backoffice: display an error on diff of snapshots that do not load (#89801)

tests/test_snapshots.py

@@ -12,7 +12,7 @@ from wcs.carddef import CardDef
 from wcs.categories import Category
 from wcs.comment_templates import CommentTemplate
 from wcs.data_sources import NamedDataSource
-from wcs.fields import CommentField, ItemField, PageField, StringField
+from wcs.fields import BlockField, CommentField, ItemField, PageField, StringField
 from wcs.formdef import FormDef
 from wcs.mail_templates import MailTemplate
 from wcs.qommon.form import UploadedFile
@@ -362,6 +362,61 @@ def test_form_snapshot_diff(pub):
     assert 'Snapshot <a href="%s/view/">%s</a> -  (Version 42.0)' % (snapshot3.id, snapshot3.id) in resp
 
 
+def test_form_snapshot_diff_with_reference_error(pub):
+    create_superuser(pub)
+    create_role(pub)
+
+    BlockDef.wipe()
+    blockdef = BlockDef()
+    blockdef.name = 'testblock'
+    blockdef.fields = []
+    blockdef.store()
+
+    FormDef.wipe()
+    formdef = FormDef()
+    formdef.name = 'testform'
+    formdef.fields = [
+        BlockField(id='1', label='block1', varname='foo', block_slug=blockdef.slug),
+    ]
+    formdef.store()
+    assert pub.snapshot_class.count() == 2
+    snapshot1 = pub.snapshot_class.get_latest('formdef', formdef.id)
+
+    formdef.fields.append(StringField(id=2, label='Test'))
+    formdef.store()
+    assert pub.snapshot_class.count() == 3
+
+    formdef.fields = formdef.fields[1:]
+    formdef.store()
+    assert pub.snapshot_class.count() == 4
+    snapshot3 = pub.snapshot_class.get_latest('formdef', formdef.id)
+
+    app = login(get_app(pub))
+    resp = app.get(
+        '/backoffice/forms/%s/history/compare?version1=%s&version2=%s'
+        % (formdef.id, snapshot1.id, snapshot3.id)
+    )
+    assert resp.pyquery('h2').text() == 'Compare snapshots (XML)'
+    resp = app.get(
+        '/backoffice/forms/%s/history/compare?version1=%s&version2=%s&mode=inspect'
+        % (formdef.id, snapshot1.id, snapshot3.id)
+    )
+    assert resp.pyquery('h2').text() == 'Compare snapshots (Inspect)'
+
+    BlockDef.wipe()
+    resp = app.get(
+        '/backoffice/forms/%s/history/compare?version1=%s&version2=%s'
+        % (formdef.id, snapshot1.id, snapshot3.id)
+    )
+    assert resp.pyquery('h2').text() == 'Compare snapshots (XML)'
+    resp = app.get(
+        '/backoffice/forms/%s/history/compare?version1=%s&version2=%s&mode=inspect'
+        % (formdef.id, snapshot1.id, snapshot3.id)
+    )
+    assert resp.pyquery('h2').text() == 'Error'
+    assert 'Can not display snapshot (Unknown referenced objects)' in resp.text
+
+
 def test_form_snapshot_comments(pub):
     create_superuser(pub)
     create_role(pub)

wcs/backoffice/snapshots.py

@@ -136,7 +136,13 @@ class SnapshotsDirectory(Directory):
         if mode == 'inspect' and not has_inspect:
             raise errors.TraversalError()
 
-        context = getattr(self, 'get_compare_%s_context' % mode)(snapshot1, snapshot2)
+        from wcs.blocks import BlockdefImportError
+
+        try:
+            context = getattr(self, 'get_compare_%s_context' % mode)(snapshot1, snapshot2)
+        except (BlockdefImportError, FormdefImportError, WorkflowImportError) as e:
+            return template.error_page(_('Can not display snapshot (%s)') % e)
+
         context.update(
             {
                 'mode': mode,