autocomplete: optimize various types of bogus input.

Optimize autocomplete to ignore various types of bogus input that will
never match anything. It turns out it's not uncommon for people to do
things like paste random URLs into autocomplete, or hold down keys, or
enter long strings of gibberish text (sometimes in other languages).
Some things, like autocorrect and slash abbreviations, become
pathologically slow when fed certain types of bad input.

Autocomplete will abort and return nothing in the following situations:

* Searching for URLs (tags that start with http:// or https://).
* Overly long tags (strings longer than the 170 char tag name limit).
* Slash abbreviations longer than 10 chars (e.g. typing `/qwoijqoiqogirqewgoi`).
* Slash abbreviations that aren't alphanumeric (e.g. typing `/////////`).
* Autocorrect input that contains too much punctuation and not enough actual letters.

app/logical/autocomplete_service.rb

@@ -65,6 +65,9 @@ class AutocompleteService
   end
 
   def autocomplete_tag(string)
+    return [] if string.size > TagNameValidator::MAX_TAG_LENGTH
+    return [] if string.start_with?("http://", "https://")
+
     if !string.ascii_only?
       results = tag_other_name_matches(string)
     elsif string.starts_with?("/")
@@ -88,8 +91,6 @@ class AutocompleteService
   end
 
   def tag_matches(string)
-    return [] unless string.ascii_only?
-
     name_matches = Tag.nonempty.name_matches(string).order(post_count: :desc).limit(limit)
     alias_matches = Tag.nonempty.alias_matches(string).order(post_count: :desc).limit(limit)
     union = "((#{name_matches.to_sql}) UNION (#{alias_matches.to_sql})) AS tags"
@@ -102,7 +103,9 @@ class AutocompleteService
     end
   end
 
-  def tag_abbreviation_matches(string)
+  def tag_abbreviation_matches(string, max_length: 10)
+    return [] if string.size > max_length
+
     tags = Tag.nonempty.abbreviation_matches(string).order(post_count: :desc).limit(limit)
 
     tags.map do |tag|
@@ -111,6 +114,9 @@ class AutocompleteService
   end
 
   def tag_autocorrect_matches(string)
+    # autocorrect uses trigram indexing, which needs at least 3 alphanumeric characters to work.
+    return [] if string.remove(/[^a-zA-Z0-9]/).size < 3
+
     tags = Tag.nonempty.autocorrect_matches(string).limit(limit)
 
     tags.map do |tag|

app/logical/tag_name_validator.rb

@@ -1,9 +1,11 @@
 class TagNameValidator < ActiveModel::EachValidator
+  MAX_TAG_LENGTH = 170
+
   def validate_each(record, attribute, value)
     value = Tag.normalize_name(value)
 
-    if value.size > 170
+    if value.size > MAX_TAG_LENGTH
-      record.errors.add(attribute, "'#{value}' cannot be more than 255 characters long")
+      record.errors.add(attribute, "'#{value}' cannot be more than #{MAX_TAG_LENGTH} characters long")
     end
 
     case value
@@ -30,6 +32,7 @@ class TagNameValidator < ActiveModel::EachValidator
     when "new", "search"
       record.errors.add(attribute, "'#{value}' is a reserved name and cannot be used")
     when /\A(.+)_\(cosplay\)\z/i
+      # XXX don't allow aliases here?
       tag_name = TagAlias.to_aliased([$1]).first
       tag = Tag.find_by_name(tag_name)
 

app/models/tag.rb

@@ -254,12 +254,13 @@ class Tag < ApplicationRecord
     end
 
     def abbreviation_matches(abbrev)
-      abbrev = abbrev.delete_prefix("/")
+      abbrev = abbrev.downcase.delete_prefix("/")
+      return none if abbrev !~ /\A[a-z0-9\*]*\z/
+
       where("regexp_replace(tags.name, ?, '\\1', 'g') LIKE ?", ABBREVIATION_REGEXP.source, abbrev.to_escaped_for_sql_like)
     end
 
