models: Change attachment.size to not be nullable.

Attachment objects in production are only created in one place, which
passses a size.  Additionally, I verified in multiple production
environments with old data that this never actually happens (or has
happened).

So we should make the data model correctly reflect the possibilities here.

zerver/migrations/0289_tighten_attachment_size.py

@@ -0,0 +1,25 @@
+# Generated by Django 2.2.13 on 2020-06-20 19:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0288_reaction_unique_on_emoji_code'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='archivedattachment',
+            name='size',
+            field=models.IntegerField(default=0),
+            preserve_default=False,
+        ),
+        migrations.AlterField(
+            model_name='attachment',
+            name='size',
+            field=models.IntegerField(default=0),
+            preserve_default=False,
+        ),
+    ]

zerver/models.py

@@ -2053,7 +2053,8 @@ class AbstractAttachment(models.Model):
     create_time: datetime.datetime = models.DateTimeField(
         default=timezone_now, db_index=True,
     )
-    size: Optional[int] = models.IntegerField(null=True)
+    # Size of the uploaded file, in bytes
+    size: int = models.IntegerField()
 
     # Whether this attachment has been posted to a public stream, and
     # thus should be available to all non-guest users in the

zerver/tests/test_attachments.py

@@ -11,7 +11,8 @@ class AttachmentsTests(ZulipTestCase):
         super().setUp()
         user_profile = self.example_user('cordelia')
         self.attachment = Attachment.objects.create(
-            file_name='test.txt', path_id='foo/bar/test.txt', owner=user_profile)
+            file_name='test.txt', path_id='foo/bar/test.txt', owner=user_profile,
+            size=512)
 
     def test_list_by_user(self) -> None:
         user_profile = self.example_user('cordelia')

zerver/tests/test_realm.py

@@ -795,12 +795,12 @@ class ScrubRealmTest(ZulipTestCase):
             self.send_stream_message(king, "Shakespeare")
 
         Attachment.objects.filter(realm=zulip).delete()
-        Attachment.objects.create(realm=zulip, owner=iago, path_id="a/b/temp1.txt")
-        Attachment.objects.create(realm=zulip, owner=othello, path_id="a/b/temp2.txt")
+        Attachment.objects.create(realm=zulip, owner=iago, path_id="a/b/temp1.txt", size=512)
+        Attachment.objects.create(realm=zulip, owner=othello, path_id="a/b/temp2.txt", size=512)
 
         Attachment.objects.filter(realm=lear).delete()
-        Attachment.objects.create(realm=lear, owner=cordelia, path_id="c/d/temp1.txt")
-        Attachment.objects.create(realm=lear, owner=king, path_id="c/d/temp2.txt")
+        Attachment.objects.create(realm=lear, owner=cordelia, path_id="c/d/temp1.txt", size=512)
+        Attachment.objects.create(realm=lear, owner=king, path_id="c/d/temp2.txt", size=512)
 
         CustomProfileField.objects.create(realm=lear)