FEATURE: Allow post/topic thumbnails to be prioritized via markdown (#12044)

Previously we would always take the first image in a post to use as the
thumbnail. On media-heavy sites, users may want to manually select a
specific image as the topic thumbnail. This commit allows this to be
done via a `|thumbnail` attribute in markdown.

For example, in this case, bbb would be chosen as the thumbnail:

```
![alttext|100x100](upload://aaa)
![alttext|100x100|thumbnail](upload://bbb)
```

app/assets/javascripts/pretty-text/addon/allow-lister.js

@@ -190,6 +190,7 @@ export const DEFAULT_LIST = [
   "img[height]",
   "img[title]",
   "img[width]",
+  "img[data-thumbnail]",
   "ins",
   "kbd",
   "li",

app/assets/javascripts/pretty-text/addon/engines/discourse-markdown-it.js

@@ -177,7 +177,7 @@ function renderImageOrPlayableMedia(tokens, idx, options, env, slf) {
   const token = tokens[idx];
   const alt = slf.renderInlineAsText(token.children, options, env);
   const split = alt.split("|");
-  const altSplit = [];
+  const altSplit = [split[0]];
 
   // markdown-it supports returning HTML instead of continuing to render the current token
   // see https://github.com/markdown-it/markdown-it/blob/master/docs/architecture.md#renderer
@@ -195,7 +195,7 @@ function renderImageOrPlayableMedia(tokens, idx, options, env, slf) {
   }
 
   // parsing ![myimage|500x300]() or ![myimage|75%]() or ![myimage|500x300, 75%]
-  for (let i = 0, match, data; i < split.length; ++i) {
+  for (let i = 1, match, data; i < split.length; ++i) {
     if ((match = split[i].match(IMG_SIZE_REGEX)) && match[1] && match[2]) {
       let width = match[1];
       let height = match[2];
@@ -238,6 +238,8 @@ function renderImageOrPlayableMedia(tokens, idx, options, env, slf) {
       }
     } else if ((data = extractDataAttribute(split[i]))) {
       token.attrs.push(data);
+    } else if (split[i] === "thumbnail") {
+      token.attrs.push(["data-thumbnail", "true"]);
     } else {
       altSplit.push(split[i]);
     }

lib/cooked_post_processor.rb

@@ -483,14 +483,20 @@ class CookedPostProcessor
 
   def update_post_image
     upload = nil
-    eligible_image_fragments = extract_images_for_post
+    images = extract_images_for_post
 
-    # Loop through those fragments until we find one with an upload record
-    @post.each_upload_url(fragments: eligible_image_fragments) do |src, path, sha1|
+    @post.each_upload_url(fragments: images.css("[data-thumbnail]")) do |src, path, sha1|
       upload = Upload.find_by(sha1: sha1)
       break if upload
     end
 
+    if upload.nil? # No specified thumbnail. Use any image:
+      @post.each_upload_url(fragments: images.css(":not([data-thumbnail])")) do |src, path, sha1|
+        upload = Upload.find_by(sha1: sha1)
+        break if upload
+      end
+    end
+
     if upload.present?
       @post.update_column(:image_upload_id, upload.id) # post
       if @post.is_first_post? # topic

spec/components/cooked_post_processor_spec.rb

@@ -772,6 +772,19 @@ describe CookedPostProcessor do
         end
       end
 
+      it "prioritizes data-thumbnail images" do
+        upload1 = Fabricate(:image_upload, width: 1750, height: 2000)
+        upload2 = Fabricate(:image_upload, width: 1750, height: 2000)
+        post = Fabricate(:post, raw: <<~MD)
+          ![alttext|1750x2000](#{upload1.url})
+          ![alttext|1750x2000|thumbnail](#{upload2.url})
+        MD
+
+        CookedPostProcessor.new(post, disable_loading_image: true).post_process
+
+        expect(post.reload.image_upload_id).to eq(upload2.id)
+      end
+
       context "post image" do
         let(:reply) { Fabricate(:post_with_uploaded_image, post_number: 2) }
         let(:cpp) { CookedPostProcessor.new(reply) }