Fix #5365: Don't allow whitespace-only text submission.

Fix bug where it was possible to submit blank text in various text fields.

Caused by `String#blank?` not considering certain Unicode characters as blank. `blank?` is defined
as `match?(/\A[[:space:]]*\z/)`, where `[[:space:]]` matches ASCII spaces (space, tab, newline, etc)
and Unicode characters in the Space category ([1]). However, there are other space-like characters
not in the Space category. This includes U+200B (Zero-Width Space), and many more.

It turns out the "Default ignorable code points" [2][3] are what we're after. These are the set of 400
or so formatting and control characters that are invisible when displayed.

Note that there are other control characters that aren't invisible when rendered, instead they're
shown with a placeholder glyph. These include the ASCII C0 and C1 control codes [4], certain Unicode
control characters [5], and unassigned, reserved, and private use codepoints.

There is one outlier: the Braille pattern blank (U+2800) [6]. This character is visually blank, but is
not considered to be a space or an ignorable code point.

[1]: https://codepoints.net/search?gc[]=Z
[2]: https://codepoints.net/search?DI=1
[3]: https://www.unicode.org/review/pr-5.html
[4]: https://codepoints.net/search?gc[]=Cc
[5]: https://codepoints.net/search?gc[]=Cf
[6]: https://codepoints.net/U+2800
[7]: https://en.wikipedia.org/wiki/Whitespace_character
[8]: https://character.construction/blanks
[9]: https://invisible-characters.com

app/logical/visible_string_validator.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+# A custom validator for ensuring a string isn't blank. Like the `presence` validator, but checks for certain invisible
+# Unicode characters such as zero-width spaces too.
+#
+# @example
+#   validates :body, visible_string: true
+#
+# @see https://invisible-characters.com/
+# @see https://guides.rubyonrails.org/active_record_validations.html#presence
+# @see https://guides.rubyonrails.org/active_record_validations.html#custom-validators
+class VisibleStringValidator < ActiveModel::EachValidator
+  def validate_each(record, attr, string)
+    return if options[:allow_empty] && string.empty?
+
+    if string.nil? || string.invisible?
+      record.errors.add(attr, "can't be blank")
+    end
+  end
+end

app/models/ban.rb

@@ -14,7 +14,7 @@ class Ban < ApplicationRecord
 
   validates :duration, presence: true
   validates :duration, inclusion: { in: [1.day, 3.days, 1.week, 1.month, 3.months, 6.months, 1.year, 100.years], message: "%{value} is not a valid ban duration" }, if: :duration_changed?
-  validates :reason, presence: true
+  validates :reason, visible_string: true
   validate :user, :validate_user_is_bannable, on: :create
 
   scope :unexpired, -> { where("bans.created_at + bans.duration > ?", Time.zone.now) }

app/models/bulk_update_request.rb

@@ -10,9 +10,9 @@ class BulkUpdateRequest < ApplicationRecord
   belongs_to :forum_post, optional: true
   belongs_to :approver, optional: true, class_name: "User"
 
-  validates :reason, presence: true, on: :create
-  validates :script, presence: true
-  validates :title, presence: true, if: ->(rec) { rec.forum_topic_id.blank? }
+  validates :reason, visible_string: true, on: :create
+  validates :script, visible_string: true
+  validates :title, visible_string: true, if: ->(rec) { rec.forum_topic_id.blank? }
   validates :forum_topic, presence: true, if: ->(rec) { rec.forum_topic_id.present? }
   validates :status, inclusion: { in: STATUSES }
   validate :validate_script, if: :script_changed?

app/models/comment.rb

@@ -13,7 +13,7 @@ class Comment < ApplicationRecord
   has_many :active_votes, -> { active }, class_name: "CommentVote"
   has_many :mod_actions, as: :subject, dependent: :destroy
 
-  validates :body, presence: true, length: { maximum: 15_000 }, if: :body_changed?
+  validates :body, visible_string: true, length: { maximum: 15_000 }, if: :body_changed?
 
   before_create :autoreport_spam
   before_save :handle_reports_on_deletion

app/models/dmail.rb

@@ -4,8 +4,8 @@ class Dmail < ApplicationRecord
   attr_accessor :creator_ip_addr, :disable_email_notifications
 
   validate :validate_sender_is_not_limited, on: :create
-  validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
-  validates :body, presence: true, length: { maximum: 50_000 }, if: :body_changed?
+  validates :title, visible_string: true, length: { maximum: 200 }, if: :title_changed?
+  validates :body, visible_string: true, length: { maximum: 50_000 }, if: :body_changed?
 
