fix(api): strip trailing slash on namespace path to avoid '//' rules (#74)

[jbbqqf]

Summary

Api.add_namespace(ns, path=path) (and Api.ns_urls) builds rule
paths by naive concatenation:

def ns_urls(self, ns, urls):
    path = self.get_ns_path(ns) or ns.path
    return [path + url for url in urls]

That is fine for path='/api' + /foo = /api/foo, but the moment the
caller does what the docs lead them to — mount a namespace at the API
root with path='/' — the rule ends up as '//foo'. path='/api/'
produces '/api//foo'. Both shapes leak into app.url_map and break
url_for/Swagger reverse routing.

Fix: normalise the prefix with rstrip('/') before concatenating, so
'/' + '/foo' becomes '' + '/foo' = '/foo' and '/api/' + '/foo'
becomes '/api' + '/foo' = '/api/foo'. This matches the normalisation
already done on Namespace.path for the namespace's own _path
(flask_restx/namespace.py:67).

Fixes #74.

Context

The OP (gyre) raised this in 2020 wanting to do exactly what the
flask-restx docs recommend for a root-mounted namespace:

base_api = Namespace("base")
api.add_namespace(base_api, path="/")

flask_restx/namespace.py:65-67 already does
(self._path or ("/" + self.name)).rstrip("/") precisely because of
this concern. The bug is that ns_urls bypasses that normalisation
when the caller specifies a path explicitly via add_namespace.

Changes

• flask_restx/api.py:486-491: add path = path.rstrip("/") in
  ns_urls, with a comment explaining why.
• tests/test_api.py: regression test test_ns_path_no_double_slash
  exercising both path="/" and path="/api/". It asserts there is no
  '//' substring in any URL rule and that url_for returns the
  expected canonical form.
• CHANGELOG.rst: 1.3.3 bug-fix entry.

Reproduce BEFORE/AFTER yourself (copy-paste)

# --- one-time setup ---
git clone https://github.com/python-restx/flask-restx.git /tmp/restx-74 && cd /tmp/restx-74
python3 -m venv .venv && . .venv/bin/activate
pip install -q -e ".[test]" tzdata

# --- BEFORE: origin/master, expect '//foo' and '/api//foo' ---
git checkout origin/master
python -c "
from flask import Flask
from flask_restx import Api, Namespace, Resource
for prefix in ('/', '/api/'):
    app = Flask(__name__)
    api = Api(app)
    ns = Namespace('ns')
    @ns.route('/foo')
    class Foo(Resource):
        def get(self): return {}
    api.add_namespace(ns, path=prefix)
    rules = sorted(str(r) for r in app.url_map.iter_rules() if 'foo' in str(r))
    print(f'path={prefix!r:8s} rules={rules}')
"
# Expected (BEFORE the fix):
#   path='/'      rules=['//foo']
#   path='/api/'  rules=['/api//foo']

# --- AFTER: this branch, expect '/foo' and '/api/foo' ---
git fetch https://github.com/jbbqqf/flask-restx.git fix/74-double-slash-namespace
git checkout FETCH_HEAD
python -c "
from flask import Flask
from flask_restx import Api, Namespace, Resource
for prefix in ('/', '/api/'):
    app = Flask(__name__)
    api = Api(app)
    ns = Namespace('ns')
    @ns.route('/foo')
    class Foo(Resource):
        def get(self): return {}
    api.add_namespace(ns, path=prefix)
    rules = sorted(str(r) for r in app.url_map.iter_rules() if 'foo' in str(r))
    print(f'path={prefix!r:8s} rules={rules}')
"
# Expected (AFTER the fix):
#   path='/'      rules=['/foo']
#   path='/api/'  rules=['/api/foo']

What I ran locally

$ pytest tests/test_api.py -k "ns_path" -v 2>&1 | tail -5
collected 29 items / 27 deselected / 2 selected
tests/test_api.py::APITest::test_ns_path_prefixes PASSED
tests/test_api.py::APITest::test_ns_path_no_double_slash PASSED

$ pytest -q --no-header 2>&1 | tail -2
FAILED tests/test_inputs.py::URLTest::test_check - Failed: DID NOT RAISE <class 'ValueError'>
1 failed, 1237 passed in 8.76s

The single failure (tests/test_inputs.py::URLTest::test_check) is
pre-existing on master and unrelated to this change — it depends on
http://this-domain-should-not-exist.com actually not resolving, which
is no longer true on the network where the suite ran.

Edge cases

#	add_namespace path	ns.route url	Resulting rule	Verified by
1	"/"	"/foo"	"/foo" (was "//foo")	new regression test
2	"/api/" (trailing /)	"/foo"	"/api/foo" (was "/api//foo")	new regression test
3	"/api_test" (no trailing /)	"/test/"	"/api_test/test/" (unchanged)	pre-existing test_ns_path_prefixes
4	omitted (no add_namespace path)	resource url	unchanged — ns.path already rstrip('/')-ed	pre-existing tests, no change in this path

Risk / blast radius

The only behavioural change is that prefix paths ending in / lose
their trailing / before being concatenated with the resource URL.
This is purely a normalisation: before this PR those paths produced
syntactically broken '//' rules that Flask still accepts but that
break url_for round-tripping and look wrong in Swagger. There is no
caller that was deliberately relying on '//foo' as a valid path.

The existing test_ns_path_prefixes (which uses "/api_test" with no
trailing slash) still passes unchanged.

---

PR drafted with assistance from Claude Code (Anthropic). The change was
reviewed manually against flask-restx's source. The reproducer block
above is the one I used during development; reviewers can paste it
verbatim.