url_for() doesn't return a working URI for a blueprint route with host

[don-makaron]

Describe the bug
When i use blueprints with host argument and try to get url with url_for i get an incorrect url - blueprint's host is used as a path, and not as a hostname

Code snippet

from sanic import Sanic, Blueprint
from sanic.response import text

bp = Blueprint('bp_app', host='bp.example.com')


@bp.route('/', name='index')
async def bp_index(request):
    url = request.app.url_for('bp_app.index')
    return text(url)


@bp.route('/internal', name='internal')
async def bp_index(request):
    url = request.app.url_for('bp_app.internal')
    return text(url)


@bp.route('/external', name='external')
async def bp_index(request):
    url = request.app.url_for('bp_app.external', _external=True)
    return text(url)


app = Sanic('app_name')
app.blueprint(bp)


if __name__ == '__main__':
    app.run(port=8000)

Output

# 1
$ curl -H "Host:bp.example.com" http://127.0.0.1:8000/
bp.example.com

# 2
$ curl -H "Host:bp.example.com" http://127.0.0.1:8000/internal
bp.example.com/internal

# 3
$ curl -H "Host:bp.example.com" http://127.0.0.1:8000/external
http:///bp.example.com/external

1 example returns hostname by like a path
2 example returns hostname and path but it's like a path
3 example return full url, but with no hostname (3 slashes)

Expected behavior
I'm expecting correct urls.
For internal urls - only path returned as stated in route.
For external urls - fully qualified domain name with full path

1 example - /
2 example - /internal
3 example - http://bp.example.com/external

Environment (please complete the following information):

• OS: macOS
• Version: 19.12.2

Additional context
Can't get correct place, but found some places, where it can be
app.py#L829-L832,
app.py#L848 (perhaps, blueprint's host should be used, and config's server name as default)

[Tronic]

Thanks for reporting and triaging this!