discord: add initial slash command integration.

Add initial support for the `/count <tags>` and `/posts <tags>` slash commands.

Slash commands are basically like webhooks; we register a command, and
when anybody types that command in Discord, Discord sends us a HTTP
request that we respond to.

* https://discord.com/developers/docs/interactions/slash-commands
* https://support.discord.com/hc/en-us/articles/1500000368501-Slash-Commands-FAQ

Gemfile

@@ -49,6 +49,7 @@ gem 'tzinfo-data'
 gem 'hsluv'
 gem 'google-cloud-bigquery', require: "google/cloud/bigquery"
 gem 'google-cloud-storage', require: "google/cloud/storage"
+gem 'ed25519'
 
 group :production, :staging do
   gem 'unicorn', :platforms => :ruby

Gemfile.lock

@@ -178,6 +178,7 @@ GEM
     dotenv-rails (2.7.6)
       dotenv (= 2.7.6)
       railties (>= 3.2)
+    ed25519 (1.2.4)
     erubi (1.10.0)
     factory_bot (6.1.0)
       activesupport (>= 5.0.0)
@@ -505,6 +506,7 @@ DEPENDENCIES
   diff-lcs
   dotenv-rails
   dtext_rb!
+  ed25519
   factory_bot
   ffaker
   flamegraph

app/controllers/webhooks_controller.rb

@@ -1,11 +1,16 @@
 class WebhooksController < ApplicationController
   skip_forgery_protection only: :receive
   rescue_with Stripe::SignatureVerificationError, status: 400
+  rescue_with DiscordSlashCommand::WebhookVerificationError, status: 401
 
   def receive
-    if params[:source] == "stripe"
+    case params[:source]
+    when "stripe"
       UserUpgrade.receive_webhook(request)
       head 200
+    when "discord"
+      json = DiscordSlashCommand.receive_webhook(request)
+      render json: json
     else
       head 400
     end

app/logical/discord_api_client.rb

@@ -0,0 +1,51 @@
+class DiscordApiClient
+  extend Memoist
+
+  BASE_URL = "https://discord.com/api/v8"
+
+  attr_reader :application_id, :guild_id, :bot_token, :http
+
+  def initialize(application_id: Danbooru.config.discord_application_client_id, guild_id: Danbooru.config.discord_guild_id, bot_token: Danbooru.config.discord_bot_token, http: Danbooru::Http.new)
+    @application_id = application_id
+    @guild_id = guild_id
+    @bot_token = bot_token
+    @http = http
+  end
+
+  def register_slash_command(name:, description:, options: [])
+    json = {
+      name: name,
+      description: description,
+      options: options
+    }
+
+    post("/applications/#{application_id}/guilds/#{guild_id}/commands", json)
+  end
+
+  def get_channel(channel_id)
+    get("/channels/#{channel_id}")
+  end
+
+  def me
+    get("/users/@me")
+  end
+
+  def get(url)
+    client.get("#{BASE_URL}/#{url}").parse
+  end
+
+  def post(url, data)
+    client.post("#{BASE_URL}/#{url}", json: data).parse
+  end
+
+  def client
+    headers = {
+      "User-Agent": "#{Danbooru.config.canonical_app_name} (#{Danbooru.config.source_code_url}, 1.0)",
+      "Authorization": "Bot #{bot_token}"
+    }
+
+    http.headers(headers)
+  end
+
+  memoize :client
+end

app/logical/discord_slash_command.rb

@@ -0,0 +1,117 @@
+class DiscordSlashCommand
+  class WebhookVerificationError < StandardError; end
+
+  COMMANDS = {
+    count: DiscordSlashCommand::CountCommand,
+    posts: DiscordSlashCommand::PostsCommand,
+  }
+
+  # https://discord.com/developers/docs/interactions/slash-commands#interaction-interactiontype
+  module InteractionType
+    Ping = 1
+    ApplicationCommand = 2
+  end
+
+  # https://discord.com/developers/docs/interactions/slash-commands#applicationcommandoptiontype
+  module ApplicationCommandOptionType
+    String = 3
+  end
+
+  attr_reader :data, :discord
+
+  # `data` is the the interaction data sent to us by Discord for the command.
+  # https://discord.com/developers/docs/interactions/slash-commands#interaction
+  def initialize(data: {}, discord: DiscordApiClient.new)
+    @data = data
+    @discord = discord
+  end
+
+  # The name of the slash command.
+  def name
+    raise NotImplementedError
+  end
+
+  # A description of the slash command.
+  def description
+    raise NotImplementedError
+  end
+
+  # The parameters of the slash command.
+  # https://discord.com/developers/docs/interactions/slash-commands#applicationcommandoption
+  def options
+    []
+  end
+
+  # Should return the response to the command.
+  def call
+    # respond_with("message")
+    raise NotImplementedError
+  end
+
+  concerning :HelperMethods do
+    # The parameters passed to the command. A hash.
+    def params
+      @params ||= data.dig(:data, :options).map do |opt|
+        [opt[:name], opt[:value]]
+      end.to_h.with_indifferent_access
+    end
+
+    # https://discord.com/developers/docs/interactions/slash-commands#responding-to-an-interaction
+    # https://discord.com/developers/docs/interactions/slash-commands#interaction-response
+    def respond_with(content = nil, type: 4, **options)
+      {
+        type: type,
+        data: {
+          content: content,
+          **options
+        }
+      }
+    end
+
+    # Register the command with the Discord API (replacing it if it already exists).
+    # https://discord.com/developers/docs/interactions/slash-commands#registering-a-command
+    def register_slash_command
+      discord.register_slash_command(name: name, description: description, options: options)
+    end
+  end
+
+  concerning :WebhookMethods do
+    class_methods do
+      # Called when we receive a command from Discord. Instantiates a
+      # DiscordSlashCommand and calls the `call` method.
+      # https://discord.com/developers/docs/interactions/slash-commands#interaction
+      def receive_webhook(request)
+        data = verify_request!(request)
+
+        case data[:type]
+        when InteractionType::Ping
+          { type: InteractionType::Ping }
+        when InteractionType::ApplicationCommand
+          name = data.dig(:data, :name)
+          klass = COMMANDS.fetch(name&.to_sym)
+          klass.new(data: data).call
+        else
+          raise NotImplementedError, "unknown Discord interaction type #{data[:type]}"
+        end
+      end
+
+      # https://discord.com/developers/docs/interactions/slash-commands#security-and-authorization
+      def verify_request!(request, public_key: Danbooru.config.discord_application_public_key)
+        timestamp = request.headers["X-Signature-Timestamp"].to_s
+        signature = request.headers["X-Signature-Ed25519"].to_s
+        signature_bytes = [signature].pack("H*")
+
+        body = request.body.read
+        message = timestamp + body
+
+        public_key_bytes = [public_key].pack("H*")
+        verify_key = Ed25519::VerifyKey.new(public_key_bytes)
+
+        verify_key.verify(signature_bytes, message)
+        JSON.parse(body).with_indifferent_access
+      rescue Ed25519::VerifyError
+        raise WebhookVerificationError
+      end
+    end
+  end
+end

