[ruby/rubygems] Fix constants in TAR to be frozen

I would like to use the tar implementation inside a Ractor, but two of
the constants are not frozen.  This patch freezes the constants so we
can use it in a Ractor.

ruby/rubygems@0ff4790f4c

Author: tenderlove

lib/rubygems/package/tar_header.rb

@@ -56,7 +56,7 @@ class Gem::Package::TarHeader
   ##
   # Pack format for a tar header
 
-  PACK_FORMAT = "a100" + # name
+  PACK_FORMAT = ("a100" + # name
                 "a8"   + # mode
                 "a8"   + # uid
                 "a8"   + # gid
@@ -71,12 +71,12 @@ class Gem::Package::TarHeader
                 "a32"  + # gname
                 "a8"   + # devmajor
                 "a8"   + # devminor
-                "a155"   # prefix
+                "a155").freeze # prefix
 
   ##
   # Unpack format for a tar header
 
-  UNPACK_FORMAT = "A100" + # name
+  UNPACK_FORMAT = ("A100" + # name
                   "A8"   + # mode
                   "A8"   + # uid
                   "A8"   + # gid
@@ -91,7 +91,7 @@ class Gem::Package::TarHeader
                   "A32"  + # gname
                   "A8"   + # devmajor
                   "A8"   + # devminor
-                  "A155"   # prefix
+                  "A155").freeze # prefix
 
   attr_reader(*FIELDS)
 

test/rubygems/test_gem_package_tar_header.rb

@@ -26,6 +26,25 @@ def setup
     @tar_header = Gem::Package::TarHeader.new header
   end
 
+  def test_decode_in_ractor
+    new_header = Ractor.new(@tar_header.to_s) do |str|
+      Gem::Package::TarHeader.from StringIO.new str
+    end.value
+
+    assert_headers_equal @tar_header, new_header
+  end if defined?(Ractor) && Ractor.instance_methods.include?(:value)
+
+  def test_encode_in_ractor
+    header_bytes = @tar_header.to_s
+
+    new_header = Ractor.new(header_bytes) do |str|
+      header = Gem::Package::TarHeader.from StringIO.new str
+      header.to_s
+    end.value
+
+    assert_headers_equal header_bytes, new_header
+  end if defined?(Ractor) && Ractor.instance_methods.include?(:value)
+
   def test_self_from
     io = TempIO.new @tar_header.to_s