From 02c8e6583b33c62b4050420234167a467f32fefa Mon Sep 17 00:00:00 2001
From: Samuel Giddins <segiddins@segiddins.me>
Date: Tue, 20 Feb 2024 11:05:30 -0800
Subject: [PATCH] [rubygems/rubygems] Use a constant empty tar header to avoid
 extra allocations

https://github.com/rubygems/rubygems/commit/716666f65f
---
 lib/rubygems/package/tar_header.rb           | 24 +++++++++++++++----
 test/rubygems/test_gem_package_tar_header.rb | 25 ++++++++++++++++++++
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/lib/rubygems/package/tar_header.rb b/lib/rubygems/package/tar_header.rb
index 087f13f6c9..dd5e835a1e 100644
--- a/lib/rubygems/package/tar_header.rb
+++ b/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 @@ class Gem::Package::TarHeader
         devminor: strict_oct(fields.shift),
         prefix: fields.shift,
 
-        empty: empty
+        empty: false
   end
 
   def self.strict_oct(str)
@@ -172,6 +172,22 @@ class Gem::Package::TarHeader
     @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 @@ class Gem::Package::TarHeader
 
     header = header.pack PACK_FORMAT
 
-    header << ("\0" * ((512 - header.size) % 512))
+    header.ljust 512, "\0"
   end
 
   def oct(num, len)
diff --git a/test/rubygems/test_gem_package_tar_header.rb b/test/rubygems/test_gem_package_tar_header.rb
index 4469750f9a..a3f95bb770 100644
--- a/test/rubygems/test_gem_package_tar_header.rb
+++ b/test/rubygems/test_gem_package_tar_header.rb
@@ -99,6 +99,31 @@ class TestGemPackageTarHeader < Gem::Package::TarTestCase
     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