   belongs_to :owner, :class_name => "User"
   belongs_to :to, :class_name => "User"

app/models/favorite_group.rb

@@ -5,7 +5,7 @@ class FavoriteGroup < ApplicationRecord
 
   before_validation :normalize_name
 
-  validates :name, presence: true
+  validates :name, visible_string: true
   validates :name, uniqueness: { case_sensitive: false, scope: :creator_id }
   validate :validate_name, if: :name_changed?
   validate :creator_can_create_favorite_groups, :on => :create

app/models/forum_post.rb

@@ -16,7 +16,7 @@ class ForumPost < ApplicationRecord
   has_one :tag_implication
   has_one :bulk_update_request
 
-  validates :body, presence: true, length: { maximum: 200_000 }, if: :body_changed?
+  validates :body, visible_string: true, length: { maximum: 200_000 }, if: :body_changed?
   validate :validate_deletion_of_original_post
   validate :validate_undeletion_of_post
 

app/models/forum_topic.rb

@@ -27,7 +27,7 @@ class ForumTopic < ApplicationRecord
   has_many :tag_implications
   has_many :mod_actions, as: :subject, dependent: :destroy
 
-  validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
+  validates :title, visible_string: true, length: { maximum: 200 }, if: :title_changed?
   validates :category_id, inclusion: { in: CATEGORIES.keys }
   validates :min_level, inclusion: { in: MIN_LEVELS.values }
 

app/models/ip_ban.rb

@@ -7,7 +7,7 @@ class IpBan < ApplicationRecord
   has_many :mod_actions, as: :subject, dependent: :destroy
 
   validate :validate_ip_addr
-  validates :reason, presence: true
+  validates :reason, visible_string: true
 
   after_save :create_mod_action
 

app/models/moderation_report.rb

@@ -10,7 +10,7 @@ class ModerationReport < ApplicationRecord
   has_many :mod_actions, as: :subject, dependent: :destroy
 
   before_validation(on: :create) { model.lock! }
-  validates :reason, presence: true
+  validates :reason, visible_string: true
   validates :model_type, inclusion: { in: MODEL_TYPES }
   validates :creator, uniqueness: { scope: [:model_type, :model_id], message: "have already reported this message." }, on: :create
 

app/models/note.rb

@@ -11,7 +11,7 @@ class Note < ApplicationRecord
   validates :y, presence: true
   validates :width, presence: true
   validates :height, presence: true
-  validates :body, presence: true
+  validates :body, visible_string: true
   validate :note_within_image
   after_save :update_post
   after_save :create_version

app/models/pool.rb

@@ -6,7 +6,7 @@ class Pool < ApplicationRecord
 
   array_attribute :post_ids, parse: /\d+/, cast: :to_i
 
-  validates :name, uniqueness: { case_sensitive: false }, if: :name_changed?
+  validates :name, visible_string: true, uniqueness: { case_sensitive: false }, if: :name_changed?
   validate :validate_name, if: :name_changed?
   validates :category, inclusion: { in: %w[series collection] }
   validate :updater_can_edit_deleted

app/models/post_appeal.rb

@@ -4,7 +4,7 @@ class PostAppeal < ApplicationRecord
   belongs_to :creator, :class_name => "User"
   belongs_to :post
 
-  validates :reason, length: { maximum: 140 }
+  validates :reason, visible_string: { allow_empty: true }, length: { maximum: 140 }
   validate :validate_post_is_appealable, on: :create
   validate :validate_creator_is_not_limited, on: :create
   validates :creator, uniqueness: { scope: :post, message: "have already appealed this post" }, on: :create

app/models/post_flag.rb

@@ -10,7 +10,7 @@ class PostFlag < ApplicationRecord
   belongs_to :post
 
   before_validation { post.lock! }
-  validates :reason, presence: true, length: { in: 1..140 }
+  validates :reason, visible_string: true, length: { in: 1..140 }
   validate :validate_creator_is_not_limited, on: :create
   validate :validate_post, on: :create
   validates :creator_id, uniqueness: { scope: :post_id, on: :create, unless: :is_deletion, message: "have already flagged this post" }

app/models/saved_search.rb

@@ -11,7 +11,7 @@ class SavedSearch < ApplicationRecord
   normalize :query, :normalize_query
   normalize :labels, :normalize_labels
 
-  validates :query, presence: true
+  validates :query, visible_string: true
   validate :validate_count, on: :create
 
   scope :labeled, ->(label) { where_array_includes_any_lower(:labels, [normalize_label(label)]) }

app/models/user_feedback.rb

@@ -7,7 +7,7 @@ class UserFeedback < ApplicationRecord
 
   belongs_to :user
   belongs_to :creator, class_name: "User"
-  validates :body, presence: true
+  validates :body, visible_string: true
   validates :category, presence: true, inclusion: { in: %w[positive negative neutral] }
   after_create :create_dmail, unless: :disable_dmail_notification
   after_update :create_mod_action

app/models/wiki_page.rb

@@ -14,7 +14,7 @@ class WikiPage < ApplicationRecord
   array_attribute :other_names # XXX must come after `normalize :other_names`
 
