[FIX] account: avoid losing product on invoice

Add restriction on product_id field to prevent the suppression of the product
if already present in an invoice.
This is to avoid the suppression of a used product variant when modifying
the list of attributes.#
Due to the constrain, the variant will be disabled instead of deleted.
Fixes #4129

Add warning message on the product form to warn users about the potential impact
of modifying variants.

addons/account/account_invoice.py

@@ -1261,7 +1261,7 @@ class account_invoice_line(models.Model):
     uos_id = fields.Many2one('product.uom', string='Unit of Measure',
         ondelete='set null', index=True)
     product_id = fields.Many2one('product.product', string='Product',
-        ondelete='set null', index=True)
+        ondelete='restrict', index=True)
     account_id = fields.Many2one('account.account', string='Account',
         required=True, domain=[('type', 'not in', ['view', 'closed'])],
         default=_default_account,

addons/product/product_view.xml

@@ -250,6 +250,11 @@
                                 <field string="List of Variants" name="product_variant_count" widget="statinfo" />
                             </button>
                         </div>
+                        <p class="oe_grey">
+                            <strong>Warning</strong>: adding or deleting attributes
+                            will delete and recreate existing variants and lead
+                            to the loss of its eventual customizations.
+                        </p>
                         <field name="attribute_line_ids" widget="one2many_list" context="{'show_attribute': False}">
                             <tree string="Variants" editable="bottom">
                                 <field name="attribute_id"/>