From 6c98e335297aacdff38b888af0c42529379fe554 Mon Sep 17 00:00:00 2001
From: Islam Elsayed <islam.elsayed.mohammed@gmail.com>
Date: Tue, 8 Sep 2026 13:23:21 +0300
Subject: [PATCH] Apply --platform in gem build when it matches the local
 platform

`gem build` never applied `--platform` itself. It relied on the platform
inference in `Gem::Specification#initialize`, which reads
`Gem.platforms.last` but deliberately skips it when that equals
`Gem::Platform.local`:

    if (platform = Gem.platforms.last) && platform != Gem::Platform::RUBY &&
       platform != Gem::Platform.local

That guard is load-bearing. `Gem.platforms` includes the local platform
by default, so without it every spec instantiated anywhere in the
process would silently become platform specific. The cost is that
`gem build --platform x86_64-linux` on x86_64-linux produced
`hello-0.1.0.gem` rather than `hello-0.1.0-x86_64-linux.gem`.

Passing `--platform` resets `Gem.platforms` to `[RUBY]` before appending
the requested value and sets `options[:added_platform]`, so an explicit
request is already distinguishable from the ambient default. Apply it in
`build_package` when that flag is set, leaving the shared guard alone.
---
 lib/rubygems/commands/build_command.rb        |  9 ++++++
 .../test_gem_commands_build_command.rb        | 30 +++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/lib/rubygems/commands/build_command.rb b/lib/rubygems/commands/build_command.rb
index 9e4d812c3626..ce24084894a7 100644
--- a/lib/rubygems/commands/build_command.rb
+++ b/lib/rubygems/commands/build_command.rb
@@ -90,6 +90,15 @@ def build_gem
   def build_package(gemspec)
     spec = Gem::Specification.load(gemspec)
     if spec
+      # Gem::Specification#initialize infers the platform from
+      # Gem.platforms.last, but skips it when that equals Gem::Platform.local,
+      # because Gem.platforms includes the local platform by default and every
+      # spec would otherwise become platform specific. Passing --platform
+      # resets Gem.platforms, so the request is unambiguous and is applied here
+      # rather than left to that inference, which would drop it whenever the
+      # requested platform happened to be the local one.
+      spec.platform = Gem.platforms.last if options[:added_platform]
+
       Gem::Package.build(
         spec,
         options[:force],
diff --git a/test/rubygems/test_gem_commands_build_command.rb b/test/rubygems/test_gem_commands_build_command.rb
index 771eb07dbc9c..fc49b32822db 100644
--- a/test/rubygems/test_gem_commands_build_command.rb
+++ b/test/rubygems/test_gem_commands_build_command.rb
@@ -122,6 +122,36 @@ def test_execute
     util_test_build_gem @gem
   end
 
+  # Regression test for https://github.com/rubygems/rubygems/issues/9344:
+  # `gem build --platform X` must produce a platform-specific gem even when X
+  # happens to equal the local platform. `Gem::Specification#initialize` infers
+  # the platform from `Gem.platforms.last` but deliberately skips it when that
+  # equals `Gem::Platform.local`, because `Gem.platforms` includes the local
+  # platform by default and every spec would otherwise become platform-specific.
+  # `gem build` relied entirely on that inference, so the one case where the
+  # requested platform matched the local one was silently dropped.
+  def test_execute_with_platform_matching_local_platform
+    gemspec_file = File.join(@tempdir, @gem.spec_name)
+
+    File.open gemspec_file, "w" do |gs|
+      gs.write @gem.to_ruby
+    end
+
+    @cmd.handle_options [gemspec_file, "--platform", Gem::Platform.local.to_s]
+
+    use_ui @ui do
+      Dir.chdir @tempdir do
+        @cmd.execute
+      end
+    end
+
+    gem_file = File.join @tempdir, "some_gem-2-#{Gem::Platform.local}.gem"
+    assert File.exist?(gem_file), "expected a platform-specific gem at #{gem_file}"
+
+    spec = Gem::Package.new(gem_file).spec
+    assert_equal Gem::Platform.local, spec.platform
+  end
+
   def test_ruby_abi_rejects_ruby_platform
     gem = util_spec "some_gem" do |s|
       s.license = "AGPL-3.0-only"