   validates :title, tag_name: true, presence: true, uniqueness: true, if: :title_changed?
-  validates :body, presence: true, unless: -> { is_deleted? || other_names.present? }
+  validates :body, visible_string: true, unless: -> { is_deleted? || other_names.present? }
   validate :validate_rename
   validate :validate_other_names
 

config/initializers/core_extensions.rb

@@ -5,6 +5,22 @@ require "danbooru"
 module Danbooru
   module Extensions
     module String
+      # https://invisible-characters.com
+      # https://character.construction/blanks
+      # https://www.unicode.org/review/pr-5.html (5.22 Default Ignorable Code Points)
+      # https://en.wikipedia.org/wiki/Whitespace_character
+      #
+      # [[:space:]] = https://codepoints.net/search?gc[]=Z (Space_Separator | Line_Separator | Paragraph_Separator | U+0009 | U+000A | U+000B | U+000C | U+000D | U+0085)
+      # \p{di} = https://codepoints.net/search?DI=1 (Default_Ignorable_Code_Point)
+      # \u2800 = https://codepoints.net/U+2800 (BRAILLE PATTERN BLANK)
+      INVISIBLE_REGEX = /\A[[:space:]\p{di}\u2800]*\z/
+
+      # Returns true if the string consists entirely of invisible characters. Like `#blank?`, but includes control
+      # characters and certain other invisible Unicode characters that aren't classified as spaces.
+      def invisible?
+        match?(INVISIBLE_REGEX)
+      end
+
       def to_escaped_for_sql_like
         string = self.gsub(/%|_|\*|\\\*|\\\\|\\/) do |str|
           case str

test/unit/comment_test.rb

@@ -202,8 +202,11 @@ class CommentTest < ActiveSupport::TestCase
     end
 
     context "during validation" do
-      subject { FactoryBot.build(:comment) }
+      subject { build(:comment) }
+
+      should_not allow_value("").for(:body)
       should_not allow_value(" ").for(:body)
+      should_not allow_value("\u200B").for(:body)
     end
   end
 end

test/unit/dmail_test.rb

@@ -149,8 +149,12 @@ class DmailTest < ActiveSupport::TestCase
     context "during validation" do
       subject { FactoryBot.build(:dmail) }
 
+      should_not allow_value("").for(:title)
       should_not allow_value(" ").for(:title)
+      should_not allow_value("\u200B").for(:title)
+      should_not allow_value("").for(:body)
       should_not allow_value(" ").for(:body)
+      should_not allow_value("\u200B").for(:body)
       should_not allow_value(nil).for(:to)
       should_not allow_value(nil).for(:from)
       should_not allow_value(nil).for(:owner)

test/unit/favorite_group_test.rb

@@ -68,5 +68,8 @@ class FavoriteGroupTest < ActiveSupport::TestCase
     should_not allow_value("_").for(:name)
     should_not allow_value("any").for(:name)
     should_not allow_value("none").for(:name)
+    should_not allow_value("").for(:name)
+    should_not allow_value("   ").for(:name)
+    should_not allow_value("\u200B").for(:name)
   end
 end

test/unit/forum_post_test.rb

@@ -165,5 +165,13 @@ class ForumPostTest < ActiveSupport::TestCase
         assert_equal(@second_user.id, @post.updater_id)
       end
     end
+
+    context "during validation" do
+      subject { build(:forum_post) }
+
+      should_not allow_value("").for(:body)
+      should_not allow_value(" ").for(:body)
+      should_not allow_value("\u200B").for(:body)
+    end
   end
 end

test/unit/forum_topic_test.rb

@@ -82,5 +82,13 @@ class ForumTopicTest < ActiveSupport::TestCase
         end
       end
     end
+
+    context "during validation" do
+      subject { build(:forum_topic) }
+
+      should_not allow_value("").for(:title)
+      should_not allow_value(" ").for(:title)
+      should_not allow_value("\u200B").for(:title)
+    end
   end
 end

test/unit/note_test.rb

@@ -145,5 +145,11 @@ class NoteTest < ActiveSupport::TestCase
         end
       end
     end
+
+    context "when validating notes" do
+      should_not allow_value("").for(:body)
+      should_not allow_value("   ").for(:body)
+      should_not allow_value("\u200B").for(:body)
+    end
   end
 end

test/unit/pool_test.rb

@@ -244,9 +244,17 @@ class PoolTest < ActiveSupport::TestCase
     end
 
     context "when validating names" do
-      ["foo,bar", "foo*bar", "123", "___", "   ", "any", "none", "series", "collection"].each do |bad_name|
-        should_not allow_value(bad_name).for(:name)
-      end
+      should_not allow_value("foo,bar").for(:name)
+      should_not allow_value("foo*bar").for(:name)
+      should_not allow_value("123").for(:name)
+      should_not allow_value("any").for(:name)
+      should_not allow_value("none").for(:name)
+      should_not allow_value("series").for(:name)
+      should_not allow_value("collection").for(:name)
+      should_not allow_value("___").for(:name)
+      should_not allow_value("   ").for(:name)
+      should_not allow_value("\u200B").for(:name)
+      should_not allow_value("").for(:name)
     end
   end
 

