Skip posted and paid invoices on set_number

When unreconciling the payment of an invoice without tax_identifier the
set_number function is called and it tries to write on tax_identifier field,
which is not allowed to be done because of check_modify.

issue6028
review33621002

invoice.py

@@ -989,6 +989,10 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
         Date = pool.get('ir.date')
 
         for invoice in invoices:
+            # Posted and paid invoices are tested by check_modify so we can
+            # not modify tax_identifier nor number
+            if invoice.state in {'posted', 'paid'}:
+                continue
             if not invoice.tax_identifier:
                 invoice.tax_identifier = invoice.get_tax_identifier()
 

tests/scenario_invoice.rst

@@ -310,3 +310,36 @@ Create some complex invoice and test its taxes base rounding::
     >>> found_invoice, = Invoice.find([('total_amount', '=', Decimal(0))])
     >>> found_invoice.id == invoice.id
     True
+
+Clear company tax_identifier::
+
+    >>> tax_identifier, = company.party.identifiers
+    >>> tax_identifier.type = None
+    >>> tax_identifier.save()
+
+Create a paid invoice::
+
+    >>> invoice = Invoice()
+    >>> invoice.party = party
+    >>> invoice.payment_term = payment_term
+    >>> line = invoice.lines.new()
+    >>> line.product = product
+    >>> line.quantity = 5
+    >>> line.unit_price = Decimal('40')
+    >>> invoice.click('post')
+    >>> pay = Wizard('account.invoice.pay', [invoice])
+    >>> pay.form.journal = journal_cash
+    >>> pay.execute('choice')
+    >>> pay.state
+    'end'
+    >>> invoice.tax_identifier
+    >>> invoice.state
+    u'paid'
+
+The invoice is posted when the reconciliation is deleted::
+
+    >>> invoice.payment_lines[0].reconciliation.delete()
+    >>> invoice.reload()
+    >>> invoice.state
+    u'posted'
+    >>> invoice.tax_identifier