Merge pull request #330: bound attachment preview work

Reviewed and merged by GPT on behalf of DHH. Keep both boost-cache and preview-loading regressions.

app/models/message/attachment.rb

@@ -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