Hand Facebook photo posts to gallery-dl as post links

CHANGELOG.md

@@ -41,6 +41,10 @@ cookie file keeps working.
   URL, so page and scripts always fit together.
 - **Fix: a partial pack after a login error reported the cookies as fine.** The cookie state now
   comes from the download, not from the pack.
+- **Fix: Facebook posts with pictures failed.** A shared post resolves to `story.php`, which
+  gallery-dl took for a profile and failed on, after yt-dlp found no video in it. gallery-dl now gets
+  such a post (and a `permalink.php` link) as `<id>/posts/<story_fbid>` and downloads the post's own
+  pictures; yt-dlp keeps the original link for videos.
 - **Hardening:** a host that only ends with `instagram.com` or `facebook.com`, such as
   `evilinstagram.com`, no longer counts as Instagram or Facebook. The host allowlist already refused
   such links.

app/fetch.py

@@ -12,12 +12,14 @@
 
 import json
 import logging
+import re
 import shutil
 import subprocess
 import sys
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from pathlib import Path
+from urllib.parse import parse_qsl, urlsplit
 
 from .config import Settings
 from .cookies import for_download
@@ -171,6 +173,28 @@ def _first(data: dict, *keys: str) -> str:
 
 # --------------------------------------------------------------------- gallery-dl
 
+FB_POST_PATHS = {"/story.php", "/permalink.php"}
+FB_STORY_ID = re.compile(r"[0-9]+|pfbid[0-9A-Za-z]+")
+FB_OWNER_ID = re.compile(r"[0-9]+")
+
+
+def gallery_dl_url(target: Target) -> str:
+    """The link gallery-dl gets. A Facebook post resolves to story.php?story_fbid=...&id=...,
+    which gallery-dl reads as a profile named "story.php" and fails on; a permalink.php link it
+    takes for the page's whole photo stream. As <id>/posts/<story_fbid> it opens the post and
+    gets the post's own pictures. yt-dlp keeps the original link, which works for videos."""
+    if target.platform != "facebook":
+        return target.url
+    parts = urlsplit(target.url)
+    if parts.path not in FB_POST_PATHS:
+        return target.url
+    query = dict(parse_qsl(parts.query))
+    story, owner = query.get("story_fbid", ""), query.get("id", "")
+    if FB_STORY_ID.fullmatch(story) and FB_OWNER_ID.fullmatch(owner):
+        return f"https://www.facebook.com/{owner}/posts/{story}"
+    return target.url
+
+
 def run_gallery_dl(target: Target, raw: Path, cookies: Path | None, settings: Settings) -> FetchResult:
     # The tool runs inside raw, so every path it gets must be absolute.
     raw = raw.resolve()
@@ -186,7 +210,7 @@ def run_gallery_dl(target: Target, raw: Path, cookies: Path | None, settings: Se
         cmd += ["--config", str(config)]
     if cookies:
         cmd += ["--cookies", str(cookies.resolve())]
-    cmd.append(target.url)
+    cmd.append(gallery_dl_url(target))
 
     proc = _run(cmd, cwd=raw)
     files = _media_files(raw)

tests/test_fetch.py

@@ -154,6 +154,55 @@ def test_complete_first_tool_needs_no_fallback(tools, tmp_path, with_cookies):
     assert [c["tool"] for c in tools.calls()] == ["gallery-dl"]
 
 
+# ------------------------------------------------------------- Facebook photo posts
+
+# A shared facebook.com/share/p/... link, resolved and normalised (app/urls.py).
+FACEBOOK_POST = Target(
+    "https://www.facebook.com/story.php?story_fbid=1128847296531599&id=100082190168714"
+    "&post_id=100082190168714_1128847296531599",
+    "facebook", "post", "1128847296531599",
+)
+
+
+def test_a_facebook_photo_post_reaches_gallery_dl_as_a_post_link(tools, tmp_path, settings):
+    # yt-dlp finds no video in a post with a picture. gallery-dl reads story.php as a profile
+    # named "story.php" and fails; as <id>/posts/<story_fbid> it gets the post's own pictures.
+    tools.set("yt-dlp", exit=1, stderr="ERROR: [facebook] 1128847296531599: No video formats found!\n")
+    tools.set("gallery-dl", files=1)
+    result = fetch(FACEBOOK_POST, tmp_path / "job", settings)
+    assert (result.tool, len(result.items)) == ("gallery-dl", 1)
+    calls = tools.calls()
+    assert [c["tool"] for c in calls] == ["yt-dlp", "gallery-dl"]
+    assert calls[0]["args"][-1] == FACEBOOK_POST.url  # yt-dlp keeps the link that works for videos
+    assert calls[1]["args"][-1] == "https://www.facebook.com/100082190168714/posts/1128847296531599"
+
+
+@pytest.mark.parametrize("url,expected", [
+    ("https://www.facebook.com/story.php?story_fbid=1128847296531599&id=100082190168714",
+     "https://www.facebook.com/100082190168714/posts/1128847296531599"),
+    # permalink.php would give gallery-dl the page's whole photo stream instead of the post.
+    ("https://www.facebook.com/permalink.php?story_fbid=1705099180979984&id=100044397550724",
+     "https://www.facebook.com/100044397550724/posts/1705099180979984"),
+    ("https://www.facebook.com/story.php?story_fbid=pfbid02AbCdEf123&id=100082190168714",
+     "https://www.facebook.com/100082190168714/posts/pfbid02AbCdEf123"),
+    ("https://www.facebook.com/story.php?id=100082190168714&story_fbid=1128847296531599&post_id=x",
+     "https://www.facebook.com/100082190168714/posts/1128847296531599"),
+    # Anything else stays as it is.
+    ("https://www.facebook.com/story.php?story_fbid=1128847296531599", None),
+    ("https://www.facebook.com/story.php?story_fbid=1128847296531599&id=../../evil", None),
+    ("https://www.facebook.com/story.php?story_fbid=11%2F28&id=100082190168714", None),
+    ("https://www.facebook.com/reel/954162721072206/", None),
+    ("https://www.facebook.com/photo/?fbid=1128847296531599&set=a.1", None),
+])
+def test_the_link_gallery_dl_gets_for_a_facebook_post(url, expected):
+    target = Target(url, "facebook", "post", "key")
+    assert fetch_module.gallery_dl_url(target) == (expected or url)
+
+
+def test_instagram_links_reach_gallery_dl_unchanged():
+    assert fetch_module.gallery_dl_url(INSTAGRAM) == INSTAGRAM.url
+
+
 # ------------------------------------------------------------------------ paths
 
 @pytest.mark.parametrize("target,tool", [(INSTAGRAM, "gallery-dl"), (FACEBOOK, "yt-dlp")])