`parent_id` fields are common in many models, and thus default values
for those fields are sometimes passed in the context.
Because mail.message also has `parent_id` field, it would automatically
use the default when an automatic message was being posted. While of
course, the parent_id value comes from a different model.
This "adoption" by a random "parent message" is unexpected,
not desired, and it can even cause a very surprising AccessError if the
parent message is not readable by the user.
Forcing the `parent_id` value during the creation of an automatic message
avoids this confusion.
One way to trigger the bug was to use the "subtask" stat button to create a
child subtask for a project task (it relies on the parent task ID
passed in the context)
Olivier Dony
e8271ffcc7