Make pg_get_expr() more bulletproof.

Since this function is defined to accept pg_node_tree values, it could
get applied to any nodetree that can appear in a cataloged pg_node_tree
column.  Some such cases can't be supported --- for example, its API
doesn't allow providing referents for more than one relation --- but
we should try to throw a user-facing error rather than an internal
error when encountering such a case.

In support of this, extend expression_tree_walker/mutator to be sure
they'll work on any such node tree (which basically means adding
support for relpartbound node types).  That allows us to run pull_varnos
and check for the case of multiple relations before we start processing
the tree.  The alternative of changing the low-level error thrown for an
out-of-range varno isn't appealing, because that could mask actual bugs
in other usages of ruleutils.

Per report from Justin Pryzby.  This is basically cosmetic, so no
back-patch.

Discussion: https://postgr.es/m/20211219205422.GT17618@telsasoft.com

1 files changed, 42 insertions, 0 deletions

diff --git a/src/backend/nodes/nodeFuncs.c b/src/backend/nodes/nodeFuncs.c
index 7d1a3aff8c..acc17da717 100644
--- a/src/backend/nodes/nodeFuncs.c
+++ b/src/backend/nodes/nodeFuncs.c
@@ -2201,6 +2201,26 @@ expression_tree_walker(Node *node,
 					return true;
 			}
 			break;
+		case T_PartitionBoundSpec:
+			{
+				PartitionBoundSpec *pbs = (PartitionBoundSpec *) node;
+
+				if (walker(pbs->listdatums, context))
+					return true;
+				if (walker(pbs->lowerdatums, context))
+					return true;
+				if (walker(pbs->upperdatums, context))
+					return true;
+			}
+			break;
+		case T_PartitionRangeDatum:
+			{
+				PartitionRangeDatum *prd = (PartitionRangeDatum *) node;
+
+				if (walker(prd->value, context))
+					return true;
+			}
+			break;
 		case T_List:
 			foreach(temp, (List *) node)
 			{
@@ -3092,6 +3112,28 @@ expression_tree_mutator(Node *node,
 				return (Node *) newnode;
 			}
 			break;
+		case T_PartitionBoundSpec:
+			{
+				PartitionBoundSpec *pbs = (PartitionBoundSpec *) node;
+				PartitionBoundSpec *newnode;
+
+				FLATCOPY(newnode, pbs, PartitionBoundSpec);
+				MUTATE(newnode->listdatums, pbs->listdatums, List *);
+				MUTATE(newnode->lowerdatums, pbs->lowerdatums, List *);
+				MUTATE(newnode->upperdatums, pbs->upperdatums, List *);
+				return (Node *) newnode;
+			}
+			break;
+		case T_PartitionRangeDatum:
+			{
+				PartitionRangeDatum *prd = (PartitionRangeDatum *) node;
+				PartitionRangeDatum *newnode;
+
+				FLATCOPY(newnode, prd, PartitionRangeDatum);
+				MUTATE(newnode->value, prd->value, Node *);
+				return (Node *) newnode;
+			}
+			break;
 		case T_List:
 			{
 				/*