Add a count preview to maintenance task UI

app/controllers/maintenance_tasks/application_controller.rb

@@ -20,6 +20,8 @@ class ApplicationController < MaintenanceTasks.parent_controller.constantize
       policy.script_src_elem(
         # <script> tag in app/views/layouts/maintenance_tasks/application.html.erb
         "'sha256-NiHKryHWudRC2IteTqmY9v1VkaDUA/5jhgXkMTkgo2w='",
+        # <script> tag in app/views/maintenance_tasks/tasks/show.html.erb
+        "'sha256-oCsB8YG3WI4aqJRWK/T7XfMAd3GEq+jhwDCOkSokj68='",
         # <script> tag for capybara-lockstep
         *capybara_lockstep_scripts,
       )

app/controllers/maintenance_tasks/tasks_controller.rb

@@ -25,6 +25,17 @@ def show
       )
     end
 
+    # Returns the estimated count of items for a Task as JSON.
+    def count
+      task_data = TaskDataShow.new(
+        params.fetch(:id),
+        arguments: params.except(:id, :controller, :action).permit!,
+      )
+      render(json: { count: task_data.count })
+    rescue StandardError
+      render(json: { count: nil })
+    end
+
     private
 
     def set_refresh

app/models/maintenance_tasks/task_data_show.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "timeout"
+
 module MaintenanceTasks
   # Class that represents the data related to a Task. Such information can be
   # sourced from a Task or from existing Run records for a Task that was since
@@ -11,6 +13,8 @@ module MaintenanceTasks
   #
   # @api private
   class TaskDataShow
+    TIMEOUT = 15
+
     # Initializes a Task Data with a name.
     #
     # @param name [String] the name of the Task subclass.
@@ -97,6 +101,11 @@ def csv_task?
       !deleted? && Task.named(name).has_csv_content?
     end
 
+    # @return [Boolean] whether the Task is collection-less.
+    def no_collection?
+      !deleted? && Task.named(name).no_collection?
+    end
+
     # @return [Array<String>] the names of parameters the Task accepts.
     def parameter_names
       if deleted?
@@ -106,6 +115,26 @@ def parameter_names
       end
     end
 
+    # @return [Integer] the count of items to be processed.
+    # @return [nil] if the count is unavailable (e.g. CSV tasks where the
+    #   collection depends on uploaded file content, tasks whose collection
+    #   requires arguments, or when the query times out).
+    def count
+      return if deleted?
+      return if csv_task?
+
+      task_instance = new
+      return if task_instance.nil?
+
+      Timeout.timeout(TIMEOUT) do
+        result = task_instance.count
+        result = task_instance.collection.count if result == :no_count
+        result if result.is_a?(Integer)
+      end
+    rescue StandardError
+      nil
+    end
+
     # @return [MaintenanceTasks::Task] an instance of the Task class.
     # @return [nil] if the Task file was deleted.
     def new

app/views/maintenance_tasks/tasks/show.html.erb

@@ -26,6 +26,11 @@
       </div>
     <% end %>
     <%= render "maintenance_tasks/tasks/custom", form: form %>
+    <% unless @task.deleted? || @task.csv_task? || @task.no_collection? %>
+      <div id="task-count" data-url="<%= count_task_path(@task) %>" class="block">
+        <p class="has-text-grey">Estimating expected items count...</p>
+      </div>
+    <% end %>
     <div class="block">
       <%= form.submit 'Run', class: "button is-success is-rounded mb-4 has-text-white-ter", disabled: @task.deleted? %>
     </div>
@@ -61,3 +66,67 @@
     <%= link_to "Next page", task_path(@task, cursor: @task.runs_page.next_cursor) unless @task.runs_page.last? %>
   <% end %>
 <% end %>
+
+<% unless @task.deleted? || @task.csv_task? || @task.no_collection? %>
+<script>
+(function() {
+  var countEl = document.getElementById('task-count');
+  if (!countEl) return;
+
+  var form = document.querySelector('form');
+  if (!form) return;
+
+  var url = countEl.dataset.url;
+  var abortCtrl;
+
+  function serializeParams() {
+    var params = new URLSearchParams();
+    new FormData(form).forEach(function(value, key) {
+      var m = key.match(/^task\[(.+)\]$/);
+      if (m) params.set(m[1], value);
+    });
+    return params.toString();
+  }
+
+  var lastParams = serializeParams();
+
+  function fetchCount() {
+    if (abortCtrl) abortCtrl.abort();
+    abortCtrl = new AbortController();
+
+    var p = countEl.querySelector('p');
+    p.textContent = 'Estimating expected items count...';
+    countEl.classList.remove('is-hidden');
+
+    var serialized = serializeParams();
+    lastParams = serialized;
+
+    fetch(url + '?' + serialized, { signal: abortCtrl.signal })
+      .then(function(r) { return r.json(); })
+      .then(function(data) {
+        if (data.count != null) {
+          p.textContent = '';
+          var strong = document.createElement('strong');
+          strong.textContent = data.count.toLocaleString();
+          p.appendChild(strong);
+          p.appendChild(document.createTextNode(
+            ' ' + (data.count === 1 ? 'item' : 'items') + ' expected to be processed'
+          ));
+          countEl.classList.remove('is-hidden');
+        } else {
+          countEl.classList.add('is-hidden');
+        }
+      })
+      .catch(function() {});
+  }
+
+  function fetchCountIfChanged() {
+    if (serializeParams() !== lastParams) fetchCount();
+  }
+
+  fetchCount();
+  form.addEventListener('focusout', fetchCountIfChanged);
+  form.addEventListener('change', fetchCount);
+})();
+</script>
+<% end %>

