feat: ruby and native gems always install nokogiri header files

This addresses part of #1788

ext/nokogiri/extconf.rb

@@ -762,6 +762,9 @@ def configure
 
 have_func('vasprintf')
 
+# When compiling during installation, install nokogiri's header files into lib/nokogiri/include
+$INSTALLFILES << ["*.h", "$(rubylibdir)/include"]
+
 create_makefile('nokogiri/nokogiri')
 
 if enable_config('clean', true)

rakelib/cross-ruby.rake

@@ -351,6 +351,21 @@ else
 
       spec.files.reject! { |path| File.fnmatch?('ports/*', path) }
       spec.dependencies.reject! { |dep| dep.name=='mini_portile2' }
+
+      # when pre-compiling a native gem, package all the C headers in lib/nokogiri/include:
+      # - nokogiri's headers
+      #
+      # (see the $INSTALLFILES section of extconf.rb for complementary behavior)
+      # (see scripts/test-gem-file-contents and scripts/test-gem-installation for tests)
+      headers_dir = "lib/nokogiri/include"
+
+      Dir.glob("ext/nokogiri/*.h").each do |header|
+        FileUtils.cp(header, headers_dir)
+      end
+
+      Dir.glob(File.join(headers_dir, "**", "*.h")).each do |header|
+        spec.files << header
+      end
     end
   end
 end

scripts/test-gem-file-contents

@@ -140,7 +140,7 @@ describe File.basename(gemfile) do
     end
 
     it "contains nokogiri header files in lib/nokogiri/include" do
-      skip "until we fix https://github.com/sparklemotion/nokogiri/pull/1788"
+      # https://github.com/sparklemotion/nokogiri/pull/1788
       assert_includes(gemfile_contents, "lib/nokogiri/include/nokogiri.h")
     end
 
@@ -186,7 +186,6 @@ describe File.basename(gemfile) do
     end
 
     it "does not contain header files in lib/nokogiri/include" do
-      skip "until we fix https://github.com/sparklemotion/nokogiri/pull/1788"
       assert_empty(gemfile_contents.grep(%r{^lib/.*\.h$}))
     end