[MankhongGarden]

Fixes #225

What

Promgen builds Alertmanager and Prometheus URLs with urllib.parse.urljoin(base, "/api/..."). When the second argument starts with /, urljoin replaces the whole path of the base URL, so a server running under a path prefix loses it:

>>> urljoin("http://example.com/alertmanager/", "/api/v2/silences")
'http://example.com/api/v2/silences'

So with alertmanager.url: http://example.com/alertmanager/ (a common setup behind a reverse proxy, or Alertmanager started with --web.route-prefix), creating, listing and deleting silences and fetching alerts all call the wrong URL. The same happens to prometheus.url (/-/reload, clean_tombstones) and to the shard URLs used by the query proxy (/api/v1/query, /api/v1/labels, ...).

rest.py already builds the shard query URL as f"{shard.url}/api/v1/query", which keeps the prefix, so the behaviour was also inconsistent between endpoints.

Change (6 files, +68/-10)

• promgen/util.py: add join_url(base, path), which appends the path to the base URL and keeps its prefix (base.rstrip("/") + "/" + path.lstrip("/")).
• promgen/prometheus.py, promgen/proxy.py, promgen/tasks.py: use util.join_url at the 7 places that used urljoin with an absolute path (Alertmanager silences/alerts, Prometheus reload and clean_tombstones, proxy shard requests). The unused urljoin imports are removed.
• Tests: test_util.test_join_url covers base URLs with and without a prefix and trailing slash, and a path with a query string. test_silence.test_alertmanager_url_with_path_prefix posts a silence with alertmanager.url set to http://example.com/alertmanager/ and checks the request goes to .../alertmanager/api/v2/silences.

Why this approach

• A small helper keeps every call site on one line and makes the intent clear. Alternatives:
  • Make the paths relative (urljoin(base, "api/v2/silences")): only works when the configured URL ends with /. http://example.com/alertmanager would still drop alertmanager.
  • Use f-strings at each call site, as rest.py does: works, but duplicates the trailing-slash handling 7 times.
• Behaviour for URLs without a path prefix (for example the default http://alertmanager:9093) is unchanged, so existing setups are not affected.

Testing

• The new silence test fails on current master (request goes to http://example.com/api/v2/silences) and passes with this change.
• promgen test: no new failures compared with master.
• ruff check promgen and ruff format --diff promgen (ruff 0.14.14, as in the Makefile): clean.
• promgen check and promgen makemigrations --check: no changes.

Scope

Only URL building is changed. It does not depend on any other open issue or PR.

[vincent-olivert-riera]

Thanks for your contribution.
Please check the review comments, thanks!

[MankhongGarden]

Thanks for the review! I've applied all three suggestions in d16438a: switched join_url to the f-string with removesuffix/removeprefix, trimmed the docstring, and removed the silence test.

[vincent-olivert-riera]

Thanks for the review! I've applied all three suggestions in d16438a: switched join_url to the f-string with removesuffix/removeprefix, trimmed the docstring, and removed the silence test.

@MankhongGarden, thanks. Please squash the fixup commit and rebase your branch on master before we can merge it.

[MankhongGarden]

Done, squashed into a single commit and rebased onto the latest master (7b8dad7). Thanks!