Don't overwrite scan key in systable_beginscan()

When systable_beginscan() and systable_beginscan_ordered() choose an
index scan, they remap the attribute numbers in the passed-in scan
keys to the attribute numbers of the index, and then write those
remapped attribute numbers back into the scan key passed by the
caller.  This second part is surprising and gratuitous.  It means that
a scan key cannot safely be used more than once (but it might
sometimes work, depending on circumstances).  Also, there is no value
in providing these remapped attribute numbers back to the caller,
since they can't do anything with that.

Fix that by making a copy of the scan keys passed by the caller and
make the modifications there.

Also, some code that had to work around the previous situation is
simplified.

Discussion: https://www.postgresql.org/message-id/flat/f8c739d9-f48d-4187-b214-df3391ba41ab@eisentraut.org

src/backend/access/index/genam.c

@@ -372,7 +372,7 @@ index_compute_xid_horizon_for_tuples(Relation irel,
  *	nkeys, key: scan keys
  *
  * The attribute numbers in the scan key should be set for the heap case.
- * If we choose to index, we reset them to 1..n to reference the index
+ * If we choose to index, we convert them to 1..n to reference the index
  * columns.  Note this means there must be one scankey qualification per
  * index column!  This is checked by the Asserts in the normal, index-using
  * case, but won't be checked if the heapscan path is taken.
@@ -420,17 +420,22 @@ systable_beginscan(Relation heapRelation,
 	if (irel)
 	{
 		int			i;
+		ScanKey		idxkey;
 
-		/* Change attribute numbers to be index column numbers. */
+		idxkey = palloc_array(ScanKeyData, nkeys);
+
+		/* Convert attribute numbers to be index column numbers. */
 		for (i = 0; i < nkeys; i++)
 		{
 			int			j;
 
+			memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
+
 			for (j = 0; j < IndexRelationGetNumberOfAttributes(irel); j++)
 			{
 				if (key[i].sk_attno == irel->rd_index->indkey.values[j])
 				{
-					key[i].sk_attno = j + 1;
+					idxkey[i].sk_attno = j + 1;
 					break;
 				}
 			}
@@ -440,7 +445,7 @@ systable_beginscan(Relation heapRelation,
 
 		sysscan->iscan = index_beginscan(heapRelation, irel,
 										 snapshot, nkeys, 0);
-		index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
+		index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
 		sysscan->scan = NULL;
 	}
 	else
@@ -648,6 +653,7 @@ systable_beginscan_ordered(Relation heapRelation,
 {
 	SysScanDesc sysscan;
 	int			i;
+	ScanKey		idxkey;
 
 	/* REINDEX can probably be a hard error here ... */
 	if (ReindexIsProcessingIndex(RelationGetRelid(indexRelation)))
@@ -679,16 +685,20 @@ systable_beginscan_ordered(Relation heapRelation,
 		sysscan->snapshot = NULL;
 	}
 
-	/* Change attribute numbers to be index column numbers. */
+	idxkey = palloc_array(ScanKeyData, nkeys);
+
+	/* Convert attribute numbers to be index column numbers. */
 	for (i = 0; i < nkeys; i++)
 	{
 		int			j;
 
+		memcpy(&idxkey[i], &key[i], sizeof(ScanKeyData));
+
 		for (j = 0; j < IndexRelationGetNumberOfAttributes(indexRelation); j++)
 		{
 			if (key[i].sk_attno == indexRelation->rd_index->indkey.values[j])
 			{
-				key[i].sk_attno = j + 1;
+				idxkey[i].sk_attno = j + 1;
 				break;
 			}
 		}
@@ -698,7 +708,7 @@ systable_beginscan_ordered(Relation heapRelation,
 
 	sysscan->iscan = index_beginscan(heapRelation, indexRelation,
 									 snapshot, nkeys, 0);
-	index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
+	index_rescan(sysscan->iscan, idxkey, nkeys, NULL, 0);
 	sysscan->scan = NULL;
 
 	return sysscan;

src/backend/utils/cache/catcache.c

@@ -1468,19 +1468,18 @@ SearchCatCacheMiss(CatCache *cache,
 	 */
 	relation = table_open(cache->cc_reloid, AccessShareLock);
 
