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

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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
This commit is contained in:
Tom Lane 2020-07-21 11:40:46 -04:00
parent 798b4faefd
commit b7103bbe34
3 changed files with 33 additions and 7 deletions
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9
doc/src/sgml/catalogs.sgml
View File

@ -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>

18
src/backend/catalog/pg_subscription.c
View File

@ -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);
}

13
src/include/catalog/pg_subscription_rel.h
View File

@ -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;