Correctly mark pg_subscription_rel.srsublsn as nullable.

The code has always set this column to NULL when it's not valid,
but the catalog header's description failed to reflect that,
as did the SGML docs, as did some of the code.  To prevent future
coding errors of the same ilk, let's hide the field from C code
as though it were variable-length (which, in a sense, it is).

As with commit 72eab84a5, we can only fix this cleanly in HEAD
and v13; the problem extends further back but we'll need some
klugery in the released branches.

Discussion: https://postgr.es/m/367660.1595202498@sss.pgh.pa.us

doc/src/sgml/catalogs.sgml

@@ -7631,7 +7631,9 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        <structfield>srsublsn</structfield> <type>pg_lsn</type>
       </para>
       <para>
-       End LSN for <literal>s</literal> and <literal>r</literal> states.
+       Remote LSN of the state change used for synchronization coordination
+       when in <literal>s</literal> or <literal>r</literal> states,
+       otherwise null
       </para></entry>
      </row>
     </tbody>

src/backend/catalog/pg_subscription.c

@@ -452,13 +452,20 @@ GetSubscriptionRelations(Oid subid)
 	{
 		Form_pg_subscription_rel subrel;
 		SubscriptionRelState *relstate;
+		Datum		d;
+		bool		isnull;
 
 		subrel = (Form_pg_subscription_rel) GETSTRUCT(tup);
 
 		relstate = (SubscriptionRelState *) palloc(sizeof(SubscriptionRelState));
 		relstate->relid = subrel->srrelid;
 		relstate->state = subrel->srsubstate;
-		relstate->lsn = subrel->srsublsn;
+		d = SysCacheGetAttr(SUBSCRIPTIONRELMAP, tup,
+							Anum_pg_subscription_rel_srsublsn, &isnull);
+		if (isnull)
+			relstate->lsn = InvalidXLogRecPtr;
+		else
+			relstate->lsn = DatumGetLSN(d);
 
 		res = lappend(res, relstate);
 	}
@@ -504,13 +511,20 @@ GetSubscriptionNotReadyRelations(Oid subid)
 	{
 		Form_pg_subscription_rel subrel;
 		SubscriptionRelState *relstate;
+		Datum		d;
+		bool		isnull;
 
 		subrel = (Form_pg_subscription_rel) GETSTRUCT(tup);
 
 		relstate = (SubscriptionRelState *) palloc(sizeof(SubscriptionRelState));
 		relstate->relid = subrel->srrelid;
 		relstate->state = subrel->srsubstate;
-		relstate->lsn = subrel->srsublsn;
+		d = SysCacheGetAttr(SUBSCRIPTIONRELMAP, tup,
+							Anum_pg_subscription_rel_srsublsn, &isnull);
+		if (isnull)
+			relstate->lsn = InvalidXLogRecPtr;
+		else
+			relstate->lsn = DatumGetLSN(d);
 
 		res = lappend(res, relstate);
 	}

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202007192
+#define CATALOG_VERSION_NO	202007202
 
 #endif

src/include/catalog/pg_subscription_rel.h

@@ -33,8 +33,18 @@ CATALOG(pg_subscription_rel,6102,SubscriptionRelRelationId)
 	Oid			srsubid;		/* Oid of subscription */
 	Oid			srrelid;		/* Oid of relation */
 	char		srsubstate;		/* state of the relation in subscription */
-	XLogRecPtr	srsublsn;		/* remote lsn of the state change used for
-								 * synchronization coordination */
+
+	/*
+	 * Although srsublsn is a fixed-width type, it is allowed to be NULL, so
+	 * we prevent direct C code access to it just as for a varlena field.
+	 */
+#ifdef CATALOG_VARLEN			/* variable-length fields start here */
+
+	XLogRecPtr	srsublsn BKI_FORCE_NULL;	/* remote LSN of the state change
+											 * used for synchronization
+											 * coordination, or NULL if not
+											 * valid */
+#endif
 } FormData_pg_subscription_rel;
 
 typedef FormData_pg_subscription_rel *Form_pg_subscription_rel;