The FB Sharing Debugger confirmed the real cause of the missing desktop-Messenger preview: the
scraper fetches og:image ASYNCHRONOUSLY on a URL's first scrape ("The provided 'og:image'
properties are not yet available… specify the dimensions using 'og:image:width'/'og:image:height'"),
so a freshly shared /watch link unfurled title-only. Read the poster JPEG's real dimensions from its
SOF marker (Pillow isn't a dep; the poster aspect differs from the video's, so the stored video
width/height won't do) and emit og:image:width/height. Only runs on a crawler hit. Unit-tested.
The iPad Messenger app unfurls a share link on-device (fetches + renders the OG tags itself),
but messenger.com desktop shows only what Facebook's server-side scraper cached — which is
pickier. Add og:image:secure_url (public HTTPS) and og:image:type=image/jpeg for our poster so
the card renders even when the scraper won't sniff the bytes. Reachability/stale-cache is a
separate, live-diagnosis step.
A remote thumbnail as og:image is unreliable for the crawler — Facebook's signed
CDN URL expires (so an old shared link's image later vanishes) and cross-origin
hotlinking is flaky. Record a self-hosted poster for EVERY download (ensure_poster
returns the existing <base>.jpg thumbnail sidecar, or cuts a frame) and point
og:image at our own /api/public/watch/{token}/poster.jpg. Backfill now fills
poster_path for all existing downloads, not just thumbnail-less ones.
Two visual gaps for non-catalog downloads:
- Channel link: YouTube already exposes channel_url; Facebook exposes none but a
numeric uploader_id that resolves at facebook.com/<id>. `_uploader_url` derives
it so the auto-detected channel renders as a real clickable link.
- Poster: a source with no thumbnail (e.g. a direct reddit HLS URL) showed a
blank image box. The worker now cuts a representative frame with ffmpeg
(`ensure_poster`) into the `<base>.jpg` sidecar and records `poster_path`
(migration 0050). The card, the public watch page (<video poster> + og:image),
and link previews fall back to it via new authed + public poster endpoints.
Adds `app.downloads.backfill` (one-off, re-run-safe) to fill uploader_url
(re-extract YouTube/Facebook metadata) and posters for pre-existing downloads.
A shared /watch/{token} link is a client-rendered SPA, so a social crawler only
saw the generic index.html — a blank link card. The server now injects per-video
Open Graph / Twitter tags (title, channel, thumbnail) into the served HTML for
that route, so links unfurl richly in Messenger and other chat apps; real
browsers ignore the extra tags and hydrate the page as usual. Password /
expired / invalid links fall back to the generic card with no metadata leak.
Also shortens the generic site description used for search engines and link
previews.
peter