(core) Migrate Attachments columns from marshalled blobs to JSON

Summary: Adds a migration in preparation for future work on tracking and deleting attachments. This includes a `_grist_Attachments.timeDeleted` column which isn't used yet, and changing the storage format of user columns of type `Attachments`. DocStorage now treats Attachments like RefList in general (since they use JSON), which also prompted a tiny bit of refactoring.

Test Plan: Added a migration test case showing the change in format.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3352

sandbox/grist/migrations.py

@@ -906,3 +906,21 @@ def migration27(tdset):
     add_column('_grist_Tables_column', 'rules', 'RefList:_grist_Tables_column'),
     add_column('_grist_Views_section_field', 'rules', 'RefList:_grist_Tables_column'),
   ])
+
+
+@migration(schema_version=28)
+def migration28(tdset):
+  doc_actions = [add_column('_grist_Attachments', 'timeDeleted', 'DateTime')]
+
+  tables = list(actions.transpose_bulk_action(tdset.all_tables["_grist_Tables"]))
+  columns = list(actions.transpose_bulk_action(tdset.all_tables["_grist_Tables_column"]))
+
+  for table in tables:
+    for col in columns:
+      if table.id == col.parentId and col.type == "Attachments":
+        # This looks like it doesn't change anything,
+        # but it makes DocStorage realise that the sqlType has changed
+        # so it converts marshalled blobs to JSON
+        doc_actions.append(actions.ModifyColumn(table.tableId, col.colId, {"type": "Attachments"}))
+
+  return tdset.apply_doc_actions(doc_actions)

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 27
+SCHEMA_VERSION = 28
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -235,6 +235,7 @@ def schema_create_actions():
       make_column("fileSize",     "Int"),  # The size in bytes
       make_column("imageHeight",  "Int"),  # height in pixels
       make_column("imageWidth",   "Int"),  # width in pixels
+      make_column("timeDeleted",  "DateTime"),
       make_column("timeUploaded", "DateTime")
     ]),