+	/*
+	 * Ok, need to make a lookup in the relation, copy the scankey and fill
+	 * out any per-call fields.
+	 */
+	memcpy(cur_skey, cache->cc_skey, sizeof(ScanKeyData) * nkeys);
+	cur_skey[0].sk_argument = v1;
+	cur_skey[1].sk_argument = v2;
+	cur_skey[2].sk_argument = v3;
+	cur_skey[3].sk_argument = v4;
+
 	do
 	{
-		/*
-		 * Ok, need to make a lookup in the relation, copy the scankey and
-		 * fill out any per-call fields.  (We must re-do this when retrying,
-		 * because systable_beginscan scribbles on the scankey.)
-		 */
-		memcpy(cur_skey, cache->cc_skey, sizeof(ScanKeyData) * nkeys);
-		cur_skey[0].sk_argument = v1;
-		cur_skey[1].sk_argument = v2;
-		cur_skey[2].sk_argument = v3;
-		cur_skey[3].sk_argument = v4;
-
 		scandesc = systable_beginscan(relation,
 									  cache->cc_indexoid,
 									  IndexScanOK(cache),
@@ -1788,19 +1787,18 @@ SearchCatCacheList(CatCache *cache,
 
 		relation = table_open(cache->cc_reloid, AccessShareLock);
 
+		/*
+		 * Ok, need to make a lookup in the relation, copy the scankey and
+		 * fill out any per-call fields.
+		 */
+		memcpy(cur_skey, cache->cc_skey, sizeof(ScanKeyData) * cache->cc_nkeys);
+		cur_skey[0].sk_argument = v1;
+		cur_skey[1].sk_argument = v2;
+		cur_skey[2].sk_argument = v3;
+		cur_skey[3].sk_argument = v4;
+
 		do
 		{
-			/*
-			 * Ok, need to make a lookup in the relation, copy the scankey and
-			 * fill out any per-call fields.  (We must re-do this when
-			 * retrying, because systable_beginscan scribbles on the scankey.)
-			 */
-			memcpy(cur_skey, cache->cc_skey, sizeof(ScanKeyData) * cache->cc_nkeys);
-			cur_skey[0].sk_argument = v1;
-			cur_skey[1].sk_argument = v2;
-			cur_skey[2].sk_argument = v3;
-			cur_skey[3].sk_argument = v4;
-
 			scandesc = systable_beginscan(relation,
 										  cache->cc_indexoid,
 										  IndexScanOK(cache),

src/backend/utils/cache/relfilenumbermap.c

@@ -141,7 +141,6 @@ RelidByRelfilenumber(Oid reltablespace, RelFileNumber relfilenumber)
 	SysScanDesc scandesc;
 	Relation	relation;
 	HeapTuple	ntp;
-	ScanKeyData skey[2];
 	Oid			relid;
 
 	if (RelfilenumberMapHash == NULL)
@@ -181,6 +180,8 @@ RelidByRelfilenumber(Oid reltablespace, RelFileNumber relfilenumber)
 	}
 	else
 	{
+		ScanKeyData skey[2];
+
 		/*
 		 * Not a shared table, could either be a plain relation or a
 		 * non-shared, nailed one, like e.g. pg_class.
@@ -189,10 +190,8 @@ RelidByRelfilenumber(Oid reltablespace, RelFileNumber relfilenumber)
 		/* check for plain relations by looking in pg_class */
 		relation = table_open(RelationRelationId, AccessShareLock);
 
-		/* copy scankey to local copy, it will be modified during the scan */
+		/* copy scankey to local copy and set scan arguments */
 		memcpy(skey, relfilenumber_skey, sizeof(skey));
-
-		/* set scan arguments */
 		skey[0].sk_argument = ObjectIdGetDatum(reltablespace);
 		skey[1].sk_argument = ObjectIdGetDatum(relfilenumber);