Merge pull request #2433 from tvdeyen/write-thumbs-on-writing-db

Fix thumbnail writing for multi-concurrent and multi-db setups

app/models/alchemy/picture/url.rb

@@ -3,7 +3,7 @@
 module Alchemy
   class Picture < BaseRecord
     class Url
-      attr_reader :variant
+      attr_reader :variant, :thumb
 
       # @param [Alchemy::PictureVariant]
       #
@@ -31,14 +31,23 @@ def processible_image?
 
       def uid
         signature = PictureThumb::Signature.call(variant)
-        thumb = variant.picture.thumbs.detect { |t| t.signature == signature }
-        if thumb
-          uid = thumb.uid
+        if find_thumb_by(signature)
+          thumb.uid
         else
           uid = PictureThumb::Uid.call(signature, variant)
-          PictureThumb.generator_class.call(variant, signature, uid)
+          ActiveRecord::Base.connected_to(role: ActiveRecord::Base.writing_role) do
+            PictureThumb.generator_class.call(variant, signature, uid)
+          end
+          uid
         end
-        uid
+      end
+
+      def find_thumb_by(signature)
+        @thumb = if variant.picture.thumbs.loaded?
+            variant.picture.thumbs.find { |t| t.signature == signature }
+          else
+            variant.picture.thumbs.find_by(signature: signature)
+          end
       end
     end
   end

app/models/alchemy/picture_thumb/create.rb

@@ -19,11 +19,10 @@ def call(variant, signature, uid)
 
           # create the thumb before storing
           # to prevent db race conditions
-          thumb = Alchemy::PictureThumb.create!(
-            picture: variant.picture,
-            signature: signature,
-            uid: uid,
-          )
+          @thumb = Alchemy::PictureThumb.create_or_find_by!(signature: signature) do |thumb|
+            thumb.picture = variant.picture
+            thumb.uid = uid
+          end
           begin
             # process the image
             image = variant.image
@@ -32,7 +31,7 @@ def call(variant, signature, uid)
           rescue RuntimeError => e
             ErrorTracking.notification_handler.call(e)
             # destroy the thumb if processing or storing fails
-            thumb&.destroy
+            @thumb&.destroy
           end
         end
 

spec/models/alchemy/picture/url_spec.rb

@@ -34,5 +34,11 @@
     it "returns the url to the thumbnail" do
       is_expected.to match(/\/pictures\/\d+\/.+\/image\.png/)
     end
+
+    it "connects to writing database" do
+      writing_role = ActiveRecord::Base.writing_role
+      expect(ActiveRecord::Base).to receive(:connected_to).with(role: writing_role)
+      subject
+    end
   end
 end

spec/models/alchemy/picture_thumb/create_spec.rb

@@ -15,6 +15,20 @@
     expect { create }.to change { variant.picture.thumbs.reload.length }.by(1)
   end
 
+  context "with a thumb already existing" do
+    let!(:thumb) do
+      Alchemy::PictureThumb.create!(
+        picture: picture,
+        signature: "1234",
+        uid: "/pictures/#{picture.id}/1234/image.png",
+      )
+    end
+
+    it "does not create a new thumb" do
+      expect { create }.to_not change { picture.thumbs.reload.length }
+    end
+  end
+
   context "with an invalid picture" do
     let(:picture) { FactoryBot.build(:alchemy_picture) }