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expand a preview card for any link, not just YouTube

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Bluesky renders no link card of its own. Its PDS never fetches the page
behind a URL, so a post whose record has no app.bsky.embed.external
shows the link as bare text; every client that shows a card builds it
before posting. We only built one for YouTube, so every other link
arrived on bluesky with nothing attached.

preview.go now reads the Open Graph tags of an ordinary page and turns
them into the same card: og:title and og:description, falling back to
the twitter:* tags and then to <title> and meta description, with
og:image fetched as the thumbnail. YouTube keeps its oEmbed path, which
answers with the handful of fields a card needs rather than the megabyte
of markup the watch page is.

Only the head of a page is read, and parsing stops at <body>, since
preview tags belong above it and a truncated page still yields what was
read. Pages are decoded through x/net/html/charset rather than assumed
to be UTF-8, which Japanese pages served as Shift_JIS are not. The
thumbnail is resolved against the URL the body came from, so a relative
og:image survives a redirect. Requests now name the bot in a User-Agent,
which some sites want before serving preview tags at all.

fetch grew a byte limit and now reports the media type and final URL its
body came with, which is what the thumbnail needs to name and resolve
itself.

Checked against go.dev, Japanese Wikipedia and a YouTube video.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
sirrow
2026-08-03 09:32:12 +09:00
parent e2f8036682
commit 953664ff1c
5 changed files with 363 additions and 62 deletions
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2
go.mod
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@@ -6,6 +6,7 @@ require (
github.com/bluesky-social/indigo v0.0.0-20250313000755-d9a74f690c90
github.com/bwmarrin/discordgo v0.28.1
golang.org/x/image v0.43.0
golang.org/x/net v0.23.0
)
require (
@@ -51,6 +52,7 @@ require (
go.uber.org/atomic v1.11.0 // indirect
go.uber.org/multierr v1.11.0 // indirect
go.uber.org/zap v1.26.0 // indirect
golang.org/x/text v0.38.0 // indirect
golang.org/x/xerrors v0.0.0-20231012003039-104605ab7028 // indirect
lukechampine.com/blake3 v1.2.1 // indirect
)