fix(windows): version info contains ldflags on windows

to allow downstream gems like nokogumbo to build correctly simply by
requiring nokogiri in the extconf and running:

  append_cflags(Nokogiri::VERSION_INFO["nokogiri"]["cppflags"])
  append_ldflags(Nokogiri::VERSION_INFO["nokogiri"]["ldflags"])

this is needed because unresolved symbols aren't allowed on windows.

Related to #2167

lib/nokogiri/version/info.rb

@@ -10,6 +10,14 @@ def jruby?
       ::JRUBY_VERSION if ::RUBY_PLATFORM == "java"
     end
 
+    def windows?
+      ::RUBY_PLATFORM =~ /mingw|mswin/
+    end
+
+    def ruby_minor
+      Gem::Version.new(::RUBY_VERSION).segments[0..1].join(".")
+    end
+
     def engine
       defined?(::RUBY_ENGINE) ? ::RUBY_ENGINE : "mri"
     end
@@ -74,18 +82,33 @@ def warnings
 
     def to_hash
       header_directory = File.expand_path(File.join(File.dirname(__FILE__), "../../../ext/nokogiri"))
+
       {}.tap do |vi|
         vi["warnings"] = []
         vi["nokogiri"] = {}.tap do |nokogiri|
           nokogiri["version"] = Nokogiri::VERSION
 
           unless jruby?
             cppflags = ["-I#{header_directory.shellescape}"]
+            ldflags = []
+
             if libxml2_using_packaged?
               cppflags << "-I#{File.join(header_directory, 'include').shellescape}"
               cppflags << "-I#{File.join(header_directory, 'include/libxml2').shellescape}"
+
+              if windows?
+                # on windows, nokogumbo needs to link against nokogiri.so to resolve symbols. see #2167
+                lib_directory = File.expand_path(File.join(File.dirname(__FILE__), "../#{ruby_minor}"))
+                unless File.exist?(lib_directory)
+                  lib_directory = File.expand_path(File.join(File.dirname(__FILE__), ".."))
+                end
+                ldflags << "-L#{lib_directory.shellescape}"
+                ldflags << "-l:nokogiri.so"
+              end
             end
+
             nokogiri["cppflags"] = cppflags
+            nokogiri["ldflags"] = ldflags
           end
         end
         vi["ruby"] = {}.tap do |ruby|

scripts/test-gem-installation

@@ -43,6 +43,8 @@ Minitest::Reporters.use!([Minitest::Reporters::SpecReporter.new])
 
 puts "Testing #{gemspec.full_name} installed in #{gemspec.base_dir}"
 describe gemspec.full_name do
+  let(:ruby_maj_min) { Gem::Version.new(::RUBY_VERSION).segments[0..1].join(".") }
+  let(:nokogiri_lib_dir) { File.join(gemspec.gem_dir, "lib/nokogiri") }
   let(:nokogiri_ext_dir) { File.join(gemspec.gem_dir, "ext/nokogiri") }
   let(:nokogiri_include_dir) { File.join(nokogiri_ext_dir, "include") }
 
@@ -108,6 +110,20 @@ describe gemspec.full_name do
             assert(found, "expected to find #{header} in one of: #{headers_dirs.inspect}")
           end
         end
+
+        it "has ldflags pointing to the shared object file" do
+          ldflags = Nokogiri::VERSION_INFO["nokogiri"]["ldflags"]
+          if ::RUBY_PLATFORM =~ /mingw|mswin/
+            if gemspec.platform.cpu
+              assert_includes(ldflags, "-L#{File.join(nokogiri_lib_dir, ruby_maj_min)}")
+            else
+              assert_includes(ldflags, "-L#{nokogiri_lib_dir}")
+            end
+            assert_includes(ldflags, "-l:nokogiri.so")
+          else
+            assert_empty(ldflags)
+          end
+        end
       end if Nokogiri::VersionInfo.instance.libxml2_using_packaged?
 
       describe "using system libraries" do

test/test_version.rb

@@ -18,8 +18,9 @@ def test_version_info_basics
     if jruby?
       refute(Nokogiri::VERSION_INFO["nokogiri"].has_key?("cppflags"), "did not expect cppflags")
     else
-      # cppflags are more fully tested in scripts/test-gem-installation
-      assert_kind_of(Array, Nokogiri::VERSION_INFO["nokogiri"]["cppflags"], "expected cppflags to be an array")
+      # cppflags/ldflags are more fully tested in scripts/test-gem-installation
+      assert_kind_of(Array, Nokogiri::VERSION_INFO["nokogiri"]["cppflags"], "cppflags should be an array")
+      assert_kind_of(Array, Nokogiri::VERSION_INFO["nokogiri"]["ldflags"], "ldflags should be an array")
     end
 
     assert_equal(::RUBY_VERSION, Nokogiri::VERSION_INFO["ruby"]["version"])