Revert: Allow table AM to store complex data structures in rd_amcache

This commit reverts 02eb07ea89 per review by Andres Freund.

Discussion: https://postgr.es/m/20240410165236.rwyrny7ihi4ddxw4%40awork3.anarazel.de

src/backend/access/heap/heapam_handler.c

@@ -2656,7 +2656,6 @@ static const TableAmRoutine heapam_methods = {
 	.index_validate_scan = heapam_index_validate_scan,
 	.relation_analyze = heapam_analyze,
 
-	.free_rd_amcache = NULL,
 	.relation_size = table_block_relation_size,
 	.relation_needs_toast_table = heapam_relation_needs_toast_table,
 	.relation_toast_am = heapam_relation_toast_am,

src/backend/utils/cache/relcache.c

@@ -2269,7 +2269,9 @@ RelationReloadIndexInfo(Relation relation)
 	RelationCloseSmgr(relation);
 
 	/* Must free any AM cached data upon relcache flush */
-	table_free_rd_amcache(relation);
+	if (relation->rd_amcache)
+		pfree(relation->rd_amcache);
+	relation->rd_amcache = NULL;
 
 	/*
 	 * If it's a shared index, we might be called before backend startup has
@@ -2489,7 +2491,8 @@ RelationDestroyRelation(Relation relation, bool remember_tupdesc)
 		pfree(relation->rd_options);
 	if (relation->rd_indextuple)
 		pfree(relation->rd_indextuple);
-	table_free_rd_amcache(relation);
+	if (relation->rd_amcache)
+		pfree(relation->rd_amcache);
 	if (relation->rd_fdwroutine)
 		pfree(relation->rd_fdwroutine);
 	if (relation->rd_indexcxt)
@@ -2551,7 +2554,9 @@ RelationClearRelation(Relation relation, bool rebuild)
 	RelationCloseSmgr(relation);
 
 	/* Free AM cached data, if any */
-	table_free_rd_amcache(relation);
+	if (relation->rd_amcache)
+		pfree(relation->rd_amcache);
+	relation->rd_amcache = NULL;
 
 	/*
 	 * Treat nailed-in system relations separately, they always need to be

src/include/access/tableam.h

@@ -687,14 +687,6 @@ typedef struct TableAmRoutine
 	 * ------------------------------------------------------------------------
 	 */
 
-	/*
-	 * This callback frees relation private cache data stored in rd_amcache.
-	 * After the call all memory related to rd_amcache must be freed,
-	 * rd_amcache must be set to NULL. If this callback is not provided,
-	 * rd_amcache is assumed to point to a single memory chunk.
-	 */
-	void		(*free_rd_amcache) (Relation rel);
-
 	/*
 	 * See table_relation_size().
 	 *
@@ -1816,32 +1808,6 @@ table_relation_analyze(Relation relation, AcquireSampleRowsFunc *func,
  * ----------------------------------------------------------------------------
  */
 
-/*
- * Frees relation private cache data stored in rd_amcache.  Uses
- * free_rd_amcache method if provided.  Assumes rd_amcache to point to single
- * memory chunk otherwise.
- */
-static inline void
-table_free_rd_amcache(Relation rel)
-{
-	if (rel->rd_tableam && rel->rd_tableam->free_rd_amcache)
-	{
-		rel->rd_tableam->free_rd_amcache(rel);
-
-		/*
-		 * We are assuming free_rd_amcache() did clear the cache and left NULL
-		 * in rd_amcache.
-		 */
-		Assert(rel->rd_amcache == NULL);
-	}
-	else
-	{
-		if (rel->rd_amcache)
-			pfree(rel->rd_amcache);
-		rel->rd_amcache = NULL;
-	}
-}
-
 /*
  * Return the current size of `rel` in bytes. If `forkNumber` is
  * InvalidForkNumber, return the relation's overall size, otherwise the size

src/include/utils/rel.h

@@ -221,12 +221,10 @@ typedef struct RelationData
 	 * rd_amcache is available for index and table AMs to cache private data
 	 * about the relation.  This must be just a cache since it may get reset
 	 * at any time (in particular, it will get reset by a relcache inval
-	 * message for the relation).  If used for table AM it must point to a
+	 * message for the relation).  If used, it must point to a single memory
-	 * single memory chunk palloc'd in CacheMemoryContext, or more complex
+	 * chunk palloc'd in CacheMemoryContext, or in rd_indexcxt for an index
-	 * data structure in that memory context to be freed by free_rd_amcache
+	 * relation.  A relcache reset will include freeing that chunk and setting
-	 * method.  If used for index AM it must point to a single memory chunk
+	 * rd_amcache = NULL.
-	 * palloc'd in rd_indexcxt memory context.  A relcache reset will include
-	 * freeing that chunk and setting rd_amcache = NULL.
 	 */
 	void	   *rd_amcache;		/* available for use by index/table AM */