Assert lack of hazardous buffer locks before possible catalog read.

Commit 0bada39c83 fixed a bug of this kind,
which existed in all branches for six days before detection.  While the
probability of reaching the trouble was low, the disruption was extreme.  No
new backends could start, and service restoration needed an immediate
shutdown.  Hence, add this to catch the next bug like it.

The new check in RelationIdGetRelation() suffices to make autovacuum detect
the bug in commit 243e9b40f1 that led to commit
0bada39.  This also checks in a number of similar places.  It replaces each
Assert(IsTransactionState()) that pertained to a conditional catalog read.

No back-patch for now, but a back-patch of commit 243e9b4 should back-patch
this, too.  A back-patch could omit the src/test/regress changes, since back
branches won't gain new index columns.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/20250410191830.0e.nmisch@google.com
Discussion: https://postgr.es/m/10ec0bc3-5933-1189-6bb8-5dec4114558e@gmail.com

src/backend/utils/cache/relcache.c

@@ -2056,6 +2056,23 @@ formrdesc(const char *relationName, Oid relationReltype,
 	relation->rd_isvalid = true;
 }
 
+#ifdef USE_ASSERT_CHECKING
+/*
+ *		AssertCouldGetRelation
+ *
+ *		Check safety of calling RelationIdGetRelation().
+ *
+ *		In code that reads catalogs in the event of a cache miss, call this
+ *		before checking the cache.
+ */
+void
+AssertCouldGetRelation(void)
+{
+	Assert(IsTransactionState());
+	AssertBufferLocksPermitCatalogRead();
+}
+#endif
+
 
 /* ----------------------------------------------------------------
  *				 Relation Descriptor Lookup Interface
@@ -2083,8 +2100,7 @@ RelationIdGetRelation(Oid relationId)
 {
 	Relation	rd;
 
-	/* Make sure we're in an xact, even if this ends up being a cache hit */
-	Assert(IsTransactionState());
+	AssertCouldGetRelation();
 
 	/*
 	 * first try to find reldesc in the cache
@@ -2373,8 +2389,7 @@ RelationReloadNailed(Relation relation)
 	Assert(relation->rd_isnailed);
 	/* nailed indexes are handled by RelationReloadIndexInfo() */
 	Assert(relation->rd_rel->relkind == RELKIND_RELATION);
-	/* can only reread catalog contents in a transaction */
-	Assert(IsTransactionState());
+	AssertCouldGetRelation();
 
 	/*
 	 * Redo RelationInitPhysicalAddr in case it is a mapped relation whose
@@ -2570,8 +2585,7 @@ static void
 RelationRebuildRelation(Relation relation)
 {
 	Assert(!RelationHasReferenceCountZero(relation));
-	/* rebuilding requires access to the catalogs */
-	Assert(IsTransactionState());
+	AssertCouldGetRelation();
 	/* there is no reason to ever rebuild a dropped relation */
 	Assert(relation->rd_droppedSubid == InvalidSubTransactionId);