Fix Logical Replication of Truncate in synchronous commit mode.

Amit Kapila · Amit Kapila · commit e7eea52b2d61 · 2021-04-27T08:28:26.000+05:30
The Truncate operation acquires an exclusive lock on the target relation and indexes. It then waits for logical replication of the operation to finish at commit. Now because we are acquiring the shared lock on the target index to get index attributes in pgoutput while sending the changes for the Truncate operation, it leads to a deadlock. Actually, we don't need to acquire a lock on the target index as we build the cache entry using a historic snapshot and all the later changes are absorbed while decoding WAL. So, we wrote a special purpose function for logical replication to get a bitmap of replica identity attribute numbers where we get that information without locking the target index. We decided not to backpatch this as there doesn't seem to be any field complaint about this issue since it was introduced in commit 5dfd1e5 in v11. Reported-by: Haiying Tang Author: Takamichi Osumi, test case by Li Japin Reviewed-by: Amit Kapila, Ajin Cherian Discussion: https://postgr.es/m/OS0PR01MB6113C2499C7DC70EE55ADB82FB759@OS0PR01MB6113.jpnprd01.prod.outlook.com
diff --git a/src/backend/replication/logical/proto.c b/src/backend/replication/logical/proto.c
@@ -668,8 +668,7 @@ logicalrep_write_attrs(StringInfo out, Relation rel)
 	/* fetch bitmap of REPLICATION IDENTITY attributes */
 	replidentfull = (rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL);
 	if (!replidentfull)
-		idattrs = RelationGetIndexAttrBitmap(rel,
-											 INDEX_ATTR_BITMAP_IDENTITY_KEY);
+		idattrs = RelationGetIdentityKeyBitmap(rel);
 
 	/* send the attributes */
 	for (i = 0; i < desc->natts; i++)
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
@@ -5206,6 +5206,81 @@ RelationGetIndexAttrBitmap(Relation relation, IndexAttrBitmapKind attrKind)
 	}
 }
 
+/*
+ * RelationGetIdentityKeyBitmap -- get a bitmap of replica identity attribute
+ * numbers
+ *
+ * A bitmap of index attribute numbers for the configured replica identity
+ * index is returned.
+ *
+ * See also comments of RelationGetIndexAttrBitmap().
+ *
+ * This is a special purpose function used during logical replication. Here,
+ * unlike RelationGetIndexAttrBitmap(), we don't acquire a lock on the required
+ * index as we build the cache entry using a historic snapshot and all the
+ * later changes are absorbed while decoding WAL. Due to this reason, we don't
+ * need to retry here in case of a change in the set of indexes.
+ */
+Bitmapset *
+RelationGetIdentityKeyBitmap(Relation relation)
+{
+	Bitmapset  *idindexattrs = NULL;	/* columns in the replica identity */
+	List	   *indexoidlist;
+	Relation	indexDesc;
+	int			i;
+	MemoryContext oldcxt;
+
+	/* Quick exit if we already computed the result */
+	if (relation->rd_idattr != NULL)
+		return bms_copy(relation->rd_idattr);
+
+	/* Fast path if definitely no indexes */
+	if (!RelationGetForm(relation)->relhasindex)
+		return NULL;
+
+	/* Historic snapshot must be set. */
+	Assert(HistoricSnapshotActive());
+
+	indexoidlist = RelationGetIndexList(relation);
+
+	/* Fall out if no indexes (but relhasindex was set) */
+	if (indexoidlist == NIL)
+		return NULL;
+
+	/* Add referenced attributes to idindexattrs */
+	indexDesc = RelationIdGetRelation(relation->rd_replidindex);
+	for (i = 0; i < indexDesc->rd_index->indnatts; i++)
+	{
+		int			attrnum = indexDesc->rd_index->indkey.values[i];
+
+		/*
+		 * We don't include non-key columns into idindexattrs bitmaps. See
+		 * RelationGetIndexAttrBitmap.
+		 */
+		if (attrnum != 0)
+		{
+			if (i < indexDesc->rd_index->indnkeyatts)
+				idindexattrs = bms_add_member(idindexattrs,
+											  attrnum - FirstLowInvalidHeapAttributeNumber);
+		}
+	}
+
+	RelationClose(indexDesc);
+	list_free(indexoidlist);
+
+	/* Don't leak the old values of these bitmaps, if any */
+	bms_free(relation->rd_idattr);
+	relation->rd_idattr = NULL;
+
+	/* Now save copy of the bitmap in the relcache entry */
+	oldcxt = MemoryContextSwitchTo(CacheMemoryContext);
+	relation->rd_idattr = bms_copy(idindexattrs);
+	MemoryContextSwitchTo(oldcxt);
+
+	/* We return our original working copy for caller to play with */
+	return idindexattrs;
+}
+
 /*
  * RelationGetExclusionInfo -- get info about index's exclusion constraint
  *
diff --git a/src/include/utils/relcache.h b/src/include/utils/relcache.h
@@ -65,6 +65,8 @@ typedef enum IndexAttrBitmapKind
 extern Bitmapset *RelationGetIndexAttrBitmap(Relation relation,
 											 IndexAttrBitmapKind attrKind);
 
+extern Bitmapset *RelationGetIdentityKeyBitmap(Relation relation);
+
 extern void RelationGetExclusionInfo(Relation indexRelation,
 									 Oid **operators,
 									 Oid **procs,
diff --git a/src/test/subscription/t/010_truncate.pl b/src/test/subscription/t/010_truncate.pl
@@ -3,7 +3,7 @@
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 9;
+use Test::More tests => 11;
 
 # setup
 
@@ -158,3 +158,28 @@
 $result = $node_subscriber->safe_psql('postgres',
 	"SELECT count(*), min(a), max(a) FROM tab2");
 is($result, qq(3|1|3), 'truncate of multiple tables some not published');
+
+# Test that truncate works for synchronous logical replication
+
+$node_publisher->safe_psql('postgres',
+	"ALTER SYSTEM SET synchronous_standby_names TO 'sub1'");
+$node_publisher->safe_psql('postgres', "SELECT pg_reload_conf()");
+
+# insert data to truncate
+
+$node_publisher->safe_psql('postgres',
+	"INSERT INTO tab1 VALUES (1), (2), (3)");
+
+$node_publisher->wait_for_catchup('sub1');
+
+$result = $node_subscriber->safe_psql('postgres',
+	"SELECT count(*), min(a), max(a) FROM tab1");
+is($result, qq(3|1|3), 'check synchronous logical replication');
+
+$node_publisher->safe_psql('postgres', "TRUNCATE tab1");
+
+$node_publisher->wait_for_catchup('sub1');
+
+$result = $node_subscriber->safe_psql('postgres',
+	"SELECT count(*), min(a), max(a) FROM tab1");
+is($result, qq(0||), 'truncate replicated in synchronous logical replication');
