prevent tests from deleting the default user

PR aiidateam#2214 stripped of safeguards that prevented the default user from
being deleted in between tests.
Since AiiDA expects a default user in many operations, leaving the DB
in a state with no default user is not ideal.

This commit reintroduces safeguards that result in the default user
remaining untouched.

aiida/backends/djsite/db/testbase.py

@@ -43,6 +43,9 @@ def setUpClass_method(self):
 
     def clean_db(self):
         from aiida.backends.djsite.db import models
+        from aiida.manage import get_config
+
+        email = get_config().current_profile.default_user_email
 
         # I first need to delete the links, because in principle I could not delete input nodes, only outputs.
         # For simplicity, since I am deleting everything, I delete the links first
@@ -52,7 +55,8 @@ def clean_db(self):
         models.DbLog.objects.all().delete()
         models.DbNode.objects.all().delete()  # pylint: disable=no-member
         models.DbWorkflow.objects.all().delete()  # pylint: disable=no-member
-        models.DbUser.objects.all().delete()  # pylint: disable=no-member
+        # The default user is needed for many operations in AiiDA; do not delete it.
+        models.DbUser.objects.exclude(email=email).delete()  # pylint: disable=no-member
         models.DbComputer.objects.all().delete()
         models.DbGroup.objects.all().delete()
 

aiida/backends/testimplbase.py

@@ -14,7 +14,7 @@
 
 import six
 
-from aiida.common.exceptions import InternalError
+from aiida.common.exceptions import InternalError, NotExistent
 from aiida import orm
 
 
@@ -87,10 +87,15 @@ def insert_data(self):
             backend=self.backend
         ).store()
 
-        config = get_config()
-        email = config.current_profile.default_user_email
-        self.user = orm.User(email=email).store()
-        self.user_email = email
+        # Since the default user is needed for many operations in AiiDA, it is not deleted by clean_db.
+        # In principle, it should therefore always exist - if not we create it anyhow.
+        default_user_email = get_config().current_profile.default_user_email
+        try:
+            self.user = orm.User.objects.get(email=default_user_email)
+        except NotExistent:
+            self.user = orm.User(email=email).store()
+
+        self.user_email = self.user.email
 
     def get_computer(self):
         """