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Bound the work of previewing an attachment

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A video's preview and a picture's thumbnail are made inside the request that posts the message, and
nothing bounded how long either could take.

- The video preview filter also selects any frame from 5 seconds on. Rails' filter takes the second
  frame it selects, which a video with a single keyframe and no scene change only gives at its end, so
  ffmpeg decoded all of it.
- TimeLimitedVideoPreviewer gives ffmpeg 10 seconds of wall-clock time, kills it past that, and reports
  a failed preview, so the message is posted without one.
- Pictures and videos above 250 megapixels, or whose size couldn't be read, get no preview: decoding
  costs in proportion to the pixels, however small the file.
- The view shows a preview only if it was made when the message was posted. Its URL used to make it on
  view, so a preview that failed or was skipped would be attempted again on every view. The cached
  presentation's version goes up, so cached messages pick this up.
- A video's poster is made, when the message is posted, at the size the view shows it. The full-size WebP
  made until now wasn't shown anywhere, and encoding it costs in proportion to the frame's pixels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bj8KnxpTf9sj2Ysa8aLAVa
This commit is contained in:
Marcello Costagliola
2026-10-06 15:12:18 +02:00
parent 91036c5085
commit 9912e63d69
8 changed files with 186 additions and 5 deletions
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app/models/message/attachment.rb
+16 -1
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@@ -4,9 +4,14 @@ module Message::Attachment
THUMBNAIL_MAX_WIDTH = 1200
THUMBNAIL_MAX_HEIGHT = 800
# Decoding a picture, or a video's frame, costs in proportion to its pixels, however small the file. The largest
# phone photos have 200 million.
THUMBNAIL_MAX_PIXELS = 250_000_000
included do
has_one_attached :attachment do |attachable|
attachable.variant :thumb, resize_to_limit: [ THUMBNAIL_MAX_WIDTH, THUMBNAIL_MAX_HEIGHT ]
attachable.variant :poster, format: :webp, resize_to_limit: [ THUMBNAIL_MAX_WIDTH, THUMBNAIL_MAX_HEIGHT ]
end
end
@@ -32,13 +37,23 @@ module Message::Attachment
# A file that ffmpeg or libvips can't decode is still the message: post it without a preview.
def process_attachment_thumbnail
return if too_many_pixels_to_preview?
case
when attachment.video?
attachment.preview(format: :webp).processed
attachment.preview(:poster).processed
when attachment.representable?
attachment.representation(:thumb).processed
end
rescue ActiveStorage::PreviewError, Vips::Error => error
Rails.logger.warn "Posted #{attachment.filename} without a preview: #{error.class}: #{error.message.lines.first&.chomp}"
end
# Without a size, the analyzer couldn't read the file's header, and the previewer wouldn't either.
def too_many_pixels_to_preview?
if attachment.image? || attachment.video?
width, height = attachment.metadata.values_at(:width, :height)
width.nil? || height.nil? || width * height > THUMBNAIL_MAX_PIXELS
end
end
end