Auto merge of #6495 - bundler:segiddins/6491-extra-gem-platform-in-lockfile, r=segiddins

[Definition] Filter out unneeded gem platforms after resolving

### What was the end-user problem that led to this PR?

The problem was the lockfile would contain platform-specific gems that it didn't need

Closes https://github.com/bundler/bundler/issues/6491

### What was your diagnosis of the problem?

My diagnosis was the resolver sometimes activates platforms it doesn't need, since it can't know it won't need them

### What is your fix for the problem, implemented in this PR?

My fix is to use `SpecSet#for` to filter out gems that won't ever be used

### Why did you choose this fix out of the possible options?

I chose this fix because it isn't re-inventing the wheel!

(cherry picked from commit 7b603f39e32a466cb1a8235a963968b2103e665e)

3 files changed, 159 insertions, 15 deletions

diff --git a/lib/bundler/definition.rb b/lib/bundler/definition.rb
index 0a02547f0d..89ae1ce926 100644
--- a/lib/bundler/definition.rb
+++ b/lib/bundler/definition.rb
@@ -246,17 +246,22 @@ module Bundler
     def resolve
       @resolve ||= begin
         last_resolve = converge_locked_specs
-        if Bundler.frozen_bundle?
-          Bundler.ui.debug "Frozen, using resolution from the lockfile"
-          last_resolve
-        elsif !unlocking? && nothing_changed?
-          Bundler.ui.debug("Found no changes, using resolution from the lockfile")
-          last_resolve
-        else
-          # Run a resolve against the locally available gems
-          Bundler.ui.debug("Found changes from the lockfile, re-resolving dependencies because #{change_reason}")
-          last_resolve.merge Resolver.resolve(expanded_dependencies, index, source_requirements, last_resolve, gem_version_promoter, additional_base_requirements_for_resolve, platforms)
-        end
+        resolve =
+          if Bundler.frozen_bundle?
+            Bundler.ui.debug "Frozen, using resolution from the lockfile"
+            last_resolve
+          elsif !unlocking? && nothing_changed?
+            Bundler.ui.debug("Found no changes, using resolution from the lockfile")
+            last_resolve
+          else
+            # Run a resolve against the locally available gems
+            Bundler.ui.debug("Found changes from the lockfile, re-resolving dependencies because #{change_reason}")
+            last_resolve.merge Resolver.resolve(expanded_dependencies, index, source_requirements, last_resolve, gem_version_promoter, additional_base_requirements_for_resolve, platforms)
+          end
+
+        # filter out gems that _can_ be installed on multiple platforms, but don't need
+        # to be
+        resolve.for(expand_dependencies(dependencies, true), [], false, false, false)
       end
     end
 
diff --git a/lib/bundler/resolver/spec_group.rb b/lib/bundler/resolver/spec_group.rb
index 40d5460643..34d043aed7 100644
--- a/lib/bundler/resolver/spec_group.rb
+++ b/lib/bundler/resolver/spec_group.rb
@@ -54,10 +54,6 @@ module Bundler
         dependencies.concat(metadata_dependencies).flatten
       end
 
-      def platforms_for_dependency_named(dependency)
-        __dependencies.select {|_, deps| deps.map(&:name).include? dependency }.keys
-      end
-
       def ==(other)
         return unless other.is_a?(SpecGroup)
         name == other.name &&
diff --git a/spec/install/gemfile/platform_spec.rb b/spec/install/gemfile/platform_spec.rb
index d7d4e0a53c..5858c3af0a 100644
--- a/spec/install/gemfile/platform_spec.rb
+++ b/spec/install/gemfile/platform_spec.rb
@@ -73,6 +73,149 @@ RSpec.describe "bundle install across platforms" do
     expect(the_bundle).not_to include_gems "weakling"
   end
 
