This scenario happens with the loader oneshot for example. This loader deals with more
than 1 type of origins to ingest in the same queue. So the computation of that function
returned negative value [1]. Which is ultimately not possible to execute in sql [1].
This commits fixes that behavior. This also explicits that the function must return
positive values in its docstring.
[1]
```
...
psycopg2.errors.InvalidRowCountInLimitClause: LIMIT must not be negative
```
[2]
```
Traceback (most recent call last):
File "/usr/bin/swh", line 11, in <module>
load_entry_point('swh.core==0.15.0', 'console_scripts', 'swh')()
File "/usr/lib/python3/dist-packages/swh/core/cli/__init__.py", line 185, in main
return swh(auto_envvar_prefix="SWH")
File "/usr/lib/python3/dist-packages/click/core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "/usr/lib/python3/dist-packages/click/core.py", line 717, in main
rv = self.invoke(ctx)
File "/usr/lib/python3/dist-packages/click/core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python3/dist-packages/click/core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python3/dist-packages/click/core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/python3/dist-packages/click/core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/lib/python3/dist-packages/click/core.py", line 555, in invoke
return callback(*args, **kwargs)
File "/usr/lib/python3/dist-packages/click/decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File "/usr/lib/python3/dist-packages/swh/scheduler/cli/origin.py", line 205, in send_to_celery
lister_uuid=lister_uuid,
File "/usr/lib/python3/dist-packages/swh/core/db/common.py", line 62, in _meth
return meth(self, *args, db=db, cur=cur, **kwargs)
File "/usr/lib/python3/dist-packages/swh/scheduler/backend.py", line 525, in grab_next_visits cur.execute(query, tuple(query_args))
File "/usr/lib/python3/dist-packages/psycopg2/extras.py", line 243, in execute
return super(RealDictCursor, self).execute(query, vars)
psycopg2.errors.InvalidRowCountInLimitClause: LIMIT must not be negative
```