hashmap: introduce hashmap_reserve()

With the current hashmap implementation that uses chaining, placing a
reservation can serve two purposes:
 - To optimize putting of entries if the number of entries to put is
   known. The reservation allocates buckets, so later resizing can be
   avoided.
 - To avoid having very long bucket chains after using
   hashmap_move(_one).

In an alternative hashmap implementation it will serve an additional
purpose:
 - To guarantee a subsequent hashmap_move(_one) will not fail with
   -ENOMEM (this never happens in the current implementation).

1 files changed, 1 insertions, 0 deletions

diff --git a/src/shared/set.h b/src/shared/set.h
index 840ee0a7e4..8fe071a326 100644
--- a/src/shared/set.h
+++ b/src/shared/set.h
@@ -50,6 +50,7 @@ void *set_remove(Set *s, void *value);
 int set_remove_and_put(Set *s, void *old_value, void *new_value);
 
 int set_merge(Set *s, Set *other);
+int set_reserve(Set *s, unsigned entries_add);
 void set_move(Set *s, Set *other);
 int set_move_one(Set *s, Set *other, void *value);