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.
  *

src/util/set.h

@@ -111,6 +111,8 @@ _mesa_set_remove_key(struct set *set, const void *key);
 
 struct set_entry *
 _mesa_set_next_entry(const struct set *set, struct set_entry *entry);
+struct set_entry *
+_mesa_set_next_entry_unsafe(const struct set *set, struct set_entry *entry);
 
 struct set_entry *
 _mesa_set_random_entry(struct set *set,
@@ -132,6 +134,15 @@ _mesa_set_intersects(struct set *a, struct set *b);
         entry != NULL;                                              \
         entry = _mesa_set_next_entry(set, entry))
 
+/**
+ * This foreach function destroys the table as it iterates.
+ * It is not safe to use when inserting or removing entries.
+ */
+#define set_foreach_remove(set, entry)                              \
+   for (struct set_entry *entry = _mesa_set_next_entry_unsafe(set, NULL);  \
+        (set)->entries;                                              \
+        entry->hash = 0, entry->key = (void*)NULL, (set)->entries--, entry = _mesa_set_next_entry_unsafe(set, entry))
+
 #ifdef __cplusplus
 } /* extern C */
 #endif

src/util/tests/set/set_test.cpp

@@ -58,6 +58,20 @@ TEST(set, basic)
       GTEST_FAIL();
    }
 
+   _mesa_set_add(s, a);
+   _mesa_set_add(s, b);
+   EXPECT_EQ(s->entries, 2);
+   unsigned count = s->entries;
+   set_foreach_remove(s, he) {
+      EXPECT_TRUE(he->key == a || he->key == b);
+      EXPECT_EQ(s->entries, count--);
+      EXPECT_EQ(s->deleted_entries, 0);
+   }
+   EXPECT_EQ(s->entries, 0);
+   set_foreach(s, he) {
+      GTEST_FAIL();
+   }
+
    _mesa_set_destroy(s, NULL);
 }