fix(computes): editing a compute keeps its password unless a new one is given

The TCP and TLS compute forms never show the stored password, so the
field is empty on every edit. Saving the form without typing the
password again replaced it with nothing, and the next connection to the
host failed to authenticate.

An empty password field on an edit now keeps the stored password; a
new value replaces it.

computes/forms.py

@@ -7,7 +7,15 @@ from computes.models import Compute
 from .validators import validate_hostname
 
 
-class TcpComputeForm(forms.ModelForm):
+class KeepPasswordMixin:
+    """The form never shows the stored password: left empty on an edit, it
+    stays as it is."""
+
+    def clean_password(self):
+        return self.cleaned_data["password"] or self.instance.password
+
+
+class TcpComputeForm(KeepPasswordMixin, forms.ModelForm):
     hostname = forms.CharField(validators=[validate_hostname])
     type = forms.IntegerField(widget=forms.HiddenInput, initial=CONN_TCP)
 
@@ -26,7 +34,7 @@ class SshComputeForm(forms.ModelForm):
         exclude = ["password"]
 
 
-class TlsComputeForm(forms.ModelForm):
+class TlsComputeForm(KeepPasswordMixin, forms.ModelForm):
     hostname = forms.CharField(validators=[validate_hostname])
     type = forms.IntegerField(widget=forms.HiddenInput, initial=CONN_TLS)
 

computes/test_compute_password.py

@@ -0,0 +1,28 @@
+"""The compute form never shows the stored password: a password left empty
+on an edit keeps it, a new one replaces it."""
+
+from django.contrib.auth import get_user_model
+from django.test import TestCase
+from django.urls import reverse
+
+from computes.models import Compute
+
+
+class ComputePasswordTests(TestCase):
+    def setUp(self):
+        self.client.force_login(get_user_model().objects.create_superuser("cp-admin", "cp@example.com", "pw"))
+
+    def edit(self, compute, password):
+        data = {"name": compute.name, "hostname": compute.hostname, "login": "admin", "password": password,
+                "type": compute.type, "details": ""}
+        self.client.post(reverse("compute_update", args=[compute.id]), data)
+        compute.refresh_from_db()
+        return compute.password
+
+    def test_an_empty_password_keeps_the_stored_one(self):
+        for conn_type in (1, 3):  # TCP, TLS
+            with self.subTest(type=conn_type):
+                compute = Compute.objects.create(name=f"cp{conn_type}", hostname="10.0.0.7", login="admin",
+                                                 password="secret", type=conn_type)
+                self.assertEqual(self.edit(compute, ""), "secret")
+                self.assertEqual(self.edit(compute, "other"), "other")