message: Add migrations to correct has_* fields.

This follows up on changes to correctly set has_link, has_attachment,
and has_image when rendering messages.

Fixes #12912.

zerver/migrations/0257_fix_has_link_attribute.py

@@ -0,0 +1,53 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.24 on 2019-10-07 05:25
+from __future__ import unicode_literals
+
+from django.db import migrations
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+import lxml
+
+def fix_has_link(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    Message = apps.get_model('zerver', 'Message')
+    for message in Message.objects.all():
+        # Because we maintain the Attachment table, this should be as
+        # simple as just just checking if there's any Attachment
+        # objects associated with this message.
+        has_attachment = message.attachment_set.exists()
+
+        # For has_link and has_image, we need to parse the messages.
+        # Links are simple -- look for a link in the message.
+        lxml_obj = lxml.html.fromstring(message.rendered_content)
+        has_link = False
+        for link in lxml_obj.xpath("//a"):
+            has_link = True
+            break
+
+        # has_image refers to inline image previews, so we just check
+        # for the relevant CSS class.
+        has_image = False
+        for img in lxml_obj.find_class("message_inline_image"):
+            has_image = True
+            break
+
+        if (message.has_link == has_link and
+                message.has_attachment == has_attachment and
+                message.has_image == has_image):
+            # No need to spend time with the database if there aren't changes.
+            continue
+        message.has_image = has_image
+        message.has_link = has_link
+        message.has_attachment = has_attachment
+        message.save(update_fields=['has_link', 'has_attachment', 'has_image'])
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0256_userprofile_stream_set_recipient_column_values'),
+    ]
+
+    operations = [
+        migrations.RunPython(fix_has_link,
+                             reverse_code=migrations.RunPython.noop),
+    ]