+  it "does not keep unneeded platforms for gems that are used" do
+    build_repo4 do
+      build_gem "empyrean", "0.1.0"
+      build_gem "coderay", "1.1.2"
+      build_gem "method_source", "0.9.0"
+      build_gem("spoon", "0.0.6") {|s| s.add_runtime_dependency "ffi" }
+      build_gem "pry", "0.11.3" do |s|
+        s.platform = "java"
+        s.add_runtime_dependency "coderay", "~> 1.1.0"
+        s.add_runtime_dependency "method_source", "~> 0.9.0"
+        s.add_runtime_dependency "spoon", "~> 0.0"
+      end
+      build_gem "pry", "0.11.3" do |s|
+        s.add_runtime_dependency "coderay", "~> 1.1.0"
+        s.add_runtime_dependency "method_source", "~> 0.9.0"
+      end
+      build_gem("ffi", "1.9.23") {|s| s.platform = "java" }
+      build_gem("ffi", "1.9.23")
+    end
+
+    simulate_platform java
+
+    install_gemfile! <<-G
+      source "file://localhost/#{gem_repo4}"
+
+      gem "empyrean", "0.1.0"
+      gem "pry"
+    G
+
+    expect(the_bundle.lockfile).to read_as strip_whitespace(<<-L)
+      GEM
+        remote: file://localhost/#{gem_repo4}/
+        specs:
+          coderay (1.1.2)
+          empyrean (0.1.0)
+          ffi (1.9.23-java)
+          method_source (0.9.0)
+          pry (0.11.3-java)
+            coderay (~> 1.1.0)
+            method_source (~> 0.9.0)
+            spoon (~> 0.0)
+          spoon (0.0.6)
+            ffi
+
+      PLATFORMS
+        java
+
+      DEPENDENCIES
+        empyrean (= 0.1.0)
+        pry
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    bundle! "lock --add-platform ruby"
+
+    good_lockfile = strip_whitespace(<<-L)
+      GEM
+        remote: file://localhost/#{gem_repo4}/
+        specs:
+          coderay (1.1.2)
+          empyrean (0.1.0)
+          ffi (1.9.23-java)
+          method_source (0.9.0)
+          pry (0.11.3)
+            coderay (~> 1.1.0)
+            method_source (~> 0.9.0)
+          pry (0.11.3-java)
+            coderay (~> 1.1.0)
+            method_source (~> 0.9.0)
+            spoon (~> 0.0)
+          spoon (0.0.6)
+            ffi
+
+      PLATFORMS
+        java
+        ruby
+
+      DEPENDENCIES
+        empyrean (= 0.1.0)
+        pry
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    expect(the_bundle.lockfile).to read_as good_lockfile
+
+    bad_lockfile = strip_whitespace <<-L
+      GEM
+        remote: file://localhost/#{gem_repo4}/
+        specs:
+          coderay (1.1.2)
+          empyrean (0.1.0)
+          ffi (1.9.23)
+          ffi (1.9.23-java)
+          method_source (0.9.0)
+          pry (0.11.3)
+            coderay (~> 1.1.0)
+            method_source (~> 0.9.0)
+          pry (0.11.3-java)
+            coderay (~> 1.1.0)
+            method_source (~> 0.9.0)
+            spoon (~> 0.0)
+          spoon (0.0.6)
+            ffi
+
+      PLATFORMS
+        java
+        ruby
+
+      DEPENDENCIES
+        empyrean (= 0.1.0)
+        pry
+
+      BUNDLED WITH
+        #{Bundler::VERSION}
+    L
+
+    aggregate_failures do
+      lockfile bad_lockfile
+      bundle! :install
+      expect(the_bundle.lockfile).to read_as good_lockfile
+
+      lockfile bad_lockfile
+      bundle! :update, :all => true
+      expect(the_bundle.lockfile).to read_as good_lockfile
+
+      lockfile bad_lockfile
+      bundle! "update ffi"
+      expect(the_bundle.lockfile).to read_as good_lockfile
+
+      lockfile bad_lockfile
+      bundle! "update empyrean"
+      expect(the_bundle.lockfile).to read_as good_lockfile
+
+      lockfile bad_lockfile
+      bundle! :lock
+      expect(the_bundle.lockfile).to read_as good_lockfile
+    end
+  end
+
   it "works the other way with gems that have different dependencies" do
     simulate_platform "ruby"
     install_gemfile <<-G