Skip the make jobserver on Windows

bundler/lib/bundler/installer/parallel_installer.rb

@@ -119,22 +119,40 @@ def install_with_worker
     end
 
     def with_jobserver
-      r, w = IO.pipe
-      r.close_on_exec = false
-      w.close_on_exec = false
-      w.write("*" * @size)
-
-      old_makeflags = ENV["MAKEFLAGS"]
-      ENV["MAKEFLAGS"] = [old_makeflags, "--jobserver-auth=#{r.fileno},#{w.fileno}"].compact.join(" ")
-
-      yield
-    ensure
-      # Restore MAKEFLAGS before closing the pipe so a close failure can't
-      # leave the process with descriptors that point at a closed pipe.
-      old_makeflags ? ENV["MAKEFLAGS"] = old_makeflags : ENV.delete("MAKEFLAGS")
+      # The jobserver hands tokens to child `make` processes through MAKEFLAGS
+      # using the GNU make `--jobserver-auth` protocol. nmake, the default make
+      # on mswin, instead reads MAKEFLAGS as bare option letters and aborts
+      # every native extension build with `fatal error U1065: invalid option
+      # '-'`. Skip the jobserver when nmake is in use. Other Windows toolchains
+      # such as mingw use GNU make and keep working through the inherited pipe.
+      return yield if nmake?
+
+      begin
+        r, w = IO.pipe
+        r.close_on_exec = false
+        w.close_on_exec = false
+        w.write("*" * @size)
+
+        old_makeflags = ENV["MAKEFLAGS"]
+        ENV["MAKEFLAGS"] = [old_makeflags, "--jobserver-auth=#{r.fileno},#{w.fileno}"].compact.join(" ")
+
+        yield
+      ensure
+        # Restore MAKEFLAGS before closing the pipe so a close failure can't
+        # leave the process with descriptors that point at a closed pipe.
+        old_makeflags ? ENV["MAKEFLAGS"] = old_makeflags : ENV.delete("MAKEFLAGS")
+
+        r&.close
+        w&.close
+      end
+    end
 
-      r&.close
-      w&.close
+    # Mirror how RubyGems' extension builder picks the make program so the
+    # jobserver is only set up when a GNU-compatible make will consume it.
+    def nmake?
+      make = ENV["MAKE"] || ENV["make"]
+      make ||= "nmake" if RUBY_PLATFORM.include?("mswin")
+      /\bnmake/i.match?(make.to_s)
     end
 
     def install_serially

spec/bundler/installer/parallel_installer_spec.rb

@@ -219,4 +219,29 @@ def redefine_build_jobs
       Bundler::RubyGemsGemInstaller.define_method(:build_jobs, old_method)
     end
   end
+
+  describe "make jobserver with nmake" do
+    # nmake reads MAKEFLAGS from the environment and treats its contents as
+    # bare option letters, so a GNU make `--jobserver-auth` aborts the build
+    # with `fatal error U1065: invalid option '-'`. The jobserver must be
+    # skipped when nmake is the make program.
+    it "leaves MAKEFLAGS untouched" do
+      parallel_installer = Bundler::ParallelInstaller.new(nil, [], 5, false, false)
+
+      makeflags_before = ENV["MAKEFLAGS"]
+      makeflags_during = :not_yielded
+
+      old_make = ENV["MAKE"]
+      ENV["MAKE"] = "nmake"
+      begin
+        parallel_installer.send(:with_jobserver) do
+          makeflags_during = ENV["MAKEFLAGS"]
+        end
+      ensure
+        ENV["MAKE"] = old_make
+      end
+
+      expect(makeflags_during).to eq(makeflags_before)
+    end
+  end
 end