fix(downloads): a shared or feed-opened Facebook group post downloads its own video

A Facebook share link (/share/v/…) redirects to a group post, but only the
permalink form went through the post-page read, so yt-dlp followed the
share link itself and downloaded an ad from the signed-in group feed
(prod job 471). The link of a post opened from the group's feed
(/groups/<g>/?multi_permalinks=<id>) was not recognised at all, and yt-dlp
answers it with "Unsupported URL" (prod job 472).

With a cookie, a share link is now followed in the same request that reads
the post page, and a feed link is read as the post it names. Without one,
the feed link goes to yt-dlp in the permalink form it knows. A post page's
"no single video" verdict now names the post for a share link too.

backend/app/downloads/adapters.py

@@ -12,7 +12,7 @@ Resolution works both before extraction (from source_kind / URL host) and after
 """
 
 import re
-from urllib.parse import urlparse
+from urllib.parse import parse_qs, urlparse
 
 from yt_dlp.networking.impersonate import ImpersonateTarget
 
@@ -183,10 +183,23 @@ class FacebookAdapter(SourceAdapter):
         # in the page scripts that carry the post id; its watch link downloads (plain first, then the
         # Chrome retry above). Needs the cookie: signed out, the page is the login wall.
         post_id = facebook_group_post_id(url)
-        if post_id is None or not ydl_opts.get("cookiefile"):
+        if not ydl_opts.get("cookiefile"):
+            # yt-dlp knows the permalink form and answers the feed's form with "Unsupported URL".
+            if post_id is not None and not _FB_GROUP_POST_PATH.match(urlparse(url).path):
+                return _group_permalink(url, post_id)
+            return url
+        # A share link is a post only once followed: handed to yt-dlp, it was followed to the group
+        # page and an ad from the signed-in feed came down (prod job 471). One request does both — the
+        # redirect lands on the post page, which is then read as any post link's.
+        if post_id is None and not _is_share_link(url):
             return url
         with egress.open_ydl(ydl_opts) as ydl:
-            raw = ydl.urlopen(url).read(self._POST_PAGE_MAX_BYTES)
+            response = ydl.urlopen(url)
+            if post_id is None:
+                post_id = facebook_group_post_id(response.url)
+                if post_id is None:
+                    return url  # a shared reel or video: yt-dlp follows the link as it always has
+            raw = response.read(self._POST_PAGE_MAX_BYTES)
         page = raw.decode("utf-8", "replace")
         # Measured: a refused or expired cookie gets the login page (~450 KB, a login
         # form, no post id at all), which is not a post without a video — the user's cookie is the cause.
@@ -246,6 +259,7 @@ class FacebookAdapter(SourceAdapter):
 
 
 _FB_GROUP_POST_PATH = re.compile(r"^/groups/[^/]+/(?:permalink|posts)/(\d+)/?$")
+_FB_GROUP_PATH = re.compile(r"^/groups/[^/]+/?$")
 _SCRIPT_BLOCK = re.compile(r"<script[^>]*>(.*?)</script>", re.S)
 _VIDEO_NODE = re.compile(r'"__typename":"Video","id":"(\d+)"')
 
@@ -256,12 +270,37 @@ def _quoted(post_id: str) -> str:
 
 
 def facebook_group_post_id(url: str) -> str | None:
-    """The post id of a Facebook group post link (`/groups/<group>/permalink|posts/<id>/`), else None."""
+    """The post id of a Facebook group post link, else None: `/groups/<group>/permalink|posts/<id>/`,
+    or a post opened from the group's feed, `/groups/<group>/?multi_permalinks=<id>` (one id only —
+    with several, which one was meant is not ours to guess)."""
     parsed = urlparse(url)
     if not _host_is(parsed.netloc.lower(), "facebook.com"):
         return None
     match = _FB_GROUP_POST_PATH.match(parsed.path)
-    return match.group(1) if match else None
+    if match:
+        return match.group(1)
+    if not _FB_GROUP_PATH.match(parsed.path):
+        return None
+    ids = parse_qs(parsed.query).get("multi_permalinks", [])
+    return ids[0] if len(ids) == 1 and ids[0].isdigit() else None
+
+
+def _group_permalink(url: str, post_id: str) -> str:
+    """The permalink form of a group post link — the one form yt-dlp itself accepts."""
+    group = urlparse(url).path.split("/")[2]
+    return f"https://www.facebook.com/groups/{group}/permalink/{post_id}/"
+
+
+def facebook_post_link(url: str) -> bool:
+    """A link that `resolve_url` may read as a group post: a post link, or a share link (which may
+    lead to one) — what the failure catalog accepts for the post page's own verdicts."""
+    return facebook_group_post_id(url) is not None or _is_share_link(url)
+
+
+def _is_share_link(url: str) -> bool:
+    # facebook.com/share/<code>/ and /share/v|r|p/<code>/ — a redirect to the shared object.
+    parsed = urlparse(url)
+    return _host_is(parsed.netloc.lower(), "facebook.com") and parsed.path.startswith("/share/")
 
 
 def facebook_post_video_id(page: str, post_id: str) -> str | None:

backend/app/downloads/errors.py

@@ -32,6 +32,7 @@ from app.downloads.adapters import (
     POST_NOT_SHOWN_MESSAGE,
     POST_WITHOUT_VIDEO_MESSAGE,
     facebook_group_post_id,
+    facebook_post_link,
 )
 from app.downloads.credentials import host_key_for_url
 
@@ -227,13 +228,14 @@ _CATALOG: tuple[_Entry, ...] = (
     # answers "No video formats found" (measured on prod jobs 66 and 145, whose videos are reshared
     # clips). With a cookie yt-dlp only ever gets the post's VIDEO link, so the same text there is about
     # that video, not the post — the category says it. Only for group post links (the adapter's own
-    # test of one): a reel with no formats is a different failure.
+    # test of one): a reel with no formats is a different failure. The adapter's own text also comes
+    # from a share link that led to a post, so its test takes those too.
     _Entry(
         host="facebook",
         markers=(POST_WITHOUT_VIDEO_MESSAGE.lower(),),
         code="format_unavailable",
         detail="facebook.group_post_video",
-        link=facebook_group_post_id,
+        link=facebook_post_link,
     ),
     _Entry(
         host="facebook",