pg_dump: Fix gathering of sequence information.

Since commit bd15b7d, pg_dump uses pg_get_sequence_data() (née
pg_sequence_read_tuple()) to gather all sequence data in a single
query as opposed to a query per sequence.  Two related bugs have
been identified:

* If the user lacks appropriate privileges on the sequence, pg_dump
generates a setval() command with garbage values instead of
failing as expected.

* pg_dump can fail due to a concurrently dropped sequence, even if
the dropped sequence's data isn't part of the dump.

This commit fixes the above issues by 1) teaching
pg_get_sequence_data() to return nulls instead of erroring for a
missing sequence and 2) teaching pg_dump to fail if it tries to
dump the data of a sequence for which pg_get_sequence_data()
returned nulls.  Note that pg_dump may still fail due to a
concurrently dropped sequence, but it should now only do so when
the sequence data is part of the dump.  This matches the behavior
before commit bd15b7d.

Bug: #19365
Reported-by: Paveł Tyślacki <[email protected]>
Suggested-by: Tom Lane <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/19365-6245240d8b926327%40postgresql.org
Discussion: https://postgr.es/m/2885944.1767029161%40sss.pgh.pa.us
Backpatch-through: 18

Author: nathan-bossart

src/backend/commands/sequence.c

@@ -1794,7 +1794,6 @@ pg_get_sequence_data(PG_FUNCTION_ARGS)
 {
 #define PG_GET_SEQUENCE_DATA_COLS	3
 	Oid			relid = PG_GETARG_OID(0);
-	SeqTable	elm;
 	Relation	seqrel;
 	Datum		values[PG_GET_SEQUENCE_DATA_COLS] = {0};
 	bool		isnull[PG_GET_SEQUENCE_DATA_COLS] = {0};
@@ -1811,13 +1810,15 @@ pg_get_sequence_data(PG_FUNCTION_ARGS)
 					   LSNOID, -1, 0);
 	resultTupleDesc = BlessTupleDesc(resultTupleDesc);
 
-	init_sequence(relid, &elm, &seqrel);
+	seqrel = try_relation_open(relid, AccessShareLock);
 
 	/*
-	 * Return all NULLs for sequences for which we lack privileges, other
-	 * sessions' temporary sequences, and unlogged sequences on standbys.
+	 * Return all NULLs for missing sequences, sequences for which we lack
+	 * privileges, other sessions' temporary sequences, and unlogged sequences
+	 * on standbys.
 	 */
-	if (pg_class_aclcheck(relid, GetUserId(), ACL_SELECT) == ACLCHECK_OK &&
+	if (seqrel && seqrel->rd_rel->relkind == RELKIND_SEQUENCE &&
+		pg_class_aclcheck(relid, GetUserId(), ACL_SELECT) == ACLCHECK_OK &&
 		!RELATION_IS_OTHER_TEMP(seqrel) &&
 		(RelationIsPermanent(seqrel) || !RecoveryInProgress()))
 	{
@@ -1838,7 +1839,8 @@ pg_get_sequence_data(PG_FUNCTION_ARGS)
 	else
 		memset(isnull, true, sizeof(isnull));
 
-	sequence_close(seqrel, NoLock);
+	if (seqrel)
+		relation_close(seqrel, AccessShareLock);
 
 	resultHeapTuple = heap_form_tuple(resultTupleDesc, values, isnull);
 	result = HeapTupleGetDatum(resultHeapTuple);

src/bin/pg_dump/pg_dump.c

@@ -137,6 +137,7 @@ typedef struct
 	int64		cache;			/* cache size */
 	int64		last_value;		/* last value of sequence */
 	bool		is_called;		/* whether nextval advances before returning */
+	bool		null_seqtuple;	/* did pg_get_sequence_data return nulls? */
 } SequenceItem;
 
 typedef enum OidOptions
@@ -18959,6 +18960,7 @@ collectSequences(Archive *fout)
 		sequences[i].cycled = (strcmp(PQgetvalue(res, i, 7), "t") == 0);
 		sequences[i].last_value = strtoi64(PQgetvalue(res, i, 8), NULL, 10);
 		sequences[i].is_called = (strcmp(PQgetvalue(res, i, 9), "t") == 0);
+		sequences[i].null_seqtuple = (PQgetisnull(res, i, 8) || PQgetisnull(res, i, 9));
 	}
 
 	PQclear(res);
@@ -19230,6 +19232,10 @@ dumpSequenceData(Archive *fout, const TableDataInfo *tdinfo)
 	bool		called;
 	PQExpBuffer query = createPQExpBuffer();
 
+	/* needn't bother if not dumping sequence data */
+	if (!fout->dopt->dumpData && !fout->dopt->sequence_data)
+		return;
+
 	/*
 	 * For versions >= 18, the sequence information is gathered in the sorted
 	 * array before any calls to dumpSequenceData().  See collectSequences()
@@ -19271,6 +19277,12 @@ dumpSequenceData(Archive *fout, const TableDataInfo *tdinfo)
 		entry = bsearch(&key, sequences, nsequences,
 						sizeof(SequenceItem), SequenceItemCmp);
 
+		if (entry->null_seqtuple)
+			pg_fatal("failed to get data for sequence \"%s\"; user may lack "
+					 "SELECT privilege on the sequence or the sequence may "
+					 "have been concurrently dropped",
+					 tbinfo->dobj.name);
+
 		last = entry->last_value;
 		called = entry->is_called;
 	}