[rubygems/rubygems] Set SOURCE_DATE_EPOCH env var if not provided.

Fixes #2290.

1. `Gem::Specification.date` returns SOURCE_DATE_EPOCH when defined,
2. this commit makes RubyGems set it _persistently_ when not provided.

This combination means that you can build a gem, check the build time,
and use that value to generate a new build -- and then verify they're
the same.

https://github.com/rubygems/rubygems/commit/d830d53f59

1 files changed, 27 insertions, 0 deletions

diff --git a/test/rubygems/test_gem_package.rb b/test/rubygems/test_gem_package.rb
index f2f712ebe0..a282e0cc5f 100644
--- a/test/rubygems/test_gem_package.rb
+++ b/test/rubygems/test_gem_package.rb
@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require 'rubygems/package/tar_test_case'
+require 'digest'
 
 class TestGemPackage < Gem::Package::TarTestCase
 
@@ -123,6 +124,32 @@ class TestGemPackage < Gem::Package::TarTestCase
     ENV["SOURCE_DATE_EPOCH"] = epoch
   end
 
+  def test_build_time_source_date_epoch_automatically_set
+    epoch = ENV["SOURCE_DATE_EPOCH"]
+    ENV["SOURCE_DATE_EPOCH"] = nil
+
+    start_time = Time.now.utc.to_i
+
+    spec = Gem::Specification.new 'build', '1'
+    spec.summary = 'build'
+    spec.authors = 'build'
+    spec.files = ['lib/code.rb']
+    spec.rubygems_version = Gem::Version.new '0'
+
+    package = Gem::Package.new spec.file_name
+
+    end_time = Time.now.utc.to_i
+
+    assert package.build_time.is_a?(Time)
+
+    build_time = package.build_time.to_i
+
+    assert(start_time <= build_time)
+    assert(build_time <= end_time)
+  ensure
+    ENV["SOURCE_DATE_EPOCH"] = epoch
+  end
+
   def test_add_files
     spec = Gem::Specification.new
     spec.files = %w[lib/code.rb lib/empty]