Use the file_fixture_path for fixture_file_upload:

- We used the `fixture_path` before `file_fixture_path` was a thing,
  but now that we have the latter we should use it.

  `fixture_path` is solely used by Active Record so it seems wrong
  to be using that in ActionPack.

actionpack/CHANGELOG.md

@@ -1,3 +1,18 @@
+*   `fixture_file_upload` now uses path relative to `file_fixture_path`
+
+    Previously the path had to be relative to `fixture_path`.
+    You can change your existing code as follow:
+
+    ```ruby
+    # Before
+    fixture_file_upload('files/dog.png')
+
+    # After
+    fixture_file_upload('dog.png')
+    ```
+
+    *Edouard Chin*
+
 *   Remove deprecated `force_ssl` at the controller level.
 
     *Rafael Mendonça França*

actionpack/lib/action_dispatch/testing/test_process.rb

@@ -17,8 +17,29 @@ module ActionDispatch
       def fixture_file_upload(path, mime_type = nil, binary = false)
         if self.class.respond_to?(:fixture_path) && self.class.fixture_path &&
             !File.exist?(path)
-          path = File.join(self.class.fixture_path, path)
+          original_path = path
+          path = Pathname.new(self.class.fixture_path).join(path)
+
+          if !self.class.file_fixture_path
+            ActiveSupport::Deprecation.warn(<<~EOM)
+              Passing a path to `fixture_file_upload` relative to `fixture_path` is deprecated.
+              In Rails 6.2, the path needs to be relative to `file_fixture_path` which you
+              haven't set yet. Set `file_fixture_path` to discard this warning.
+            EOM
+          elsif path.exist?
+            non_deprecated_path = path.relative_path_from(Pathname(self.class.file_fixture_path))
+            ActiveSupport::Deprecation.warn(<<~EOM)
+              Passing a path to `fixture_file_upload` relative to `fixture_path` is deprecated.
+              In Rails 6.2, the path needs to be relative to `file_fixture_path`.
+
+              Please modify the call from
+              `fixture_file_upload(#{original_path})` to `fixture_file_upload(#{non_deprecated_path})`.
+            EOM
           end
+        elsif self.class.file_fixture_path && !File.exist?(path)
+          path = file_fixture(path)
+        end
+
         Rack::Test::UploadedFile.new(path, mime_type, binary)
       end
     end

actionpack/test/controller/integration_test.rb

@@ -1205,7 +1205,7 @@ class IntegrationFileUploadTest < ActionDispatch::IntegrationTest
     self.class
   end
 
-  def self.fixture_path
+  def self.file_fixture_path
     File.expand_path("../fixtures/multipart", __dir__)
   end
 

actionpack/test/controller/test_case_test.rb

@@ -7,6 +7,7 @@ require "rails/engine"
 
 class TestCaseTest < ActionController::TestCase
   def self.fixture_path; end
+  self.file_fixture_path = File.expand_path("../fixtures/multipart", __dir__)
 
   class TestController < ActionController::Base
     def no_op
@@ -872,11 +873,15 @@ XML
   end
 
   def test_fixture_path_is_accessed_from_self_instead_of_active_support_test_case
-    TestCaseTest.stub :fixture_path, FILES_DIR do
-      uploaded_file = fixture_file_upload("/ruby_on_rails.jpg", "image/png")
+    TestCaseTest.stub :fixture_path, File.expand_path("../fixtures", __dir__) do
+      expected = "`fixture_file_upload(multipart/ruby_on_rails.jpg)` to `fixture_file_upload(ruby_on_rails.jpg)`"
+
+      assert_deprecated(expected) do
+        uploaded_file = fixture_file_upload("multipart/ruby_on_rails.jpg", "image/png")
         assert_equal File.open("#{FILES_DIR}/ruby_on_rails.jpg", READ_PLAIN).read, uploaded_file.read
       end
     end
+  end
 
   def test_test_uploaded_file_with_binary
     filename = "ruby_on_rails.jpg"
@@ -915,12 +920,26 @@ XML
     assert_equal "45142", @response.body
   end
 
+  def test_fixture_file_upload_output_deprecation_when_file_fixture_path_is_not_set
+    TestCaseTest.stub :fixture_path, File.expand_path("../fixtures", __dir__) do
+      TestCaseTest.stub :file_fixture_path, nil do
+        assert_deprecated(/In Rails 6.2, the path needs to be relative to `file_fixture_path`/) do
+          fixture_file_upload("multipart/ruby_on_rails.jpg", "image/jpg")
+        end
+      end
+    end
+  end
+
   def test_fixture_file_upload_relative_to_fixture_path
-    TestCaseTest.stub :fixture_path, FILES_DIR do
-      uploaded_file = fixture_file_upload("ruby_on_rails.jpg", "image/jpg")
+    TestCaseTest.stub :fixture_path, File.expand_path("../fixtures", __dir__) do
+      expected = "`fixture_file_upload(multipart/ruby_on_rails.jpg)` to `fixture_file_upload(ruby_on_rails.jpg)`"
+
+      assert_deprecated(expected) do
+        uploaded_file = fixture_file_upload("multipart/ruby_on_rails.jpg", "image/jpg")
         assert_equal File.open("#{FILES_DIR}/ruby_on_rails.jpg", READ_PLAIN).read, uploaded_file.read
       end
     end
+  end
 
   def test_fixture_file_upload_ignores_fixture_path_given_full_path
     TestCaseTest.stub :fixture_path, __dir__ do