From 17c564b7a015a616ee58726ace77b96775bd6e0a Mon Sep 17 00:00:00 2001
From: Matt Westcott <matt@west.co.tt>
Date: Tue, 16 Oct 2018 12:07:09 +0100
Subject: [PATCH] Catch empty non-StreamValues in get_prep_value. Fixes #4809

---
 wagtail/core/blocks/stream_block.py           |  5 +-
 .../0035_streamfieldmigrationmodels.py        | 47 +++++++++++++++++++
 ...36_streamfieldmigrationmodels_add_field.py | 33 +++++++++++++
 wagtail/tests/testapp/models.py               | 19 ++++++++
 4 files changed, 102 insertions(+), 2 deletions(-)
 create mode 100644 wagtail/tests/testapp/migrations/0035_streamfieldmigrationmodels.py
 create mode 100644 wagtail/tests/testapp/migrations/0036_streamfieldmigrationmodels_add_field.py

diff --git a/wagtail/core/blocks/stream_block.py b/wagtail/core/blocks/stream_block.py
index d47f827e3..7deeff408 100644
--- a/wagtail/core/blocks/stream_block.py
+++ b/wagtail/core/blocks/stream_block.py
@@ -243,8 +243,9 @@ class BaseStreamBlock(Block):
         ], is_lazy=True)
 
     def get_prep_value(self, value):
-        if value is None:
-            # treat None as identical to an empty stream
+        if not value:
+            # Falsy values (including None, empty string, empty list, and
+            # empty StreamValue) become an empty stream
             return []
         else:
             # value is a StreamValue - delegate to its get_prep_value() method
diff --git a/wagtail/tests/testapp/migrations/0035_streamfieldmigrationmodels.py b/wagtail/tests/testapp/migrations/0035_streamfieldmigrationmodels.py
new file mode 100644
index 000000000..08172b22b
--- /dev/null
+++ b/wagtail/tests/testapp/migrations/0035_streamfieldmigrationmodels.py
@@ -0,0 +1,47 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.15 on 2018-10-16 10:58
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0040_page_draft_title'),
+        ('tests', '0034_advertwithuuidcustomprimarykey'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='AddedStreamFieldWithEmptyListDefaultPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+        migrations.CreateModel(
+            name='AddedStreamFieldWithEmptyStringDefaultPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+        migrations.CreateModel(
+            name='AddedStreamFieldWithoutDefaultPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+    ]
diff --git a/wagtail/tests/testapp/migrations/0036_streamfieldmigrationmodels_add_field.py b/wagtail/tests/testapp/migrations/0036_streamfieldmigrationmodels_add_field.py
new file mode 100644
index 000000000..bca667e0d
--- /dev/null
+++ b/wagtail/tests/testapp/migrations/0036_streamfieldmigrationmodels_add_field.py
@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.15 on 2018-10-16 11:00
+from __future__ import unicode_literals
+
+from django.db import migrations
+import wagtail.core.blocks
+import wagtail.core.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tests', '0035_streamfieldmigrationmodels'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='addedstreamfieldwithemptylistdefaultpage',
+            name='body',
+            field=wagtail.core.fields.StreamField([('title', wagtail.core.blocks.CharBlock())], default=[]),
+        ),
+        migrations.AddField(
+            model_name='addedstreamfieldwithemptystringdefaultpage',
+            name='body',
+            field=wagtail.core.fields.StreamField([('title', wagtail.core.blocks.CharBlock())], default=''),
+        ),
+        migrations.AddField(
+            model_name='addedstreamfieldwithoutdefaultpage',
+            name='body',
+            field=wagtail.core.fields.StreamField([('title', wagtail.core.blocks.CharBlock())], default=''),
+            preserve_default=False,
+        ),
+    ]
diff --git a/wagtail/tests/testapp/models.py b/wagtail/tests/testapp/models.py
index a29b4a51e..835da832d 100644
--- a/wagtail/tests/testapp/models.py
+++ b/wagtail/tests/testapp/models.py
@@ -1232,3 +1232,22 @@ class TabbedSettings(TestSetting):
 
 class AlwaysShowInMenusPage(Page):
     show_in_menus_default = True
+
+
+# test for AddField migrations on StreamFields using various default values
+class AddedStreamFieldWithoutDefaultPage(Page):
+    body = StreamField([
+        ('title', CharBlock())
+    ])
+
+
+class AddedStreamFieldWithEmptyStringDefaultPage(Page):
+    body = StreamField([
+        ('title', CharBlock())
+    ], default='')
+
+
+class AddedStreamFieldWithEmptyListDefaultPage(Page):
+    body = StreamField([
+        ('title', CharBlock())
+    ], default=[])