git a/src/datastore/plugin_datastore_template.c b/src/datastore/plugin_datastore_template.c
index a1e03e8ee..187221798 100644
--- a/src/datastore/plugin_datastore_template.c
+++ b/src/datastore/plugin_datastore_template.c
@@ -89,8 +89,8 @@ template_plugin_put (void *cls, const struct GNUNET_HashCode * key, uint32_t siz
  * Get one of the results for a particular key in the datastore.
  *
  * @param cls closure
- * @param offset offset of the result (modulo num-results);
- *               specific ordering does not matter for the offset
+ * @param next_uid return the result with lowest uid >= next_uid
+ * @param random if true, return a random result instead of using next_uid
  * @param key maybe NULL (to match all entries)
  * @param vhash hash of the value, maybe NULL (to
  *        match all values that have the right key).
@@ -104,7 +104,7 @@ template_plugin_put (void *cls, const struct GNUNET_HashCode * key, uint32_t siz
  * @param proc_cls closure for proc
  */
 static void
-template_plugin_get_key (void *cls, uint64_t offset,
+template_plugin_get_key (void *cls, uint64_t next_uid, bool random,
                          const struct GNUNET_HashCode * key,
                          const struct GNUNET_HashCode * vhash,
                          enum GNUNET_BLOCK_Type type, PluginDatumProcessor proc,
@@ -185,16 +185,15 @@ template_plugin_update (void *cls, uint64_t uid, uint32_t delta,
  * Call the given processor on an item with zero anonymity.
  *
  * @param cls our "struct Plugin*"
- * @param offset offset of the result (modulo num-results);
- *               specific ordering does not matter for the offset
+ * @param next_uid return the result with lowest uid >= next_uid
  * @param type entries of which type should be considered?
- *        Use 0 for any type.
- * @param proc function to call on each matching value;
- *        will be called  with NULL if no value matches
+ *        Must not be zero (ANY).
+ * @param proc function to call on the matching value;
+ *        will be called with NULL if no value matches
  * @param proc_cls closure for proc
  */
 static void
-template_plugin_get_zero_anonymity (void *cls, uint64_t offset,
+template_plugin_get_zero_anonymity (void *cls, uint64_t next_uid,
                                     enum GNUNET_BLOCK_Type type,
                                     PluginDatumProcessor proc, void *proc_cls)
 {