config/routes.rb

@@ -2,6 +2,9 @@
 
 MaintenanceTasks::Engine.routes.draw do
   resources :tasks, only: [:index, :show], format: false do
+    member do
+      get :count
+    end
     resources :runs, only: [:create], format: false do
       member do
         post "pause"

test/models/maintenance_tasks/task_data_show_test.rb

@@ -91,6 +91,18 @@ class TaskDataShowTest < ActiveSupport::TestCase
       refute_predicate TaskDataShow.new("Maintenance::DoesNotExist"), :csv_task?
     end
 
+    test "#no_collection? returns true for a no-collection Task" do
+      assert_predicate TaskDataShow.new("Maintenance::NoCollectionTask"), :no_collection?
+    end
+
+    test "#no_collection? returns false for a Task with a collection" do
+      refute_predicate TaskDataShow.new("Maintenance::UpdatePostsTask"), :no_collection?
+    end
+
+    test "#no_collection? returns false if the Task is deleted" do
+      refute_predicate TaskDataShow.new("Maintenance::DoesNotExist"), :no_collection?
+    end
+
     test "#refresh? returns true if there are active runs" do
       assert_predicate TaskDataShow.new("Maintenance::UpdatePostsTask"), :refresh?
     end
@@ -153,5 +165,54 @@ class TaskDataShowTest < ActiveSupport::TestCase
       task_data = TaskDataShow.prepare("Maintenance::ParamsTask", arguments: { unknown: nil })
       assert_nothing_raised { task_data.new }
     end
+
+    test "#count returns the count for a Task with a count override" do
+      task_data = TaskDataShow.new("Maintenance::UpdatePostsThrottledTask")
+      assert_equal Post.count, task_data.count
+    end
+
+    test "#count returns the count for a custom enumerating Task" do
+      task_data = TaskDataShow.new("Maintenance::CustomEnumeratingTask")
+      assert_equal 3, task_data.count
+    end
+
+    test "#count falls back to collection count for a Task without a count override" do
+      task_data = TaskDataShow.new("Maintenance::TestTask")
+      assert_equal 2, task_data.count
+    end
+
+    test "#count returns nil when collection count errors" do
+      task_data = TaskDataShow.new("Maintenance::ParamsTask")
+      assert_nil task_data.count
+    end
+
+    test "#count returns the count for a no-collection Task" do
+      task_data = TaskDataShow.new("Maintenance::NoCollectionTask")
+      assert_equal 1, task_data.count
+    end
+
+    test "#count returns nil for a CSV Task" do
+      task_data = TaskDataShow.new("Maintenance::ImportPostsTask")
+      assert_nil task_data.count
+    end
+
+    test "#count returns nil for a deleted Task" do
+      task_data = TaskDataShow.new("Maintenance::DoesNotExist")
+      assert_nil task_data.count
+    end
+
+    test "#count returns nil when collection count times out" do
+      task_data = TaskDataShow.new("Maintenance::UpdatePostsThrottledTask")
+      Task.named("Maintenance::UpdatePostsThrottledTask").any_instance.stubs(:count).raises(Timeout::Error)
+      assert_nil task_data.count
+    end
+
+    test "#count returns the count for a Task with arguments" do
+      task_data = TaskDataShow.new(
+        "Maintenance::ParamsTask",
+        arguments: { post_ids: Post.first.id.to_s },
+      )
+      assert_equal 1, task_data.count
+    end
   end
 end

test/system/maintenance_tasks/tasks_test.rb

@@ -258,6 +258,32 @@ class TasksTest < ApplicationSystemTestCase
       assert_button "Run", disabled: true
     end
 
+    test "show a Task with count indicator" do
+      visit maintenance_tasks.task_path("Maintenance::UpdatePostsThrottledTask")
+      assert_text "items expected to be processed"
+    end
+
+    test "count updates dynamically when params change" do
+      visit maintenance_tasks.task_path("Maintenance::ParamsTask")
+      assert_selector "#task-count:not(.is-hidden)"
+
+      fill_in "task[post_ids]", with: Post.first.id.to_s
+      find_field("task[post_ids]").send_keys(:tab)
+      within("#task-count") do
+        assert_text "1 item expected to be processed"
+      end
+    end
+
+    test "show a Task without count indicator" do
+      visit maintenance_tasks.task_path("Maintenance::ImportPostsTask")
+      assert_no_text "expected to be processed"
+    end
+
+    test "show a no-collection Task without count indicator" do
+      visit maintenance_tasks.task_path("Maintenance::NoCollectionTask")
+      assert_no_selector "#task-count"
+    end
+
     test "visit main page through iframe" do
       visit root_path