[rubygems/rubygems] Fix issue with bundle update with an out of sync lockfile

An old platform related bug fix made some existing lockfiles no longer
work because they included invalid platforms. So to make it backwards
compatible, code was added to remove invalid platforms from the lockfile
before resolution. This is skipped though when Gemfile has changed
dependencies because in that case we will be re-resolving anyways.
However, in the `bundle update` case, the detection of "dependencies
have changed" was not actually working making Bundler remove all
platforms and not be able to resolve.

https://github.com/rubygems/rubygems/commit/6452adfd62

lib/bundler/definition.rb

@@ -92,11 +92,12 @@ module Bundler
         @platforms = @locked_platforms.dup
         @locked_bundler_version = @locked_gems.bundler_version
         @locked_ruby_version = @locked_gems.ruby_version
+        @originally_locked_deps = @locked_gems.dependencies
         @originally_locked_specs = SpecSet.new(@locked_gems.specs)
         @locked_checksums = @locked_gems.checksums
 
         if unlock != true
-          @locked_deps    = @locked_gems.dependencies
+          @locked_deps    = @originally_locked_deps
           @locked_specs   = @originally_locked_specs
           @locked_sources = @locked_gems.sources
         else
@@ -111,6 +112,7 @@ module Bundler
         @locked_gems    = nil
         @locked_deps    = {}
         @locked_specs   = SpecSet.new([])
+        @originally_locked_deps = {}
         @originally_locked_specs = @locked_specs
         @locked_sources = []
         @locked_platforms = []
@@ -835,9 +837,7 @@ module Bundler
           dep.source = sources.get(dep.source)
         end
 
-        next if unlocking?
-
-        unless locked_dep = @locked_deps[dep.name]
+        unless locked_dep = @originally_locked_deps[dep.name]
           changes = true
           next
         end

spec/bundler/commands/update_spec.rb

@@ -1954,6 +1954,52 @@ RSpec.describe "bundle update conservative" do
     end
   end
 
+  context "when Gemfile dependencies have changed" do
+    before do
+      build_repo4 do
+        build_gem "nokogiri", "1.16.4" do |s|
+          s.platform = "arm64-darwin"
+        end
+
+        build_gem "nokogiri", "1.16.4" do |s|
+          s.platform = "x86_64-linux"
+        end
+
+        build_gem "prism", "0.25.0"
+      end
+
+      gemfile <<~G
+        source "#{file_uri_for(gem_repo4)}"
+        gem "nokogiri", ">=1.16.4"
+        gem "prism", ">=0.25.0"
+      G
+
+      lockfile <<~L
+        GEM
+          remote: #{file_uri_for(gem_repo4)}/
+          specs:
+            nokogiri (1.16.4-arm64-darwin)
+            nokogiri (1.16.4-x86_64-linux)
+
+        PLATFORMS
+          arm64-darwin
+          x86_64-linux
+
+        DEPENDENCIES
+          nokogiri (>= 1.16.4)
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+    end
+
+    it "still works" do
+      simulate_platform "arm64-darwin-23" do
+        bundle "update"
+      end
+    end
+  end
+
   context "error handling" do
     before do
       gemfile "source \"#{file_uri_for(gem_repo1)}\""