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/tests/test_facebook_impersonation.py

@@ -27,6 +27,18 @@ _GROUP_LINK = f"https://www.facebook.com/groups/983554479435596/permalink/{_POST
 _POST_VIDEO = "1408015447857753"
 _FEED_VIDEO = "1056963063816966"  # what yt-dlp downloaded from this post's link: a feed video
 _CHROME = ImpersonateTarget.from_str("chrome")
+# The two other ways a group post reaches us (prod jobs 471 and 472, the same post): Facebook's "Share"
+# link, which redirects to the post's permalink (measured), and the link of a post opened from the
+# group's feed, which yt-dlp does not know at all ("Unsupported URL").
+_SHARE_LINK = "https://www.facebook.com/share/v/19bCAHKyrd/"
+_LANDED = (
+    f"https://www.facebook.com/groups/it.humor.and.memes/permalink/{_POST}/"
+    "?rdid=y9HAMntF7aCoAKZE&share_url=https%3A%2F%2Fwww.facebook.com%2Fshare%2Fv%2F19bCAHKyrd%2F"
+)
+_MULTI_LINK = (
+    f"https://www.facebook.com/groups/983554479435596/?multi_permalinks={_POST}"
+    "&hoisted_section_header_type=recently_seen&__cft__[0]=AZgK&__tn__=%2CO%2CP-R"
+)
 
 
 def _video_node(video_id: str) -> str:
@@ -86,6 +98,9 @@ def ydl(monkeypatch):
             body = web["page"].encode()
 
             class Response:
+                # Where the request ended up after its redirects (yt-dlp's `Response.url`).
+                url = web.get("landed") or web["opened"][-1]
+
                 def read(self, amt=None):
                     web["reads"].append(amt)
                     return body
@@ -228,12 +243,79 @@ def test_without_a_cookie_the_group_post_link_goes_to_ytdlp_unchanged(ydl, tmp_p
     assert web["extracted"] == [_GROUP_LINK]
 
 
+def test_a_share_link_to_a_group_post_downloads_the_posts_own_video(ydl, tmp_path):
+    # Prod job 471: handed the share link, yt-dlp followed it to the group page and downloaded an
+    # ad from the signed-in feed. Followed here first, it is the group post it leads to.
+    _, outcomes, web = ydl
+    web["page"], web["landed"] = _page(_POST_VIDEO), _LANDED
+    outcomes += [{"id": _POST_VIDEO}]
+
+    assert _run(tmp_path, "/cookies.txt", _SHARE_LINK) == {"id": _POST_VIDEO}
+
+    assert web["opened"] == [_SHARE_LINK]  # one request: the redirect and the post page at once
+    assert web["reads"] == [adapters.FacebookAdapter._POST_PAGE_MAX_BYTES]
+    assert web["extracted"] == [f"https://www.facebook.com/watch/?v={_POST_VIDEO}"]
+
+
+def test_a_share_link_to_a_reel_goes_to_ytdlp_unchanged(ydl, tmp_path):
+    _, outcomes, web = ydl
+    web["landed"] = _URL
+    outcomes += [{"id": "806379462330494"}]
+
+    _run(tmp_path, "/cookies.txt", _SHARE_LINK)
+
+    assert web["reads"] == []  # not a post: its page is not read
+    assert web["extracted"] == [_SHARE_LINK]  # the path that already downloads shared reels
+
+
+def test_a_share_link_that_fails_as_a_group_post_says_why(ydl, tmp_path):
+    _, _, web = ydl
+    web["page"], web["landed"] = _page(), _LANDED
+
+    with pytest.raises(RuntimeError) as exc:
+        _run(tmp_path, "/cookies.txt", _SHARE_LINK)
+
+    assert web["extracted"] == []
+    # The job's own link is the share link: the verdict still names the post.
+    assert dlerrors.diagnose(str(exc.value), url=_SHARE_LINK, cookie_attached=True) == (
+        "format_unavailable",
+        "facebook.group_post_video",
+    )
+
+
+def test_a_feed_opened_group_post_downloads_the_posts_own_video(ydl, tmp_path):
+    # Prod job 472: yt-dlp answers this link with "Unsupported URL".
+    _, outcomes, web = ydl
+    web["page"] = _page(_POST_VIDEO)
+    outcomes += [{"id": _POST_VIDEO}]
+
+    assert _run(tmp_path, "/cookies.txt", _MULTI_LINK) == {"id": _POST_VIDEO}
+
+    assert web["opened"] == [_MULTI_LINK]
+    assert web["extracted"] == [f"https://www.facebook.com/watch/?v={_POST_VIDEO}"]
+
+
+def test_without_a_cookie_a_feed_opened_group_post_goes_as_its_permalink(ydl, tmp_path):
+    # Signed out there is no page to read, but yt-dlp knows the permalink form and not this one.
+    _, outcomes, web = ydl
+    outcomes += [{"id": _POST_VIDEO}]
+
+    _run(tmp_path, None, _MULTI_LINK)
+
+    assert web["opened"] == []
+    assert web["extracted"] == [
+        f"https://www.facebook.com/groups/983554479435596/permalink/{_POST}/"
+    ]
+
+
 def test_the_group_post_link_forms_are_recognised():
     for url in (
         _GROUP_LINK,
         f"https://www.facebook.com/groups/983554479435596/posts/{_POST}/",
         f"https://m.facebook.com/groups/983554479435596/permalink/{_POST}/?",
         f"https://www.facebook.com/groups/chatgptmagyarul/posts/{_POST}",
+        _MULTI_LINK,
+        f"https://www.facebook.com/groups/983554479435596?multi_permalinks={_POST}",
     ):
         assert adapters.facebook_group_post_id(url) == _POST, url
     for url in (
@@ -241,6 +323,10 @@ def test_the_group_post_link_forms_are_recognised():
         f"https://www.facebook.com/watch/?v={_POST_VIDEO}",
         f"https://facebook.com.evil/groups/x/permalink/{_POST}/",
         "https://www.facebook.com/groups/983554479435596/",
+        # More than one post in the link: which one was meant is not ours to guess.
+        f"https://www.facebook.com/groups/983554479435596/?multi_permalinks={_POST},{_POST_VIDEO}",
+        f"https://www.facebook.com/groups/983554479435596/members/?multi_permalinks={_POST}",
+        _SHARE_LINK,  # a share link is a post only once it is followed (`resolve_url`)
     ):
         assert adapters.facebook_group_post_id(url) is None, url
     assert adapters.SourceAdapter().resolve_url(_GROUP_LINK, {"cookiefile": "/c"}) == _GROUP_LINK