util/set: add macro for destructively iterating set entries

a common usage for sets is for tracking exactly one instance of a pointer
for a given period of time, after which the set's entries are purged and it
is reused

this macro enables the purge phase of such usage to reset the table to a
pristine state, avoiding future rehashing due to ballooning of deleted entries

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8498>

src/util/set.c

@@ -560,6 +560,27 @@ _mesa_set_remove_key(struct set *set, const void *key)
    _mesa_set_remove(set, _mesa_set_search(set, key));
 }
 
+/**
+ * This function is an iterator over the set when no deleted entries are present.
+ *
+ * Pass in NULL for the first entry, as in the start of a for loop.
+ */
+struct set_entry *
+_mesa_set_next_entry_unsafe(const struct set *ht, struct set_entry *entry)
+{
+   assert(!ht->deleted_entries);
+   if (!ht->entries)
+      return NULL;
+   if (entry == NULL)
+      entry = ht->table;
+   else
+      entry = entry + 1;
+   if (entry != ht->table + ht->size)
+      return entry->key ? entry : _mesa_set_next_entry_unsafe(ht, entry);
+
+   return NULL;
+}
+
 /**
  * This function is an iterator over the hash table.
  *