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

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
index 76ed2fbea8bdb5aed0ef56ee2236d093a711eee2..582f6f66c63366ea98d1f5219807bc288acd88e9 100644 (file)
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -6786,8 +6786,9 @@ 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>
@@ -6869,7 +6870,9 @@ 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
index afee2838cc24e8ab1808fae625ef31c550b512ea..a3a8b4e599625c5dfab2a7e53def34e2d880b59d 100644 (file)
--- 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
index f7df814a1818931d0d3ab6c3d72cdfa618f1f658..a1389ae46d426bccd5d36f664f274022f5ca08ba 100644 (file)
--- 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)
    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;