Fix several recently introduced issues around handling new relation forks.

Most of these stem from d25f519107 "tableam: relation creation, VACUUM
FULL/CLUSTER, SET TABLESPACE.".

1) To pass data to the relation_set_new_filenode()
   RelationSetNewRelfilenode() was made to update RelationData.rd_rel
   directly. That's not OK however, as it makes the relcache entries
   temporarily inconsistent. Which among other scenarios is a problem
   if a REINDEX targets an index on pg_class - the
   CatalogTupleUpdate() in RelationSetNewRelfilenode().  Presumably
   that was introduced because other places in the code do so - while
   those aren't "good practice" they don't appear to be actively
   buggy (e.g. because system tables may not be targeted).

   I (Andres) should have caught this while reviewing and signficantly
   evolving the code in that commit, mea culpa.

   Fix that by instead passing in the new RelFileNode as separate
   argument to relation_set_new_filenode() and rely on the relcache to
   update the catalog entry. Also revert that the
   RelationMapUpdateMap() call was changed to immediate, and undo some
   other more unnecessary changes.

2) Document that the relation_set_new_filenode cannot rely on the
   whole relcache entry to be valid. It might be worthwhile to
   refactor the code to never have to rely on that, but given the way
   heap_create() is currently coded, that'd be a large change.

3) ATExecSetTableSpace() shouldn't do FlushRelationBuffers() itself. A
   table AM might not use shared buffers at all. Move to
   index_copy_data() and heapam_relation_copy_data().

4) heapam_relation_set_new_filenode() previously sometimes accessed
   rel->rd_rel->relpersistence rather than the `persistence`
   argument. Code movement mistake.

5) Previously heapam_relation_set_new_filenode() re-opened the smgr
   relation to create the init for, if necesary. Instead have
   RelationCreateStorage() return the SMgrRelation and use it to
   create the init fork.

6) Add a note about the danger of modifying the relcache directly to
   ATExecSetTableSpace() - it's currently not a bug because there's a
   check ERRORing for catalog tables.

Regression tests and assertion improvements that together trigger the
bug described in 1) will be added in a later commit, as there is a
related bug on all branches.

Reported-By: Michael Paquier
Diagnosed-By: Tom Lane and Andres Freund
Author: Andres Freund
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/20190418011430.GA19133@paquier.xyz

src/backend/catalog/storage.c

@@ -75,7 +75,7 @@ static PendingRelDelete *pendingDeletes = NULL; /* head of linked list */
  * This function is transactional. The creation is WAL-logged, and if the
  * transaction aborts later on, the storage will be destroyed.
  */
-void
+SMgrRelation
 RelationCreateStorage(RelFileNode rnode, char relpersistence)
 {
 	PendingRelDelete *pending;
@@ -99,7 +99,7 @@ RelationCreateStorage(RelFileNode rnode, char relpersistence)
 			break;
 		default:
 			elog(ERROR, "invalid relpersistence: %c", relpersistence);
-			return;				/* placate compiler */
+			return NULL;				/* placate compiler */
 	}
 
 	srel = smgropen(rnode, backend);
@@ -117,13 +117,15 @@ RelationCreateStorage(RelFileNode rnode, char relpersistence)
 	pending->nestLevel = GetCurrentTransactionNestLevel();
 	pending->next = pendingDeletes;
 	pendingDeletes = pending;
+
+	return srel;
 }
 
 /*
  * Perform XLogInsert of an XLOG_SMGR_CREATE record to WAL.
  */
 void
-log_smgrcreate(RelFileNode *rnode, ForkNumber forkNum)
+log_smgrcreate(const RelFileNode *rnode, ForkNumber forkNum)
 {
 	xl_smgr_create xlrec;
 
@@ -294,6 +296,10 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
 
 /*
  * Copy a fork's data, block by block.
+ *
+ * Note that this requires that there is no dirty data in shared buffers. If
+ * it's possible that there are, callers need to flush those using
+ * e.g. FlushRelationBuffers(rel).
  */
 void
 RelationCopyStorage(SMgrRelation src, SMgrRelation dst,