test/unit/post_appeal_test.rb

@@ -52,6 +52,15 @@ class PostAppealTest < ActiveSupport::TestCase
           end
         end
       end
+
+      context "validation" do
+        subject { build(:post_appeal) }
+
+        should allow_value("").for(:reason)
+
+        should_not allow_value(" ").for(:reason)
+        should_not allow_value("\u200B").for(:reason)
+      end
     end
   end
 end

test/unit/post_flag_test.rb

@@ -114,5 +114,13 @@ class PostFlagTest < ActiveSupport::TestCase
         end
       end
     end
+
+    context "during validation" do
+      subject { build(:post_flag) }
+
+      should_not allow_value("").for(:reason)
+      should_not allow_value(" ").for(:reason)
+      should_not allow_value("\u200B").for(:reason)
+    end
   end
 end

test/unit/string_test.rb

@@ -41,4 +41,26 @@ class StringTest < ActiveSupport::TestCase
       assert_equal("foo\r\nbar", "foo\u2029bar".normalize_whitespace)
     end
   end
+
+  context "String#invisible?" do
+    should "work" do
+      assert_equal(true, "".invisible?)
+      assert_equal(true, " ".invisible?)
+      assert_equal(true, "\v\t\f\r\n".invisible?)
+      assert_equal(true, "\u00A0\u00AD\u034F\u061C\u115F\u1160\u17B4\u17B5\u180B\u180C\u180D\u180E".invisible?)
+      assert_equal(true, "\u200B\u200C\u200D\u200E\u200F\u2028\u2029\u2060\u206F\u2800\u3000\u3164".invisible?)
+      assert_equal(true, "\uFE00\uFE0F\uFEFF\uFFA0".invisible?)
+      assert_equal(true, "\u{E0001}\u{E007F}".invisible?)
+
+      assert_equal(false, "foo".invisible?)
+      assert_equal(false, "\u0000".invisible?) # https://codepoints.net/U+0000 (NULL)
+      assert_equal(false, "\u0001".invisible?) # https://codepoints.net/U+0001 (START OF HEADING)
+      assert_equal(false, "\uE000".invisible?) # https://codepoints.net/U+E000 (PRIVATE USE CHARACTER)
+      assert_equal(false, "\uFDD0".invisible?) # https://codepoints.net/U+FDD0 (NONCHARACTER)
+      assert_equal(false, "\uFFF9".invisible?) # https://codepoints.net/U+FFF9 (INTERLINEAR ANNOTATION ANCHOR)
+      assert_equal(false, "\uFFFF".invisible?) # https://codepoints.net/U+FFFF (NONCHARACTER)
+      assert_equal(false, "\u{1D159}".invisible?) # https://codepoints.net/U+1D159 (MUSICAL SYMBOL NULL NOTEHEAD)
+      assert_equal(false, "\u{1D455}".invisible?) # https://codepoints.net/U+1D455 (<reserved>)
+    end
+  end
 end

test/unit/user_feedback_test.rb

@@ -17,5 +17,11 @@ class UserFeedbackTest < ActiveSupport::TestCase
         assert_equal(dmail, user.dmails.last.body)
       end
     end
+
+    context "on validation" do
+      should_not allow_value("").for(:body)
+      should_not allow_value("   ").for(:body)
+      should_not allow_value("\u200B").for(:body)
+    end
   end
 end

test/unit/wiki_page_test.rb

@@ -114,6 +114,8 @@ class WikiPageTest < ActiveSupport::TestCase
       should normalize_attribute(:title).from(" Foo___   Bar ").to("foo_bar")
 
       should_not allow_value("").for(:title).on(:create)
+      should_not allow_value(" ").for(:title).on(:create)
+      should_not allow_value("\u200B").for(:title).on(:create)
       should_not allow_value("___").for(:title).on(:create)
       should_not allow_value("-foo").for(:title).on(:create)
       should_not allow_value("/foo").for(:title).on(:create)
@@ -126,6 +128,12 @@ class WikiPageTest < ActiveSupport::TestCase
       should_not allow_value("X"*171).for(:title).on(:create)
     end
 
+    context "during body validation" do
+      should_not allow_value("").for(:body)
+      should_not allow_value(" ").for(:body)
+      should_not allow_value("\u200B").for(:body)
+    end
+
     context "with other names" do
       should "not allow artist wikis to have other names" do
         tag = create(:artist_tag)