blueprints are registered with nested names, can change registered name

[pgjones]

• Blueprints are registered with the full nested name, not only the name they were created with. This allows the same blueprint to be nested in different locations. Fixes Nested blueprint should be able to have same name #4069
• name can be passed as an option to register_blueprint. It changes the name the blueprint is registered with (or nested with). This allows url_for to work if the same blueprint is registered with different prefixes. Fixes url_for can't distinguish a blueprint mounted two times #1091

[davidism]

I've added a _split_blueprint_path helper function that uses functools.lru_cache. I figured that splitting the blueprint path over and over again across requests would be wasteful, so this will cache as long as a WSGI worker is still active. @pgjones do you think this is worth it? We can remove it if not.

I also think we should start issuing a deprecation warning if the same blueprint was registered with the same name, and eventually show the error for any name collisions, not just for different blueprints with the same name.

[davidism]

from flask import Blueprint
from flask import Flask
from flask import url_for

app = Flask(__name__)
bp = Blueprint("user", __name__)


@bp.get("/")
def index():
    return url_for(".index")


app.register_blueprint(bp, url_prefix="/a")
app.register_blueprint(bp, name="user_b", url_prefix="/b")

Navigating to both /a/ and /b/ outputs /a/, so #1091 isn't solved yet. It seems request.blueprint is "user" instead of "user_b" when navigating to /b/.

Looking at app.url_map shows that both were registered with the same endpoint still:

Map([<Rule '/a/' (GET, HEAD, OPTIONS) -> user.index>,
 <Rule '/b/' (GET, HEAD, OPTIONS) -> user.index>,
 <Rule '/static/<filename>' (GET, HEAD, OPTIONS) -> static>])

app.blueprints shows the two names, so there's something going wrong with registration.