FIX: Sharing to Discord forum channels requires a thread_name (#251)

Discourse is able to share topics and replies to normal Discord channels, but it doesn't work with forum channels, which require an additional `thread_name` parameter.

This change fixes the issue by checking for the specific error the Discord returns when trying to share to a forum channel, adding the `thread_name`, then trying again.

This is intentionally a minimal fix, additional work would be required to make the sharing more closely fit the style of Discord forum channels.

lib/discourse_chat_integration/provider/discord/discord_provider.rb

@@ -88,6 +88,16 @@ module DiscourseChatIntegration
         message = generate_discord_message(post)
         response = send_message(webhook_url, message)
 
+        # If the message fails to send, it might be because it's a forum channel.
+        if !response.kind_of?(Net::HTTPSuccess)
+          error = JSON.parse(response.body)
+          # Error code 220001 occurs when trying to post to a forum channel without a thread_name.
+          if error["code"] == 220_001
+            message[:thread_name] = message[:embeds][0][:title]
+            response = send_message(webhook_url, message)
+          end
+        end
+
         if !response.kind_of?(Net::HTTPSuccess)
           raise ::DiscourseChatIntegration::ProviderError.new(
                   info: {

spec/lib/discourse_chat_integration/provider/discord/discord_provider_spec.rb

@@ -41,6 +41,7 @@ RSpec.describe DiscourseChatIntegration::Provider::DiscordProvider do
       stub1 =
         stub_request(:post, "https://discord.com/api/webhooks/1234/abcd?wait=true").to_return(
           status: 400,
+          body: '{"message": "This is an error!", "code": 400}',
         )
       expect(stub1).to have_been_requested.times(0)
       expect { described_class.trigger_notification(post, chan1, nil) }.to raise_exception(
@@ -48,6 +49,20 @@ RSpec.describe DiscourseChatIntegration::Provider::DiscordProvider do
       )
       expect(stub1).to have_been_requested.once
     end
+
+    it "handles posting to forum channels" do
+      stub1 =
+        stub_request(:post, "https://discord.com/api/webhooks/1234/abcd?wait=true")
+          .with { |request| !JSON.parse(request.body)["thread_name"].present? }
+          .to_return(status: 400, body: '{"message": "This is an error!", "code": 220001}')
+      stub2 =
+        stub_request(:post, "https://discord.com/api/webhooks/1234/abcd?wait=true")
+          .with { |request| JSON.parse(request.body)["thread_name"].present? }
+          .to_return(status: 200)
+      described_class.trigger_notification(post, chan1, nil)
+      expect(stub1).to have_been_requested.once
+      expect(stub2).to have_been_requested.once
+    end
   end
 
   describe ".get_channel_by_name" do