Simplify gem downloading inside bundler

We can skip most stuff in `Gem::RemoteFetcher#download`, and use
`Gem::RemoteFetcher#update_cache_path` directly.

This has the benefit of allowing us to remove some workarounds to
support several rubygems versions, but also allows us to pass the target
folder where the gem should be downloaded directly and skip the logic
inside `Gem::RemoteFetcher#download` to infer the cache path. This will
be useful later to fix some issues with the `global_gem_cache` feature
flag.

bundler/lib/bundler/rubygems_integration.rb

@@ -518,13 +518,32 @@ def fetch_all_remote_specs(remote)
       specs.concat(pres)
     end
 
-    def download_gem(spec, uri, path)
+    def download_gem(spec, uri, cache_dir)
       require "rubygems/remote_fetcher"
       uri = Bundler.settings.mirror_for(uri)
       fetcher = gem_remote_fetcher
       fetcher.headers = { "X-Gemfile-Source" => spec.remote.original_uri.to_s } if spec.remote.original_uri
       Bundler::Retry.new("download gem from #{uri}").attempts do
-        fetcher.download(spec, uri, path)
+        gem_file_name = spec.file_name
+        local_gem_path = File.join cache_dir, gem_file_name
+        return if File.exist? local_gem_path
+
+        begin
+          remote_gem_path = uri + "gems/#{gem_file_name}"
+          remote_gem_path = remote_gem_path.to_s if provides?("< 3.2.0.rc.1")
+
+          SharedHelpers.filesystem_access(local_gem_path) do
+            fetcher.cache_update_path remote_gem_path, local_gem_path
+          end
+        rescue Gem::RemoteFetcher::FetchError
+          raise if spec.original_platform == spec.platform
+
+          original_gem_file_name = "#{spec.original_name}.gem"
+          raise if gem_file_name == original_gem_file_name
+
+          gem_file_name = original_gem_file_name
+          retry
+        end
       end
     rescue Gem::RemoteFetcher::FetchError => e
       raise Bundler::HTTPError, "Could not download gem from #{uri} due to underlying error <#{e.message}>"

bundler/lib/bundler/source/rubygems.rb

@@ -521,17 +521,7 @@ def download_gem(spec, download_cache_path)
         else
           uri = spec.remote.uri
           Bundler.ui.confirm("Fetching #{version_message(spec)}")
-          rubygems_local_path = Bundler.rubygems.download_gem(spec, uri, File.dirname(download_cache_path))
-
-          # older rubygems return varying file:// variants depending on version
-          rubygems_local_path = rubygems_local_path.gsub(/\Afile:/, "") unless Bundler.rubygems.provides?(">= 3.2.0.rc.2")
-          rubygems_local_path = rubygems_local_path.gsub(%r{\A//}, "") if Bundler.rubygems.provides?("< 3.1.0")
-
-          if rubygems_local_path != local_path
-            SharedHelpers.filesystem_access(local_path) do
-              FileUtils.mv(rubygems_local_path, local_path)
-            end
-          end
+          Bundler.rubygems.download_gem(spec, uri, download_cache_path)
           cache_globally(spec, local_path)
         end
       end

bundler/spec/bundler/rubygems_integration_spec.rb

@@ -43,11 +43,10 @@
 
   describe "#download_gem" do
     let(:bundler_retry) { double(Bundler::Retry) }
-    let(:uri) {  Bundler::URI.parse("https://foo.bar") }
-    let(:path) { Gem.path.first }
+    let(:uri) { Bundler::URI.parse("https://foo.bar") }
+    let(:cache_dir) { "#{Gem.path.first}/cache" }
     let(:spec) do
-      spec = Bundler::RemoteSpecification.new("Foo", Gem::Version.new("2.5.2"),
-        Gem::Platform::RUBY, nil)
+      spec = Gem::Specification.new("Foo", Gem::Version.new("2.5.2"))
       spec.remote = Bundler::Source::Rubygems::Remote.new(uri.to_s)
       spec
     end
@@ -59,9 +58,9 @@
       expect(Bundler::Retry).to receive(:new).with("download gem from #{uri}/").
         and_return(bundler_retry)
       expect(bundler_retry).to receive(:attempts).and_yield
-      expect(fetcher).to receive(:download).with(spec, uri, path)
+      expect(fetcher).to receive(:cache_update_path)
 
-      Bundler.rubygems.download_gem(spec, uri, path)
+      Bundler.rubygems.download_gem(spec, uri, cache_dir)
     end
   end