Use a constant empty tar header to avoid extra allocations

lib/rubygems/package/tar_header.rb

@@ -95,14 +95,14 @@ class Gem::Package::TarHeader
 
   attr_reader(*FIELDS)
 
-  EMPTY_HEADER = ("\0" * 512).freeze # :nodoc:
+  EMPTY_HEADER = ("\0" * 512).b.freeze # :nodoc:
 
   ##
   # Creates a tar header from IO +stream+
 
   def self.from(stream)
     header = stream.read 512
-    empty = (header == EMPTY_HEADER)
+    return EMPTY if header == EMPTY_HEADER
 
     fields = header.unpack UNPACK_FORMAT
 
@@ -123,7 +123,7 @@ def self.from(stream)
         devminor: strict_oct(fields.shift),
         prefix: fields.shift,
 
-        empty: empty
+        empty: false
   end
 
   def self.strict_oct(str)
@@ -172,6 +172,22 @@ def initialize(vals)
     @empty = vals[:empty]
   end
 
+  EMPTY = new({ # :nodoc:
+    checksum: 0,
+    gname: "",
+    linkname: "",
+    magic: "",
+    mode: 0,
+    name: "",
+    prefix: "",
+    size: 0,
+    uname: "",
+    version: 0,
+
+    empty: true,
+  }).freeze
+  private_constant :EMPTY
+
   ##
   # Is the tar entry empty?
 
@@ -241,7 +257,7 @@ def header(checksum = @checksum)
 
     header = header.pack PACK_FORMAT
 
-    header << ("\0" * ((512 - header.size) % 512))
+    header.ljust 512, "\0"
   end
 
   def oct(num, len)

test/rubygems/test_gem_package_tar_header.rb

@@ -99,6 +99,31 @@ def test_empty_eh
     assert_empty @tar_header
   end
 
+  def test_empty
+    @tar_header = Gem::Package::TarHeader.from(StringIO.new(Gem::Package::TarHeader::EMPTY_HEADER))
+
+    assert_empty @tar_header
+    assert_equal Gem::Package::TarHeader.new(
+      checksum: 0,
+      devmajor: 0,
+      devminor: 0,
+      empty: true,
+      gid: 0,
+      gname: "",
+      linkname: "",
+      magic: "",
+      mode: 0,
+      mtime: 0,
+      name: "",
+      prefix: "",
+      size: 0,
+      typeflag: "0",
+      uid: 0,
+      uname: "",
+      version: 0,
+    ), @tar_header
+  end
+
   def test_equals2
     assert_equal @tar_header, @tar_header
     assert_equal @tar_header, @tar_header.dup