Use caller's memory context for radix tree iteration state

Typically only one iterator is present at any time, so it's overkill
to devote an entire context for this. Get rid of it and use the
caller's context.

This is tidy-up work, so no backpatch in this form. However, a
hypothetical extension to v17 that tried to start iteration from
an attaching backend would result in a crash, so that'll be fixed
separately in a way that doesn't change behavior in core.

Patch by me, reported and reviewed by Masahiko Sawada

Discussion: https://postgr.es/m/CAD21AoBB2U47V=F+wQRB1bERov_of5=BOZGaybjaV8FLQyqG3Q@mail.gmail.com

src/include/lib/radixtree.h

@@ -719,7 +719,6 @@ struct RT_RADIX_TREE
 	/* leaf_context is used only for single-value leaves */
 	MemoryContextData *leaf_context;
 #endif
-	MemoryContextData *iter_context;
 };
 
 /*
@@ -1836,14 +1835,6 @@ RT_CREATE(MemoryContext ctx)
 	tree = (RT_RADIX_TREE *) palloc0(sizeof(RT_RADIX_TREE));
 	tree->context = ctx;
 
-	/*
-	 * Separate context for iteration in case the tree context doesn't support
-	 * pfree
-	 */
-	tree->iter_context = AllocSetContextCreate(ctx,
-											   RT_STR(RT_PREFIX) "_radix_tree iter context",
-											   ALLOCSET_SMALL_SIZES);
-
 #ifdef RT_SHMEM
 	tree->dsa = dsa;
 	dp = dsa_allocate0(dsa, sizeof(RT_RADIX_TREE_CONTROL));
@@ -2075,7 +2066,8 @@ RT_FREE(RT_RADIX_TREE * tree)
 /***************** ITERATION *****************/
 
 /*
- * Create and return the iterator for the given radix tree.
+ * Create and return an iterator for the given radix tree
+ * in the caller's memory context.
  *
  * Taking a lock in shared mode during the iteration is the caller's
  * responsibility.
@@ -2086,8 +2078,7 @@ RT_BEGIN_ITERATE(RT_RADIX_TREE * tree)
 	RT_ITER    *iter;
 	RT_CHILD_PTR root;
 
-	iter = (RT_ITER *) MemoryContextAllocZero(tree->iter_context,
-											  sizeof(RT_ITER));
+	iter = (RT_ITER *) palloc0(sizeof(RT_ITER));
 	iter->tree = tree;
 
 	Assert(RT_PTR_ALLOC_IS_VALID(tree->ctl->root));