Always use the caller-provided context for radix tree leaves

Previously, it would not have worked for a caller to pass a slab
context, since it would have been used for other things which likely
had incompatible size. In an attempt to be helpful and avoid possible
space wastage due to aset's power-of-two rounding, RT_CREATE would
create an additional slab context if the value type was fixed-length
and larger than pointer size. The problem was, we have since added
the bump context type, and the generation context was a possibility as
well, so silently overriding the caller's choice may actually be worse.

Commit e8a6f1f908d arranged so that the caller-provided context is
used only for leaves, so it's safe for the caller to use slab here
if they wish. As demonstration, use slab in one of the radix tree
regression tests.

Reviewed by Masahiko Sawada

Discussion: https://postgr.es/m/CANWCAZZDCo4k5oURg_pPxM6+WZ1oiG=sqgjmQiELuyP0Vtrwig@mail.gmail.com

src/include/lib/radixtree.h

@@ -1849,21 +1849,7 @@ RT_CREATE(MemoryContext ctx)
 												size_class.allocsize);
 	}
 
-	/* By default we use the passed context for leaves. */
 	tree->leaf_context = ctx;
-
-#ifndef RT_VARLEN_VALUE_SIZE
-
-	/*
-	 * For leaves storing fixed-length values, we use a slab context to avoid
-	 * the possibility of space wastage by power-of-2 rounding up.
-	 */
-	if (sizeof(RT_VALUE_TYPE) > sizeof(RT_PTR_ALLOC))
-		tree->leaf_context = SlabContextCreate(ctx,
-											   RT_STR(RT_PREFIX) "_radix_tree leaf context",
-											   RT_SLAB_BLOCK_SIZE(sizeof(RT_VALUE_TYPE)),
-											   sizeof(RT_VALUE_TYPE));
-#endif							/* !RT_VARLEN_VALUE_SIZE */
 #endif							/* RT_SHMEM */
 
 	/* add root node now so that RT_SET can assume it exists */

src/test/modules/test_radixtree/test_radixtree.c

@@ -313,9 +313,10 @@ test_random(void)
 #else
 	MemoryContext radixtree_ctx;
 
-	radixtree_ctx = AllocSetContextCreate(CurrentMemoryContext,
+	radixtree_ctx = SlabContextCreate(CurrentMemoryContext,
 									  "test_radix_tree",
-										  ALLOCSET_SMALL_SIZES);
+									  SLAB_DEFAULT_BLOCK_SIZE,
+									  sizeof(TestValueType));
 	radixtree = rt_create(radixtree_ctx);
 #endif