[FIX] point_of_sale: fix memory error

When there are too many (millions) of POS order lines associated to
opened POS sessions we get too many taxes, most are duplicated. This
causes a MemorryError.

Example queries from a real DB:
```
> select count(distinct r.account_tax_id) from pos_order_line l join pos_order o on o.id = l.order_id join pos_session s on s.id = o.session_id join account_tax_pos_order_line_rel r on r.pos_order_
 line_id = l.id where s.state != 'closed'
+-------+
| count |
|-------|
| 24    |
+-------+
> select count(r.account_tax_id) from pos_order_line l join pos_order o on o.id = l.order_id join pos_session s on s.id = o.session_id join account_tax_pos_order_line_rel r on r.pos_order_line_id =
  l.id where s.state != 'closed'
+---------+
| count   |
|---------|
| 2504539 |
+---------+
```

opw-3295467

closes odoo/odoo#124180

X-original-commit: 45d19b265033aa626e110e51d8b1d01408717a3e
Signed-off-by: Christophe Simonis (chs) <chs@odoo.com>

addons/point_of_sale/models/account_tax.py

@@ -2,27 +2,29 @@
 
 from odoo import _, api, models
 from odoo.exceptions import UserError
+from odoo.tools import split_every
 
 
 class AccountTax(models.Model):
     _inherit = 'account.tax'
 
     def write(self, vals):
-        forbidden_fields = set([
+        forbidden_fields = {
             'amount_type', 'amount', 'type_tax_use', 'tax_group_id', 'price_include',
             'include_base_amount'
-        ])
+        }
         if forbidden_fields & set(vals.keys()):
-            tax_ids = self.env['pos.order.line'].sudo().search([
+            lines = self.env['pos.order.line'].sudo().search([
                 ('order_id.session_id.state', '!=', 'closed')
-            ]).read(['tax_ids'])
-            # Flatten the list of taxes, see https://stackoverflow.com/questions/952914
-            tax_ids = set([i for sl in [t['tax_ids'] for t in tax_ids] for i in sl])
-            if tax_ids & set(self.ids):
-                raise UserError(_(
-                    'It is forbidden to modify a tax used in a POS order not posted. '
-                    'You must close the POS sessions before modifying the tax.'
-                ))
+            ])
+            self_ids = set(self.ids)
+            for lines_chunk in map(self.env['pos.order.line'].browse, split_every(100000, lines.ids)):
+                if any(tid in self_ids for ts in lines_chunk.read(['tax_ids']) for tid in ts['tax_ids']):
+                    raise UserError(_(
+                        'It is forbidden to modify a tax used in a POS order not posted. '
+                        'You must close the POS sessions before modifying the tax.'
+                    ))
+                lines_chunk.invalidate_cache(['tax_ids'], lines_chunk.ids)
         return super(AccountTax, self).write(vals)
 
     def get_real_tax_amount(self):