DEV: Introduce Auth::ManagedAuthenticator

A generic implementation of Auth::Authenticator which stores data in the
new UserAssociatedAccount model. This should help significantly reduce the duplicated
logic across different auth providers.

lib/auth/managed_authenticator.rb

@@ -0,0 +1,105 @@
+class Auth::ManagedAuthenticator < Auth::Authenticator
+  def description_for_user(user)
+    info = UserAssociatedAccount.find_by(provider_name: name, user_id: user.id)&.info
+    return "" if info.nil?
+    info["email"] || info["nickname"] || info["name"] || ""
+  end
+
+  # These three methods are designed to be overriden by child classes
+  def match_by_email
+    true
+  end
+
+  def can_revoke?
+    true
+  end
+
+  def can_connect_existing_user?
+    true
+  end
+
+  def revoke(user, skip_remote: false)
+    association = UserAssociatedAccount.find_by(provider_name: name, user_id: user.id)
+    raise Discourse::NotFound if association.nil?
+    association.destroy!
+    true
+  end
+
+  def after_authenticate(auth_token, existing_account: nil)
+    result = Auth::Result.new
+
+    # Store all the metadata for later, in case the `after_create_account` hook is used
+    result.extra_data = {
+      provider: auth_token[:provider],
+      uid: auth_token[:uid],
+      info: auth_token[:info],
+      extra: auth_token[:extra],
+      credentials: auth_token[:credentials]
+    }
+
+    # Build the Auth::Result object
+    info = auth_token[:info]
+    result.email = email = info[:email]
+    result.name = name = "#{info[:first_name]} #{info[:last_name]}"
+    result.username = info[:nickname]
+
+    # Try and find an association for this account
+    association = UserAssociatedAccount.find_by(provider_name: auth_token[:provider], provider_uid: auth_token[:uid])
+    result.user = association&.user
+
+    # Reconnecting to existing account
+    if can_connect_existing_user? && existing_account && (association.nil? || existing_account.id != association.user_id)
+      association.destroy! if association
+      association = nil
+      result.user = existing_account
+    end
+
+    # Matching an account by email
+    if match_by_email && association.nil? && (user = User.find_by_email(email))
+      UserAssociatedAccount.where(user: user, provider_name: auth_token[:provider]).destroy_all # Destroy existing associations for the new user
+      result.user = user
+    end
+
+    # Add the association to the database if it doesn't already exist
+    if association.nil? && result.user
+      association = create_association!(result.extra_data.merge(user: result.user))
+    end
+
+    # Update all the metadata in the association:
+    if association
+      association.update!(
+        info: auth_token[:info] || {},
+        credentials: auth_token[:credentials] || {},
+        extra: auth_token[:extra] || {}
+      )
+      retrieve_avatar(result.user, auth_token[:info][:image])
+    end
+
+    result.email_valid = true if result.email
+
+    result
+  end
+
+  def create_association!(hash)
+    association = UserAssociatedAccount.create!(
+      user: hash[:user],
+      provider_name: hash[:provider],
+      provider_uid: hash[:uid],
+      info: hash[:info] || {},
+      credentials: hash[:credentials] || {},
+      extra: hash[:extra] || {}
+    )
+  end
+
+  def after_create_account(user, auth)
+    data = auth[:extra_data]
+    create_association!(data.merge(user: user))
+    retrieve_avatar(user, data["info"]["image"])
+  end
+
+  def retrieve_avatar(user, url)
+    return unless user && url
+    return if user.user_avatar.try(:custom_upload_id).present?
+    Jobs.enqueue(:download_avatar_from_url, url: url, user_id: user.id, override_gravatar: false)
+  end
+end