simplehash: Add support for resetting a hashtable's contents.

A hashtable reset just reset the hashtable entries, but does not free
memory.

Author: Andres Freund
Discussion: https://postgr.es/m/20190114180423.ywhdg2iagzvh43we@alap3.anarazel.de
Bug: #15592 #15486
Backpatch: 11, this is a prerequisite for other fixes

src/include/lib/simplehash.h

@@ -72,6 +72,7 @@
 /* function declarations */
 #define SH_CREATE SH_MAKE_NAME(create)
 #define SH_DESTROY SH_MAKE_NAME(destroy)
+#define SH_RESET SH_MAKE_NAME(reset)
 #define SH_INSERT SH_MAKE_NAME(insert)
 #define SH_DELETE SH_MAKE_NAME(delete)
 #define SH_LOOKUP SH_MAKE_NAME(lookup)
@@ -140,6 +141,7 @@ typedef struct SH_ITERATOR
 SH_SCOPE	SH_TYPE *SH_CREATE(MemoryContext ctx, uint32 nelements,
 		  void *private_data);
 SH_SCOPE void SH_DESTROY(SH_TYPE * tb);
+SH_SCOPE void SH_RESET(SH_TYPE * tb);
 SH_SCOPE void SH_GROW(SH_TYPE * tb, uint32 newsize);
 SH_SCOPE	SH_ELEMENT_TYPE *SH_INSERT(SH_TYPE * tb, SH_KEY_TYPE key, bool *found);
 SH_SCOPE	SH_ELEMENT_TYPE *SH_LOOKUP(SH_TYPE * tb, SH_KEY_TYPE key);
@@ -368,6 +370,14 @@ SH_DESTROY(SH_TYPE * tb)
 	pfree(tb);
 }
 
+/* reset the contents of a previously created hash table */
+SH_SCOPE void
+SH_RESET(SH_TYPE * tb)
+{
+	memset(tb->data, 0, sizeof(SH_ELEMENT_TYPE) * tb->size);
+	tb->members = 0;
+}
+
 /*
  * Grow a hash table to at least `newsize` buckets.
  *
@@ -959,6 +969,7 @@ SH_STAT(SH_TYPE * tb)
 /* external function names */
 #undef SH_CREATE
 #undef SH_DESTROY
+#undef SH_RESET
 #undef SH_INSERT
 #undef SH_DELETE
 #undef SH_LOOKUP