Implement review comments for !12445 from @godfat and @rymai.

- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
  fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
  visibility level is not restricted.

- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
  the `username` parameter is passed.

- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
  route + method, rather than the description.

- Change the type of `current_user` check in `UsersFinder` to be more
  compatible with EE.

4 files changed, 12 insertions, 14 deletions

diff --git a/app/finders/users_finder.rb b/app/finders/users_finder.rb
index 0534317df8f..07deceb827b 100644
--- a/app/finders/users_finder.rb
+++ b/app/finders/users_finder.rb
@@ -27,11 +27,8 @@ class UsersFinder
     users = by_search(users)
     users = by_blocked(users)
     users = by_active(users)
-
-    if current_user
-      users = by_external_identity(users)
-      users = by_external(users)
-    end
+    users = by_external_identity(users)
+    users = by_external(users)
 
     users
   end
@@ -63,13 +60,13 @@ class UsersFinder
   end
 
   def by_external_identity(users)
-    return users unless current_user.admin? && params[:extern_uid] && params[:provider]
+    return users unless current_user&.admin? && params[:extern_uid] && params[:provider]
 
     users.joins(:identities).merge(Identity.with_extern_uid(params[:provider], params[:extern_uid]))
   end
 
   def by_external(users)
-    return users = users.where.not(external: true) unless current_user.admin?
+    return users = users.where.not(external: true) unless current_user&.admin?
     return users unless params[:external]
 
     users.external
diff --git a/app/policies/base_policy.rb b/app/policies/base_policy.rb
index 623424c63e0..261a2e780c5 100644
--- a/app/policies/base_policy.rb
+++ b/app/policies/base_policy.rb
@@ -1,4 +1,6 @@
 class BasePolicy
+  include Gitlab::CurrentSettings
+
   class RuleSet
     attr_reader :can_set, :cannot_set
     def initialize(can_set, cannot_set)
@@ -124,4 +126,8 @@ class BasePolicy
     yield
     @rule_set
   end
+
+  def restricted_public_level?
+    current_application_settings.restricted_visibility_levels.include?(Gitlab::VisibilityLevel::PUBLIC)
+  end
 end
diff --git a/app/policies/global_policy.rb b/app/policies/global_policy.rb
index 2683aaad981..e9be43a5037 100644
--- a/app/policies/global_policy.rb
+++ b/app/policies/global_policy.rb
@@ -1,9 +1,10 @@
 class GlobalPolicy < BasePolicy
   def rules
+    can! :read_users_list unless restricted_public_level?
+
     return unless @user
 
     can! :create_group if @user.can_create_group
-    can! :read_users_list
 
     unless @user.blocked? || @user.internal?
       can! :log_in unless @user.access_locked?
diff --git a/app/policies/user_policy.rb b/app/policies/user_policy.rb
index 229846e368c..265c56aba53 100644
--- a/app/policies/user_policy.rb
+++ b/app/policies/user_policy.rb
@@ -1,6 +1,4 @@
 class UserPolicy < BasePolicy
-  include Gitlab::CurrentSettings
-
   def rules
     can! :read_user if @user || !restricted_public_level?
 
@@ -12,8 +10,4 @@ class UserPolicy < BasePolicy
       cannot! :destroy_user if @subject.ghost?
     end
   end
-
-  def restricted_public_level?
-    current_application_settings.restricted_visibility_levels.include?(Gitlab::VisibilityLevel::PUBLIC)
-  end
 end