[IMP] snippets: do not return unneeded data from mp worker

In `convert_html_columns()`, we select 100MiB worth of DB tuples and pass them
to a ProcessPoolExecutor together with a converter callable. So far, the
converter returns all tuples, changed or unchanged together with the
information if it has changed something. All this is returned through IPC to
the parent process. In the parent process, the caller only acts on the changed
tuples, though, the rest is ignored. In any scenario I've seen, only a small
proportion of the input tuples is actually changed, meaning that a large
proportion is returned through IPC unnecessarily.

What makes it worse is that processing of the converted results in the parent
process is often slower than the conversion, leading to two effects:
1) The results of all workers sit in the parent process's memory, possibly
   leading to MemoryError (upg-2021031)
2) The parallel processing is being serialized on the feedback, defeating a
   large part of the intended performance gains

This commit fixes (1) by only returning `None` for unchanged tuples. An
improvement for (2) follows in the next commit.

Author: cawo-odoo

src/util/snippets.py

@@ -267,7 +267,7 @@ def __call__(self, row):
             changes[column] = new_content
             if has_changed:
                 changes["id"] = res_id
-        return changes
+        return changes if "id" in changes else None
 
 
 def convert_html_columns(cr, table, columns, converter_callback, where_column="IS NOT NULL", extra_where="true"):
@@ -310,7 +310,7 @@ def convert_html_columns(cr, table, columns, converter_callback, where_column="I
         for query in log_progress(split_queries, logger=_logger, qualifier=f"{table} updates"):
             cr.execute(query)
             for data in executor.map(convert, cr.fetchall(), chunksize=1000):
-                if "id" in data:
+                if data:
                     cr.execute(update_query, data)