CI: Failed test_with_ruby_keyword(ConfigDSLTest) with Ruby HEAD

[Watson1978]

Describe the bug

CI in Ruby HEAD causes error with following message:

PS C:\src\fluentd> bundle exec ruby -v -I"lib;test" test/test_configdsl.rb -v
ruby 4.0.0dev (2025-11-08 master 9bbe4b600b) +PRISM [x64-mingw-ucrt]
C:/Ruby40-x64/lib/ruby/gems/4.0.0+0/gems/serverengine-2.4.0/lib/serverengine/socket_manager_win.rb:141: warning: assigned but unused variable - type
C:/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
C:/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
Loaded suite test/test_configdsl
Started
ConfigDSLTest: 
  test_config_error:																		.: (0.000403)
  test_parse:																			.: (0.000730)
  test_parse2:																			.: (0.000127)
  test_with_ruby_keyword:																	E
==================================================================================================================================================================================
Error: test_with_ruby_keyword(ConfigDSLTest): Errno::EINVAL: Invalid argument @ rb_sysopen - |uname -a
config.rb:1:in 'File#initialize'
config.rb:1:in 'Kernel.open'
config.rb:1:in 'Fluent::Config::DSL::Proxy#eval'
C:/src/fluentd/lib/fluent/config/dsl.rb:54:in 'BasicObject#instance_eval'
C:/src/fluentd/lib/fluent/config/dsl.rb:54:in 'Fluent::Config::DSL::Proxy#eval'
C:/src/fluentd/lib/fluent/config/dsl.rb:35:in 'Fluent::Config::DSL::Parser.parse'
test/test_configdsl.rb:110:in 'ConfigDSLTest#test_with_ruby_keyword'
     107: 
     108:   def test_with_ruby_keyword
     109:     uname_string = `uname -a`
  => 110:     root1 = Fluent::Config::DSL::Parser.parse(<<DSL)
     111: uname_str = ruby.open('|uname -a'){|out| out.read}
     112: source {
     113:   uname uname_str
==================================================================================================================================================================================
: (0.069793)

Finished in 0.0731801 seconds.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
4 tests, 22 assertions, 0 failures, 1 errors, 0 pendings, 0 omissions, 0 notifications
75% passed
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
54.66 tests/s, 300.63 assertions/s

Ref. https://github.com/fluent/fluentd/actions/runs/19209763728/job/54910244654

To Reproduce

Run bundle exec ruby -v -I"lib;test" test/test_configdsl.rb -v with Ruby HEAD on Windows

Expected behavior

No error

Your Environment

- Fluentd version:
- Package version:
- Operating system:
- Kernel version:

Your Configuration

none

Your Error Log

See above

Additional context

You can download Ruby HEAD windows installer from https://github.com/oneclick/rubyinstaller2/releases/tag/rubyinstaller-head

[Watson1978]

It fails on non-Windows systems as well.

$ bundle exec ruby -v -I"lib;test" test/test_configdsl.rb -v
ruby 4.0.0dev (2025-11-13T16:02:01Z master d58960a912) +PRISM [arm64-darwin25]
/Users/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
/Users/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
Loaded suite test/test_configdsl
Started
ConfigDSLTest:
  test_config_error:													.: (0.000272)
  test_parse:														.: (0.000196)
  test_parse2:														.: (0.000048)
  test_with_ruby_keyword:												E
============================================================================================================================================
Error: test_with_ruby_keyword(ConfigDSLTest): Errno::ENOENT: No such file or directory @ rb_sysopen - |uname -a
config.rb:1:in 'File#initialize'
config.rb:1:in 'Kernel.open'
config.rb:1:in 'Fluent::Config::DSL::Proxy#eval'
/Users/watson/src/fluentd/lib/fluent/config/dsl.rb:54:in 'BasicObject#instance_eval'
/Users/watson/src/fluentd/lib/fluent/config/dsl.rb:54:in 'Fluent::Config::DSL::Proxy#eval'
/Users/watson/src/fluentd/lib/fluent/config/dsl.rb:35:in 'Fluent::Config::DSL::Parser.parse'
test/test_configdsl.rb:110:in 'ConfigDSLTest#test_with_ruby_keyword'
     107:
     108:   def test_with_ruby_keyword
     109:     uname_string = `uname -a`
  => 110:     root1 = Fluent::Config::DSL::Parser.parse(<<DSL)
     111: uname_str = ruby.open('|uname -a'){|out| out.read}
     112: source {
     113:   uname uname_str
============================================================================================================================================
: (0.005369)

Finished in 0.00631 seconds.
--------------------------------------------------------------------------------------------------------------------------------------------
4 tests, 22 assertions, 0 failures, 1 errors, 0 pendings, 0 omissions, 0 notifications
75% passed
--------------------------------------------------------------------------------------------------------------------------------------------
633.91 tests/s, 3486.53 assertions/s

[Watson1978]

The Ruby HEAD in CI does not appear to be head...
At least, it has the commit stopped at a month ago.

[Watson1978]

Related to

• ruby/ruby@a1366f2
• ruby/ruby@996cae6#diff-92194f057884b3287a3a6bf84e6e3b2bf433a556b68562799252a091744e7854

In Ruby 4.0, as a security, seems that Kernel.open no longer executes commands starting with |.