systempayv2: do not use filesystem for unique vads_trans_id (#47534)

vads_trans_id character space is larger than what we use, using 6
alphanumeric characters probability of collision on a day is small, 1 on
2*10^9.

https://paiement.systempay.fr/doc/fr-FR/form-payment/reference/vads-trans-id.html

eopayment/systempayv2.py

@@ -20,7 +20,10 @@ import pytz
 import datetime as dt
 import hashlib
 import hmac
+import random
+import re
 import string
+import six
 from six.moves.urllib import parse as urlparse
 import warnings
 from gettext import gettext as _
@@ -315,6 +318,21 @@ class Payment(PaymentCommon):
         options = add_vads(options)
         self.options = options
 
+    def make_vads_trans_id(self):
+        # vads_trans_id must be 6 alphanumeric characters,
+        # trans_id starting with 9 are reserved for the systempay backoffice
+        # https://paiement.systempay.fr/doc/fr-FR/form-payment/reference/vads-trans-id.html
+        gen = random.SystemRandom()
+        if six.PY3:
+            alphabet = string.ascii_letters + string.digits
+        else:
+            alphabet = string.letters + string.digits
+        first_letter_alphabet = alphabet.replace('9', '')
+        vads_trans_id = (
+            gen.choice(first_letter_alphabet)
+            + ''.join(gen.choice(alphabet) for i in range(5))
+        )
+        return vads_trans_id
 
     def request(self, amount, name=None, first_name=None, last_name=None,
                 address=None, email=None, phone=None, orderid=None, info1=None,
@@ -369,9 +387,10 @@ class Payment(PaymentCommon):
                     '%s value %s is not of the type %s' % (name, orderid, ptype))
             kwargs[name] = orderid
 
-        transaction_id = self.transaction_id(6, string.digits, 'systempay',
-                                             self.options[VADS_SITE_ID])
-        kwargs[VADS_TRANS_ID] = force_text(transaction_id)
+        vads_trans_id = self.make_vads_trans_id()
+        assert re.match(r'^[0-9a-zA-Z]{6}$', vads_trans_id)
+
+        kwargs[VADS_TRANS_ID] = vads_trans_id
         fields = kwargs
         for parameter in PARAMETERS:
             name = parameter.name
@@ -393,7 +412,7 @@ class Payment(PaymentCommon):
         check_vads(fields)
         fields[SIGNATURE] = force_text(self.signature(fields))
         self.logger.debug('%s request contains fields: %s', __name__, fields)
-        transaction_id = '%s_%s' % (fields[VADS_TRANS_DATE], transaction_id)
+        transaction_id = '%s_%s' % (fields[VADS_TRANS_DATE], vads_trans_id)
         self.logger.debug('%s transaction id: %s', __name__, transaction_id)
         form = Form(
             url=self.service_url,