[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, 19 insertions, 22 deletions

diff --git a/src/include/gnunet_datastore_plugin.h b/src/include/gnunet_datastore_plugin.h
index 2295d4e72..b1c9cb7c3 100644
--- a/src/include/gnunet_datastore_plugin.h
+++ b/src/include/gnunet_datastore_plugin.h
@@ -204,9 +204,9 @@ typedef void
  * 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 key key to match, never NULL
+ * @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).
  *        Note that for DBlocks there is no difference
@@ -215,17 +215,18 @@ typedef void
  * @param type entries of which type are relevant?
  *     Use 0 for any type.
  * @param proc function to call on the matching value;
- *        proc should be called with NULL if there is no result
+ *        will be called with NULL if nothing matches
  * @param proc_cls closure for @a proc
  */
 typedef void
 (*PluginGetKey) (void *cls,
-                 uint64_t offset,
-                 const struct GNUNET_HashCode *key,
-                 const struct GNUNET_HashCode *vhash,
-                 enum GNUNET_BLOCK_Type type,
-                 PluginDatumProcessor proc,
-                 void *proc_cls);
+                 uint64_t next_uid,
+                 bool random,
+                 const struct GNUNET_HashCode *key,
+                 const struct GNUNET_HashCode *vhash,
+                 enum GNUNET_BLOCK_Type type,
+                 PluginDatumProcessor proc,
+                 void *proc_cls);
 
 
 /**
@@ -285,23 +286,22 @@ typedef void
 
 
 /**
- * Select a single item from the datastore at the specified offset
- * (among those applicable).
+ * Select a single item from the datastore (among those applicable).
  *
  * @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 type entries of which type should be considered?
  *        Must not be zero (ANY).
- * @param proc function to call on the matching value
+ * @param proc function to call on the matching value;
+ *        will be called with NULL if no value matches
  * @param proc_cls closure for @a proc
  */
 typedef void
 (*PluginGetType) (void *cls,
-                  uint64_t offset,
-                  enum GNUNET_BLOCK_Type type,
-                  PluginDatumProcessor proc,
-                  void *proc_cls);
+                  uint64_t next_uid,
+                  enum GNUNET_BLOCK_Type type,
+                  PluginDatumProcessor proc,
+                  void *proc_cls);
 
 
 /**
@@ -354,9 +354,6 @@ struct GNUNET_DATASTORE_PluginFunctions
 
   /**
    * Get datum (of the specified type) with anonymity level zero.
-   * This function is allowed to ignore the 'offset' argument
-   * and instead return a random result (with zero anonymity of
-   * the correct type) if implementing an offset is expensive.
    */
   PluginGetType get_zero_anonymity;