[rubygems/rubygems] Fix generic platform gems getting incorrectly removed when locked for a specific platform

If they are already in the lockfile as the most specific variant for a
platform, we shouldn't change that unless explicitly unlocking.

https://github.com/rubygems/rubygems/commit/a901660498

lib/bundler/gem_helpers.rb

@@ -46,13 +46,18 @@ module Bundler
     end
     module_function :platform_specificity_match
 
-    def select_best_platform_match(specs, platform, force_ruby: false)
+    def select_best_platform_match(specs, platform, force_ruby: false, prefer_locked: false)
       matching = if force_ruby
         specs.select {|spec| spec.match_platform(Gem::Platform::RUBY) && spec.force_ruby_platform! }
       else
         specs.select {|spec| spec.match_platform(platform) }
       end
 
+      if prefer_locked
+        locked_originally = matching.select {|spec| spec.is_a?(LazySpecification) }
+        return locked_originally if locked_originally.any?
+      end
+
       sort_best_platform_match(matching, platform)
     end
     module_function :select_best_platform_match

lib/bundler/resolver.rb

@@ -255,7 +255,7 @@ module Bundler
       results = filter_matching_specs(results, locked_requirement) if locked_requirement
 
       results.group_by(&:version).reduce([]) do |groups, (version, specs)|
-        platform_specs = package.platforms.map {|platform| select_best_platform_match(specs, platform) }
+        platform_specs = package.platform_specs(specs)
 
         # If package is a top-level dependency,
         #   candidate is only valid if there are matching versions for all resolution platforms.

lib/bundler/resolver/package.rb

@@ -25,6 +25,10 @@ module Bundler
         @prerelease = @dependency.prerelease? || @locked_version&.prerelease? || prerelease ? :consider_first : :ignore
       end
 
+      def platform_specs(specs)
+        platforms.map {|platform| GemHelpers.select_best_platform_match(specs, platform, prefer_locked: !unlock?) }
+      end
+
       def to_s
         @name.delete("\0")
       end

spec/bundler/install/gemfile/specific_platform_spec.rb

@@ -1521,6 +1521,102 @@ RSpec.describe "bundle install with specific platforms" do
     end
   end
 
+  it "does not remove generic platform gems locked for a specific platform from lockfile when unlocking an unrelated gem" do
+    build_repo4 do
+      build_gem "ffi"
+
+      build_gem "ffi" do |s|
+        s.platform = "x86_64-linux"
+      end
+
+      build_gem "nokogiri"
+    end
+
+    gemfile <<~G
+      source "https://gem.repo4"
+
+      gem "ffi"
+      gem "nokogiri"
+    G
+
+    original_lockfile = <<~L
+      GEM
+        remote: https://gem.repo4/
+        specs:
+          ffi (1.0)
+          nokogiri (1.0)
+
+      PLATFORMS
+        x86_64-linux
+
+      DEPENDENCIES
+        ffi
+        nokogiri
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    lockfile original_lockfile
+
+    simulate_platform "x86_64-linux" do
+      bundle "lock --update nokogiri"
+
+      expect(lockfile).to eq(original_lockfile)
+    end
+  end
+
+  it "does not remove generic platform gems locked for a specific platform from lockfile when unlocking an unrelated gem, and variants for other platform also locked" do
+    build_repo4 do
+      build_gem "ffi"
+
+      build_gem "ffi" do |s|
+        s.platform = "x86_64-linux"
+      end
+
+      build_gem "ffi" do |s|
+        s.platform = "java"
+      end
+
+      build_gem "nokogiri"
+    end
+
+    gemfile <<~G
+      source "https://gem.repo4"
+
+      gem "ffi"
+      gem "nokogiri"
+    G
+
+    original_lockfile = <<~L
+      GEM
+        remote: https://gem.repo4/
+        specs:
+          ffi (1.0)
+          ffi (1.0-java)
+          nokogiri (1.0)
+
+      PLATFORMS
+        java
+        x86_64-linux
+
+      DEPENDENCIES
+        ffi
+        nokogiri
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    lockfile original_lockfile
+
+    simulate_platform "x86_64-linux" do
+      bundle "lock --update nokogiri"
+
+      expect(lockfile).to eq(original_lockfile)
+    end
+  end
+
   private
 
   def setup_multiplatform_gem