fix: guard against NULL scanTupleSlot in apply_update_list (#2537)

regress/expected/cypher_merge.out

@@ -2237,6 +2237,55 @@ SELECT * FROM cypher('merge_actions', $$ MATCH (n) DETACH DELETE n $$) AS (a agt
 ---
 (0 rows)
 
+--
+-- Regression test for #2537: DELETE edge + full-path MERGE segfault
+-- (apply_update_list scanTupleSlot NULL deref)
+--
+SELECT * FROM create_graph('issue_2537');
+NOTICE:  graph "issue_2537" has been created
+ create_graph 
+--------------
+
+(1 row)
+
+SELECT * FROM cypher('issue_2537', $$
+CREATE (a:base {id: 1})-[r1:r {k0: false}]->
+       (n:base {id: 2})-[r0:s {k5: 'x'}]->(b:base {id: 3})
+$$) AS (created agtype);
+ created 
+---------
+(0 rows)
+
+SELECT * FROM cypher('issue_2537', $$
+MATCH (a:base)-[r1:r]->(n:base)-[r0:s]->(b:base)
+SET r0.k5 = 'J', r1.k0 = true
+WITH range(0, 0) + [0] AS keep, r1 AS old
+WHERE 0 IN keep
+DELETE old
+CREATE (:x)
+MERGE p0 = ({id: 129})-[r2:r]->(n2 {id: 130})-[:s]->(:c {id: 131})
+RETURN 1
+$$) AS (value agtype);
+ value 
+-------
+ 1
+(1 row)
+
+SELECT * FROM drop_graph('issue_2537', true);
+NOTICE:  drop cascades to 7 other objects
+DETAIL:  drop cascades to table issue_2537._ag_label_vertex
+drop cascades to table issue_2537._ag_label_edge
+drop cascades to table issue_2537.base
+drop cascades to table issue_2537.r
+drop cascades to table issue_2537.s
+drop cascades to table issue_2537.x
+drop cascades to table issue_2537.c
+NOTICE:  graph "issue_2537" has been dropped
+ drop_graph 
+------------
+
+(1 row)
+
 --
 -- delete graphs
 --

regress/sql/cypher_merge.sql

@@ -1088,6 +1088,30 @@ $$) AS (src agtype, last agtype);
 -- cleanup
 SELECT * FROM cypher('merge_actions', $$ MATCH (n) DETACH DELETE n $$) AS (a agtype);
 
+--
+-- Regression test for #2537: DELETE edge + full-path MERGE segfault
+-- (apply_update_list scanTupleSlot NULL deref)
+--
+SELECT * FROM create_graph('issue_2537');
+
+SELECT * FROM cypher('issue_2537', $$
+CREATE (a:base {id: 1})-[r1:r {k0: false}]->
+       (n:base {id: 2})-[r0:s {k5: 'x'}]->(b:base {id: 3})
+$$) AS (created agtype);
+
+SELECT * FROM cypher('issue_2537', $$
+MATCH (a:base)-[r1:r]->(n:base)-[r0:s]->(b:base)
+SET r0.k5 = 'J', r1.k0 = true
+WITH range(0, 0) + [0] AS keep, r1 AS old
+WHERE 0 IN keep
+DELETE old
+CREATE (:x)
+MERGE p0 = ({id: 129})-[r2:r]->(n2 {id: 130})-[:s]->(:c {id: 131})
+RETURN 1
+$$) AS (value agtype);
+
+SELECT * FROM drop_graph('issue_2537', true);
+
 --
 -- delete graphs
 --

src/backend/executor/cypher_set.c

@@ -416,6 +416,10 @@ void apply_update_list(CustomScanState *node,
     HTAB *index_cache = NULL;
     HASHCTL idx_hashctl;
 
+    /* if scanTupleSlot is NULL, there is no data to update */
+    if (scanTupleSlot == NULL)
+        return;
+
     /* allocate an array to hold the last update index of each 'entity' */
     luindex = palloc0(sizeof(int) * scanTupleSlot->tts_nvalid);