Detect the MultiXML constant to avoid the multi_xml deprecation

multi_xml 0.9.0 deprecated the `MultiXml` constant in favor of `MultiXML`
(removed in v1.0); the legacy constant warns on access/use. Grape detected
the backend via `::MultiXml` and called `Grape::Xml.parse`, so users on
multi_xml >= 0.9 would hit the deprecation.

Detect `::MultiXML` first, falling back to the legacy `::MultiXml` constant
and then `ActiveSupport::XmlMini`. Unlike multi_json, no facade is needed:
`parse` is canonical (not renamed) and Grape only calls `parse`, so a direct
alias suffices on every backend.

Cover both multi_xml lines in CI: pin gemfiles/multi_xml.gemfile to >= 0.9
and add gemfiles/multi_xml_0_8.gemfile (< 0.9), each running the
spec/integration/multi_xml suite. The integration spec now drives a real
Grape API that parses an XML request body through Grape::Xml.parse.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ericproulx

.github/workflows/test.yml

@@ -53,6 +53,9 @@ jobs:
           - ruby: '4.0'
             gemfile: gemfiles/multi_xml.gemfile
             specs: 'spec/integration/multi_xml'
+          - ruby: '4.0'
+            gemfile: gemfiles/multi_xml_0_8.gemfile
+            specs: 'spec/integration/multi_xml'
     runs-on: ubuntu-latest
     env:
       BUNDLE_GEMFILE: ${{ github.workspace }}/${{ matrix.gemfile }}

CHANGELOG.md

@@ -75,6 +75,7 @@
 * [#2706](https://github.com/ruby-grape/grape/pull/2706): Fix `optional :foo, message: 'oops'` raising `UnknownValidator` - [@ericproulx](https://github.com/ericproulx).
 * [#2751](https://github.com/ruby-grape/grape/pull/2751): Fix structured error messages leaking the raw i18n key for an undefined optional step such as `summary` (closes #2748) - [@ericproulx](https://github.com/ericproulx).
 * [#2759](https://github.com/ruby-grape/grape/pull/2759): Use `create_additions: false` in `Grape::Json.load` to prevent object instantiation via the `json_class` key when using the stdlib JSON fallback - [@dblock](https://github.com/dblock).
+* [#2765](https://github.com/ruby-grape/grape/pull/2765): Detect the `MultiXML` constant to avoid the multi_xml 0.9 `MultiXml` deprecation - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.2.1 (2026-04-16)

gemfiles/multi_xml.gemfile

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
-gem 'multi_xml'
+gem 'multi_xml', '>= 0.9'
 
 eval_gemfile '../Gemfile'

gemfiles/multi_xml_0_8.gemfile

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+gem 'multi_xml', '< 0.9'
+
+eval_gemfile '../Gemfile'

lib/grape/xml.rb

@@ -1,7 +1,14 @@
 # frozen_string_literal: true
 
 module Grape
-  if defined?(::MultiXml)
+  # Since multi_xml 0.9.0 the canonical constant is MultiXML; MultiXml is a
+  # deprecated alias (removed in v1.0) that warns on use. Prefer MultiXML so
+  # Grape::Xml.parse doesn't trip the deprecation, falling back to the legacy
+  # constant and then ActiveSupport::XmlMini.
+  # https://github.com/sferik/multi_xml/blob/v0.9.1/CHANGELOG.md
+  if defined?(::MultiXML)
+    Xml = ::MultiXML
+  elsif defined?(::MultiXml)
     Xml = ::MultiXml
   else
     Xml = ::ActiveSupport::XmlMini

spec/integration/multi_xml/xml_spec.rb

@@ -1,7 +1,22 @@
 # frozen_string_literal: true
 
 describe Grape::Xml, if: defined?(MultiXml) do
-  subject { described_class }
+  # Exercise the full request stack: a Grape API parses an XML body through the
+  # active multi_xml backend (MultiXML on >= 0.9, the legacy MultiXml alias on
+  # < 0.9). Calling parse through the deprecated MultiXml constant would raise
+  # via the suite's deprecation handler (see spec/support/deprecated_warning_handlers.rb).
+  let(:app) do
+    Class.new(Grape::API) do
+      post '/request_body' do
+        params[:user]
+      end
+    end
+  end
 
-  it { is_expected.to eq(MultiXml) }
+  it 'parses an XML request body into params' do
+    post '/request_body', '<user>Bobby T.</user>', 'CONTENT_TYPE' => 'application/xml'
+
+    expect(last_response.status).to eq(201)
+    expect(last_response.body).to eq('Bobby T.')
+  end
 end