app/logical/discord_slash_command/count_command.rb

@@ -0,0 +1,28 @@
+class DiscordSlashCommand
+  class CountCommand < DiscordSlashCommand
+    def name
+      "count"
+    end
+
+    def description
+      "Do a tag search and return the number of results"
+    end
+
+    def options
+      [{
+        name: "tags",
+        description: "The tags to search",
+        required: true,
+        type: ApplicationCommandOptionType::String
+      }]
+    end
+
+    def call
+      tags = params[:tags]
+      query = PostQueryBuilder.new(tags, User.anonymous).normalized_query
+      count = query.fast_count(estimate_count: true, skip_cache: true)
+
+      respond_with("`#{tags}`: #{count} posts")
+    end
+  end
+end

app/logical/discord_slash_command/posts_command.rb

@@ -0,0 +1,93 @@
+class DiscordSlashCommand
+  class PostsCommand < DiscordSlashCommand
+    extend Memoist
+
+    def name
+      "posts"
+    end
+
+    def description
+      "Do a tag search"
+    end
+
+    def options
+      [{
+        name: "tags",
+        description: "The tags to search",
+        required: true,
+        type: ApplicationCommandOptionType::String
+      }]
+    end
+
+    def call
+      tags = params[:tags]
+      query = PostQueryBuilder.new(tags, User.anonymous).normalized_query
+
+      limit = query.find_metatag(:limit) || 3
+      limit = limit.to_i.clamp(1, 10)
+      posts = query.build.paginate(1, limit: limit)
+      embeds = posts.map { |post| post_embed(post) }
+
+      respond_with(embeds: embeds)
+    end
+
+    def post_embed(post)
+      {
+        title: post.dtext_shortlink,
+        url: Routes.url_for(post),
+        timestamp: post.created_at.iso8601,
+        color: post_embed_color(post),
+        footer: post_embed_footer(post),
+        image: {
+          width: post.image_width,
+          height: post.image_height,
+          url: post_embed_image(post),
+        },
+      }
+    end
+
+    def post_embed_image(post, blur: 50)
+      if is_censored?(post)
+        nil
+      elsif post.file_ext.match?(/jpe?g|png|gif/)
+        post.file_url
+      else
+        post.preview_file_url
+      end
+    end
+
+    def post_embed_color(post)
+      if post.is_flagged?
+        0xC41C19
+      elsif post.is_pending?
+        0x0000FF
+      elsif post.parent_id.present?
+        0xC0C000
+      elsif post.has_active_children?
+        0x00FF00
+      elsif post.is_deleted?
+        0xFFFFFF
+      else
+        nil
+      end
+    end
+
+    def post_embed_footer(post)
+      dimensions = "#{post.image_width}x#{post.image_height}"
+      file_size = post.file_size.to_s(:human_size, precision: 4)
+      text = "Rating: #{post.rating.upcase} | #{dimensions} (#{file_size} #{post.file_ext})"
+
+      { text: text }
+    end
+
+    def is_censored?(post)
+      post.rating != "s" && !is_nsfw_channel?
+    end
+
+    def is_nsfw_channel?
+      discord.get_channel(data[:channel_id]).fetch("nsfw")
+    end
+
+    memoize :is_nsfw_channel?
+  end
+end

config/danbooru_default_config.rb

@@ -402,6 +402,29 @@ module Danbooru
     def discord_webhook_secret
     end
 
+    # Settings used for Discord slash commands.
+    #
+    # * Go to https://discord.com/developers/applications
+    # * Create an application.
+    # * Copy the client ID and public key.
+    # * Create a bot user.
+    # * Copy the bot token.
+    # * Go to the OAuth2 page, select the `bot` and `applications.commands`
+    #   scopes, and the `Administrator` permission, then follow the oauth2
+    #   link to add the bot to the Discord server.
+    def discord_application_client_id
+    end
+
+    def discord_application_public_key
+    end
+
+    def discord_bot_token
+    end
+
+    # The ID of the Discord server to register slash commands for.
+    def discord_guild_id
+    end
+
     # you should override this
     def email_key
       "zDMSATq0W3hmA5p3rKTgD"