Disable variant options when false or nil present

In response to https://github.com/rails/rails/issues/32917

In the current implementation, ActiveStorage passes all options to the underlying processor,
including when a key has a value of false.

For example, passing:

```
avatar.variant(resize: "100x100", monochrome: false, flip: "-90")
```

will return a monochrome image (or an error, pending on ImageMagick configuration) because
it passes `-monochrome false` to the command (but the command line does not allow disabling
flags this way, as usually a user would omit the flag entirely to disable that feature).

This fix only passes those keys forward to the underlying processor if the value responds to
`present?`. In practice, this means that `false` or `nil` will be filtered out before going
to the processor.

One possible use case would be for a user to be able to apply different filters to an avatar.
The code might look something like:

```
  variant_options = {
    monochrome: params[:monochrome],
    resize:     params[:resize]
  }

  avatar.variant(*variant_options)
```

Obviously some sanitization may be beneficial in a real-world scenario, but this type of
configuration object could be used in many other places as well.

- Add removing falsy values from varaints to changelog

- The entirety of #image_processing_transformation inject block was wrapped in `list.tap`
 to guard against the default `nil` being returned if no conditional was called.

- add test for explicitly true variant options

1 files changed, 10 insertions, 12 deletions

diff --git a/activestorage/app/models/active_storage/variation.rb b/activestorage/app/models/active_storage/variation.rb
index 42f00beb82..806af6366d 100644
--- a/activestorage/app/models/active_storage/variation.rb
+++ b/activestorage/app/models/active_storage/variation.rb
@@ -66,11 +66,13 @@ class ActiveStorage::Variation
     # Applies image transformations using the ImageProcessing gem.
     def image_processing_transform(file, format)
       operations = transformations.inject([]) do |list, (name, argument)|
-        if name.to_s == "combine_options"
-          ActiveSupport::Deprecation.warn("The ImageProcessing ActiveStorage variant backend doesn't need :combine_options, as it already generates a single MiniMagick command. In Rails 6.1 :combine_options will not be supported anymore.")
-          list.concat argument.to_a
-        else
-          list << [name, argument]
+        list.tap do |list|
+          if name.to_s == "combine_options"
+            ActiveSupport::Deprecation.warn("The ImageProcessing ActiveStorage variant backend doesn't need :combine_options, as it already generates a single MiniMagick command. In Rails 6.1 :combine_options will not be supported anymore.")
+            list.concat argument.keep_if { |key, value| value.present? }.to_a
+          elsif argument.present?
+            list << [name, argument]
+          end
         end
       end
 
@@ -116,14 +118,10 @@ class ActiveStorage::Variation
     end
 
     def pass_transform_argument(command, method, argument)
-      if eligible_argument?(argument)
-        command.public_send(method, argument)
-      else
+      if argument == true
         command.public_send(method)
+      elsif argument.present?
+        command.public_send(method, argument)
       end
     end
-
-    def eligible_argument?(argument)
-      argument.present? && argument != true
-    end
 end