DEV: simplify username suggester (#14531)

This PR doesn't change any behavior, but just removes code that wasn't in use. This is a pretty dangerous place to change, since it gets called during user's registration. At the same time the refactoring is very straightforward, it's clear that this code wasn't doing any work (it still needs to be double-checked during review though). Also, the test coverage of UserNameSuggester is good.

lib/user_name_suggester.rb

@@ -4,9 +4,9 @@ module UserNameSuggester
   GENERIC_NAMES = ['i', 'me', 'info', 'support', 'admin', 'webmaster', 'hello', 'mail', 'office', 'contact', 'team']
   LAST_RESORT_USERNAME = "user"
 
-  def self.suggest(name_or_email, allowed_username = nil)
+  def self.suggest(name_or_email)
     name = parse_name_from_email(name_or_email)
-    find_available_username_based_on(name, allowed_username)
+    find_available_username_based_on(name)
   end
 
   def self.parse_name_from_email(name_or_email)
@@ -20,22 +20,13 @@ module UserNameSuggester
     name
   end
 
-  def self.find_available_username_based_on(name, allowed_username = nil)
+  def self.find_available_username_based_on(name)
     name = fix_username(name)
     offset = nil
     i = 1
 
     attempt = name
-    normalized_attempt = User.normalize_username(attempt)
-
-    original_allowed_username = allowed_username
-    allowed_username = User.normalize_username(allowed_username) if allowed_username
-
-    until (
-      normalized_attempt == allowed_username ||
-      User.username_available?(attempt) ||
-      i > 100
-    )
+    until User.username_available?(attempt) || i > 100
 
       if offset.nil?
         normalized = User.normalize_username(name)
@@ -51,8 +42,7 @@ module UserNameSuggester
 
           params = {
             count: count + 10,
-            name: normalized,
-            allowed_normalized: allowed_username || ''
+            name: normalized
           }
 
           # increasing the search space a bit to allow for some extra noise
@@ -60,11 +50,7 @@ module UserNameSuggester
             WITH numbers AS (SELECT generate_series(1, :count) AS n)
 
             SELECT n FROM numbers
-            LEFT JOIN users ON (
-              username_lower = :name || n::varchar
-            ) AND (
-              username_lower <> :allowed_normalized
-            )
+            LEFT JOIN users ON (username_lower = :name || n::varchar)
             WHERE users.id IS NULL
             ORDER by n ASC
             LIMIT 1
@@ -82,22 +68,15 @@ module UserNameSuggester
 
       max_length = User.username_length.end - suffix.length
       attempt = "#{truncate(name, max_length)}#{suffix}"
-      normalized_attempt = User.normalize_username(attempt)
       i += 1
     end
 
-    until normalized_attempt == allowed_username || User.username_available?(attempt) || i > 200
+    until User.username_available?(attempt) || i > 200
       attempt = SecureRandom.hex[1..SiteSetting.max_username_length]
-      normalized_attempt = User.normalize_username(attempt)
       i += 1
     end
 
-    if allowed_username == normalized_attempt
-      original_allowed_username
-    else
-      attempt
-    end
-
+    attempt
   end
 
   def self.fix_username(name)