Freeze more constants for Ractor compatibility

Freeze Net::HTTP::SSL_ATTRIBUTES and IDEMPOTENT_METHODS_. Both constants
have been marked as :nodoc:.

Together with ruby/openssl#521, this enables
HTTPS clients in non-main Ractors on Ruby 4.0.

Author: rhenium

lib/net/http.rb

@@ -1531,7 +1531,7 @@ def use_ssl=(flag)
       :verify_depth,
       :verify_mode,
       :verify_hostname,
-    ] # :nodoc:
+    ].freeze # :nodoc:
 
     SSL_IVNAMES = SSL_ATTRIBUTES.map { |a| "@#{a}".to_sym }.freeze # :nodoc:
 
@@ -2428,7 +2428,7 @@ def send_entity(path, data, initheader, dest, type, &block)
 
     # :stopdoc:
 
-    IDEMPOTENT_METHODS_ = %w/GET HEAD PUT DELETE OPTIONS TRACE/ # :nodoc:
+    IDEMPOTENT_METHODS_ = %w/GET HEAD PUT DELETE OPTIONS TRACE/.freeze # :nodoc:
 
     def transport_request(req)
       count = 0

test/net/http/test_http.rb

@@ -1400,3 +1400,28 @@ def test_partial_response
     assert_raise(EOFError) {http.get('/')}
   end
 end
+
+class TestNetHTTPInRactor < Test::Unit::TestCase
+  CONFIG = {
+    'host' => '127.0.0.1',
+    'proxy_host' => nil,
+    'proxy_port' => nil,
+  }
+
+  include TestNetHTTPUtils
+
+  def test_get
+    assert_ractor(<<~RUBY, require: 'net/http')
+      expected = #{$test_net_http_data.dump}.b
+      ret = Ractor.new {
+        host = #{config('host').dump}
+        port = #{config('port')}
+        Net::HTTP.start(host, port) { |http|
+          res = http.get('/')
+          res.body
+        }
+      }.value
+      assert_equal expected, ret
+    RUBY
+  end
+end if defined?(Ractor) && Ractor.method_defined?(:value)

test/net/http/test_https.rb

@@ -266,6 +266,24 @@ def test_max_version
     assert_match(re_msg, ex.message)
   end
 
+  def test_ractor
+    assert_ractor(<<~RUBY, require: 'net/https')
+      expected = #{$test_net_http_data.dump}.b
+      ret = Ractor.new {
+        host = #{HOST.dump}
+        port = #{config('port')}
+        ca_cert_pem = #{CA_CERT.to_pem.dump}
+        cert_store = OpenSSL::X509::Store.new.tap { |s|
+          s.add_cert(OpenSSL::X509::Certificate.new(ca_cert_pem))
+        }
+        Net::HTTP.start(host, port, use_ssl: true, cert_store: cert_store) { |http|
+          res = http.get('/')
+          res.body
+        }
+      }.value
+      assert_equal expected, ret
+    RUBY
+  end if defined?(Ractor) && Ractor.method_defined?(:value)
 end
 
 class TestNetHTTPSIdentityVerifyFailure < Test::Unit::TestCase