Ensure that blueprint subdomains suffix-chain

This ensures that a child's subdomain prefixs any parent subdomain
such that the full domain is child.parent.domain.tld and onwards with
further nesting. This makes the most sense to users and mimics how
url_prefixes work (although subdomains suffix).

CHANGES.rst

@@ -3,6 +3,8 @@ Version 2.3.0
 
 Unreleased
 
+-   Ensure subdomains are applied with nested blueprints. :issue:`4834`
+
 
 Version 2.2.3
 -------------

docs/blueprints.rst

@@ -140,6 +140,19 @@ name, and child URLs will be prefixed with the parent's URL prefix.
     url_for('parent.child.create')
     /parent/child/create
 
+In addition a child blueprint's will gain their parent's subdomain,
+with their subdomain as prefix if present i.e.
+
+.. code-block:: python
+
+    parent = Blueprint('parent', __name__, subdomain='parent')
+    child = Blueprint('child', __name__, subdomain='child')
+    parent.register_blueprint(child)
+    app.register_blueprint(parent)
+
+    url_for('parent.child.create', _external=True)
+    "child.parent.domain.tld"
+
 Blueprint-specific before request functions, etc. registered with the
 parent will trigger for the child. If a child does not have an error
 handler that can handle a given exception, the parent's will be tried.

src/flask/blueprints.py

@@ -358,6 +358,9 @@ def register(self, app: "Flask", options: dict) -> None:
         :param options: Keyword arguments forwarded from
             :meth:`~Flask.register_blueprint`.
 
+        .. versionchanged:: 2.3
+            Nested blueprints now correctly apply subdomains.
+
         .. versionchanged:: 2.0.1
             Nested blueprints are registered with their dotted name.
             This allows different blueprints with the same name to be
@@ -458,10 +461,12 @@ def extend(bp_dict, parent_dict):
             if bp_subdomain is None:
                 bp_subdomain = blueprint.subdomain
 
-            if state.subdomain is not None and bp_subdomain is None:
-                bp_options["subdomain"] = state.subdomain
+            if state.subdomain is not None and bp_subdomain is not None:
+                bp_options["subdomain"] = bp_subdomain + "." + state.subdomain
             elif bp_subdomain is not None:
                 bp_options["subdomain"] = bp_subdomain
+            elif state.subdomain is not None:
+                bp_options["subdomain"] = state.subdomain
 
             if bp_url_prefix is None:
                 bp_url_prefix = blueprint.url_prefix

tests/test_blueprints.py

@@ -971,7 +971,7 @@ def index():
     assert response.status_code == 200
 
 
-def test_child_overrides_parent_subdomain(app, client) -> None:
+def test_child_and_parent_subdomain(app, client) -> None:
     child_subdomain = "api"
     parent_subdomain = "parent"
     parent = flask.Blueprint("parent", __name__)
@@ -988,7 +988,9 @@ def index():
 
     domain_name = "domain.tld"
     app.config["SERVER_NAME"] = domain_name
-    response = client.get("/", base_url=f"http://{child_subdomain}.{domain_name}")
+    response = client.get(
+        "/", base_url=f"http://{child_subdomain}.{parent_subdomain}.{domain_name}"
+    )
 
     assert response.status_code == 200