Add webmentions helper

app/utils/webmentions.py

@@ -0,0 +1,62 @@
+from urllib.parse import urlparse
+
+import httpx
+from bs4 import BeautifulSoup  # type: ignore
+from loguru import logger
+
+from app import config
+from app.utils.url import is_url_valid
+
+
+def _make_abs(url: str | None, parent: str) -> str | None:
+    if url is None:
+        return None
+
+    if url.startswith("http"):
+        return url
+
+    return (
+        urlparse(parent)._replace(path=url, params="", query="", fragment="").geturl()
+    )
+
+
+async def _discover_webmention_endoint(url: str) -> str | None:
+    async with httpx.AsyncClient() as client:
+        try:
+            resp = await client.get(
+                url,
+                headers={
+                    "User-Agent": config.USER_AGENT,
+                },
+                follow_redirects=True,
+            )
+            resp.raise_for_status()
+        except (httpx.HTTPError, httpx.HTTPStatusError):
+            logger.exception(f"Failed to discover webmention endpoint for {url}")
+            return None
+
+    for k, v in resp.links.items():
+        if k and "webmention" in k:
+            return _make_abs(resp.links[k].get("url"), url)
+
+    soup = BeautifulSoup(resp.text, "html5lib")
+    wlinks = soup.find_all(["link", "a"], attrs={"rel": "webmention"})
+    for wlink in wlinks:
+        if "href" in wlink.attrs:
+            return _make_abs(wlink.attrs["href"], url)
+
+    return None
+
+
+async def discover_webmention_endpoint(url: str) -> str | None:
+    """Discover the Webmention endpoint of a given URL, if any.
+
+    Passes all the tests at https://webmention.rocks!
+
+    """
+    wurl = await _discover_webmention_endoint(url)
+    if wurl is None:
+        return None
+    if not is_url_valid(wurl):
+        return None
+    return wurl