[server] 500 Error with wrong parent

[AlekseyLobanov]

Hello.
I use latest stable version with Docker setup.
I found that internal sqlite table does not use any constraints on parent column of table comments which may be a problem in two cases.

1. DB is corrupted and one of the parents is not valid.
2. Someone trying to add comment with invalid parent.

I tried second case with curl and got 500 Error on client and stacktrace below on server

2022-01-29 22:57:29,824 ERROR: POST /new
Traceback (most recent call last):
  File "/isso/lib/python3.8/site-packages/isso-0.12.5-py3.8.egg/isso/__init__.py", line 147, in dispatch
    response = handler(request.environ, request, **values)
  File "/isso/lib/python3.8/site-packages/isso-0.12.5-py3.8.egg/isso/views/comments.py", line 88, in dec
    return func(self, env, req, *args, **kwargs)
  File "/isso/lib/python3.8/site-packages/isso-0.12.5-py3.8.egg/isso/views/__init__.py", line 45, in dec
    return func(cls, env, req, *args, **kwargs)
  File "/isso/lib/python3.8/site-packages/isso-0.12.5-py3.8.egg/isso/views/comments.py", line 307, in new
    rv = self.comments.add(uri, data)
  File "/isso/lib/python3.8/site-packages/isso-0.12.5-py3.8.egg/isso/db/comments.py", line 57, in add
    if ref.get("parent") is not None:
AttributeError: 'NoneType' object has no attribute 'get'

Steps to reproduce:

1. Start Docker instance of isso (on http://127.0.0.1:8080/ for example)
2. POST request http://127.0.0.1:8080/new?uri=%2Fbase.html with valid comment and wrong parent. I used "Copy as cURL" button on Firefox Web Tools and replaced null with some invalid id (42 for example)

[ix5]

@AlekseyLobanov thank you for your report.
Would you mind working on a fix and submitting a PR?

[ix5]

Related: #531

[ix5]

@AlekseyLobanov would you like to work on a PR?

[ix5]

Copying over from #531, quoting @budui

I am porting isso from Python to Go. While testing I noticed that if I specify a wrong parent(existed but different thread) for the new comment, the new comment will be added successfully but can't be retrievaled with uri.

Maybe we need to check whether the tid of new comment and parent's tid are consistent
https://github.com/posativ/isso/blob/6092881bb205c999a8f7f086dd5d20f686d641ea/isso/db/comments.py#L49-L52

[ix5]

Tracked in #885