Avoid direct C access to possibly-null pg_subscription_rel.srsublsn.

tglsfdc · tglsfdc · commit 99b0c5da3e10 · 2020-07-21T11:40:47.000-04:00
This coding technique is unsafe, since we'd be accessing off the end of the tuple if the field is null. SIGSEGV is pretty improbable, but perhaps not impossible. Also, returning garbage for the LSN doesn't seem like a great idea, even if callers aren't looking at it today. Also update docs to point out explicitly that pg_subscription.subslotname and pg_subscription_rel.srsublsn can be null. Perhaps we should mark these two fields BKI_FORCE_NULL, so that they'd be correctly labeled in databases that are initdb'd in the future. But we can't force that for existing databases, and on balance it's not too clear that having a mix of different catalog contents in the field would be wise. Apply to v10 (where this code came in) through v12. Already fixed in v13 and HEAD. Discussion: https://postgr.es/m/732838.1595278439@sss.pgh.pa.us
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
@@ -6695,8 +6695,9 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <entry><structfield>subslotname</structfield></entry>
       <entry><type>name</type></entry>
       <entry></entry>
-      <entry>Name of the replication slot in the upstream database. Also used
-       for local replication origin name.</entry>
+      <entry>Name of the replication slot in the upstream database (also used
+       for the local replication origin name);
+       null represents <literal>NONE</literal></entry>
      </row>
 
      <row>
@@ -6778,7 +6779,9 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <entry><type>pg_lsn</type></entry>
       <entry></entry>
       <entry>
-       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
       </entry>
      </row>
     </tbody>
diff --git a/src/backend/catalog/pg_subscription.c b/src/backend/catalog/pg_subscription.c
@@ -457,13 +457,20 @@ 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);
 	}
@@ -509,13 +516,20 @@ 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);
 	}
diff --git a/src/include/catalog/pg_subscription_rel.h b/src/include/catalog/pg_subscription_rel.h
@@ -34,8 +34,17 @@ CATALOG(pg_subscription_rel,6102,SubscriptionRelRelationId) BKI_WITHOUT_OIDS
 	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;		/* 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;
