fix(stock): serialize pick list allocation per item on postgres

Two simultaneous allocations for the same item can both claim the same stock
on postgres: the picked-items locking read cannot see the rows another
in-flight creation is inserting, while MariaDB's gap locks make the creations
take turns. Advisory-gate set_item_locations per item (sorted against
deadlocks) so the second allocation waits, then subtracts the first's claim.
MariaDB unchanged.

erpnext/stock/doctype/pick_list/pick_list.py

@@ -550,6 +550,15 @@ class PickList(TransactionBase):
 	def set_item_locations(self, save: bool = False):
 		self.validate_for_qty()
 		items = self.aggregate_item_qty()
+
+		# Serialize concurrent allocations per item on postgres. MariaDB's gap locks on the
+		# picked-items locking read below already make two simultaneous allocations take turns;
+		# postgres locking reads can't see the rows another in-flight allocation is inserting, so
+		# both could claim the same stock. Sorted so overlapping documents can't deadlock.
+		if frappe.db.db_type == "postgres" and hasattr(frappe.db, "transaction_advisory_lock"):
+			for item_code in sorted({d.item_code for d in items}):
+				frappe.db.transaction_advisory_lock(("pick-allocate", item_code))
+
 		picked_items_details = self.get_picked_items_details(items)
 		self.item_location_map = frappe._dict()
 

erpnext/stock/doctype/pick_list/test_pick_list.py

@@ -29,6 +29,23 @@ from erpnext.tests.utils import ERPNextTestSuite
 
 
 class TestPickList(ERPNextTestSuite):
+	def test_pick_list_allocation_takes_advisory_gate(self):
+		if frappe.db.db_type != "postgres":
+			return
+
+		item = make_item(properties={"is_stock_item": 1}).name
+		make_stock_entry(item=item, to_warehouse="_Test Warehouse - _TC", qty=5, basic_rate=100)
+		sales_order = make_sales_order(item_code=item, warehouse="_Test Warehouse - _TC", qty=2, rate=100)
+
+		def held_advisory_locks():
+			return frappe.db.sql(
+				"SELECT count(*) FROM pg_locks WHERE locktype = 'advisory' AND pid = pg_backend_pid()"
+			)[0][0]
+
+		before = held_advisory_locks()
+		create_pick_list(sales_order.name)
+		self.assertGreater(held_advisory_locks(), before)
+
 	def test_pick_list_picks_warehouse_for_each_item(self):
 		item_code = make_item().name
 		try: