Do not return NULL for error cases in satisfies_hash_partition().

Since this function is used as a CHECK constraint condition,
returning NULL is tantamount to returning TRUE, which would have the
effect of letting in a row that doesn't satisfy the hash condition.
Admittedly, the cases for which this is done should be unreachable
in practice, but that doesn't make it any less a bad idea.  It also
seems like a dartboard was used to decide which error cases should
throw errors as opposed to returning NULL.

For the checks for NULL input values, I just switched it to returning
false.  There's some argument that an error would be better; but the
case really should be can't-happen in a generated hash constraint,
so it's likely not worth more code for.

For the parent-relation-open-failure case, it seems like we might
as well let relation_open throw an error, instead of having an
impossible-to-diagnose constraint failure.

Back-patch to v11 where this code came in.

Discussion: https://postgr.es/m/24067.1605134819@sss.pgh.pa.us

src/backend/partitioning/partbounds.c

@@ -4653,6 +4653,8 @@ compute_partition_hash_value(int partnatts, FmgrInfo *partsupfunc, Oid *partcoll
  *
  * Returns true if remainder produced when this computed single hash value is
  * divided by the given modulus is equal to given remainder, otherwise false.
+ * NB: it's important that this never return null, as the constraint machinery
+ * would consider that to be a "pass".
  *
  * See get_qual_for_hash() for usage.
  */
@@ -4677,9 +4679,9 @@ satisfies_hash_partition(PG_FUNCTION_ARGS)
 	ColumnsHashData *my_extra;
 	uint64		rowHash = 0;
 
-	/* Return null if the parent OID, modulus, or remainder is NULL. */
+	/* Return false if the parent OID, modulus, or remainder is NULL. */
 	if (PG_ARGISNULL(0) || PG_ARGISNULL(1) || PG_ARGISNULL(2))
-		PG_RETURN_NULL();
+		PG_RETURN_BOOL(false);
 	parentId = PG_GETARG_OID(0);
 	modulus = PG_GETARG_INT32(1);
 	remainder = PG_GETARG_INT32(2);
@@ -4709,14 +4711,11 @@ satisfies_hash_partition(PG_FUNCTION_ARGS)
 		int			j;
 
 		/* Open parent relation and fetch partition key info */
-		parent = try_relation_open(parentId, AccessShareLock);
+		parent = relation_open(parentId, AccessShareLock);
-		if (parent == NULL)
-			PG_RETURN_NULL();
 		key = RelationGetPartitionKey(parent);
 
 		/* Reject parent table that is not hash-partitioned. */
-		if (parent->rd_rel->relkind != RELKIND_PARTITIONED_TABLE ||
+		if (key == NULL || key->strategy != PARTITION_STRATEGY_HASH)
-			key->strategy != PARTITION_STRATEGY_HASH)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("\"%s\" is not a hash partitioned table",

src/test/regress/expected/hash_part.out

@@ -10,11 +10,7 @@ CREATE TABLE mchash1
   PARTITION OF mchash FOR VALUES WITH (MODULUS 4, REMAINDER 0);
 -- invalid OID, no such table
 SELECT satisfies_hash_partition(0, 4, 0, NULL);
- satisfies_hash_partition 
+ERROR:  could not open relation with OID 0
---------------------------
- 
-(1 row)
-
 -- not partitioned
 SELECT satisfies_hash_partition('tenk1'::regclass, 4, 0, NULL);
 ERROR:  "tenk1" is not a hash partitioned table
@@ -34,14 +30,14 @@ ERROR:  remainder for hash partition must be less than modulus
 SELECT satisfies_hash_partition('mchash'::regclass, NULL, 0, NULL);
  satisfies_hash_partition 
 --------------------------
- 
+ f
 (1 row)
 
 -- remainder is null
 SELECT satisfies_hash_partition('mchash'::regclass, 4, NULL, NULL);
  satisfies_hash_partition 
 --------------------------
- 
+ f
 (1 row)
 
 -- too many arguments