     def find_by_abbreviation(abbrev)
-      abbrev = abbrev.delete_prefix("/")
       abbreviation_matches(abbrev.escape_wildcards).order(post_count: :desc).first
     end
 

test/unit/autocomplete_service_test.rb

@@ -87,15 +87,20 @@ class AutocompleteServiceTest < ActiveSupport::TestCase
           create(:tag, name: "mole", post_count: 150)
           create(:tag, name: "mole_under_eye", post_count: 100)
           create(:tag, name: "mole_under_mouth", post_count: 50)
+          create(:tag, name: "x_x_x_x_x_x_x_x_x_x_x_x", post_count: 1)
 
           assert_autocomplete_equals(%w[mole mole_under_eye mole_under_mouth], "/m", :tag_query)
           assert_autocomplete_equals(%w[mole_under_eye mole_under_mouth], "/mu", :tag_query)
           assert_autocomplete_equals(%w[mole_under_mouth], "/mum", :tag_query)
           assert_autocomplete_equals(%w[mole_under_eye], "/mue", :tag_query)
           assert_autocomplete_equals(%w[mole_under_eye], "/*ue", :tag_query)
+          assert_autocomplete_equals(%w[mole_under_eye], "/MUE", :tag_query)
 
           assert_autocomplete_includes("mole_under_eye", "-/mue", :tag_query)
           assert_autocomplete_includes("mole_under_eye", "~/mue", :tag_query)
+
+          assert_autocomplete_equals([], "/xxxxxxxxxx", :tag_query)
+          assert_autocomplete_equals([], "/_", :tag_query)
         end
 
         should "list aliases before abbreviations" do
@@ -117,19 +122,19 @@ class AutocompleteServiceTest < ActiveSupport::TestCase
         assert_autocomplete_equals(["touhou"], "东", :tag_query)
         assert_autocomplete_equals(["touhou"], "동", :tag_query)
 
-        assert_autocomplete_equals(["touhou"], "*東*", :tag_query)
+        assert_autocomplete_equals([], "*東*", :tag_query)
-        assert_autocomplete_equals(["touhou"], "東*", :tag_query)
+        assert_autocomplete_equals([], "東*", :tag_query)
         assert_autocomplete_equals([], "*東", :tag_query)
 
-        assert_autocomplete_equals(["touhou"], "*方*", :tag_query)
+        assert_autocomplete_equals([], "*方*", :tag_query)
-        assert_autocomplete_equals(["touhou"], "*方", :tag_query)
+        assert_autocomplete_equals([], "*方", :tag_query)
         assert_autocomplete_equals([], "方", :tag_query)
 
-        assert_autocomplete_equals(["bkub"], "*大*", :tag_query)
+        assert_autocomplete_equals([], "*大*", :tag_query)
         assert_autocomplete_equals(["bkub"], "大", :tag_query)
         assert_autocomplete_equals([], "*大", :tag_query)
 
-        assert_autocomplete_equals(["bkub"], "*川*", :tag_query)
+        assert_autocomplete_equals([], "*川*", :tag_query)
         assert_autocomplete_equals([], "*川", :tag_query)
         assert_autocomplete_equals([], "川", :tag_query)
       end
@@ -148,6 +153,8 @@ class AutocompleteServiceTest < ActiveSupport::TestCase
         create(:tag, name: "touhou")
 
         assert_autocomplete_equals(%w[touhou], "touhuo", :tag_query)
+        assert_autocomplete_equals(%w[], ".....", :tag_query)
+        assert_autocomplete_equals(%w[], "t___", :tag_query)
       end
 
       should "ignore unsupported metatags" do
@@ -193,6 +200,11 @@ class AutocompleteServiceTest < ActiveSupport::TestCase
 
         assert_autocomplete_equals(["order:score", "order:score_asc"], "order:sco", :tag_query)
       end
+
+      should "ignore bogus tags" do
+        assert_autocomplete_equals([], "x"*200, :tag_query)
+        assert_autocomplete_equals([], "http://www.google.com", :tag_query)
+      end
     end
   end
 end