From: "luke-gru (Luke Gruber) via ruby-core" <ruby-core@...>
Date: 2023-01-25T23:17:45+00:00
Subject: [ruby-core:112043] [Ruby master Bug#19112] Ractor garbage collection breaks/disables all active tracepoints

Issue #19112 has been updated by luke-gru (Luke Gruber).





Pull request is here if you're interested: https://github.com/ruby/ruby/pull/7184



----------------------------------------

Bug #19112: Ractor garbage collection breaks/disables all active tracepoints

https://bugs.ruby-lang.org/issues/19112#change-101477



* Author: ivoanjo (Ivo Anjo)

* Status: Open

* Priority: Normal

* ruby -v: ruby 3.2.0dev (2022-11-08T08:08:48Z master cdb3ec3af8) [x86_64-linux]

* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

----------------------------------------

Howdy ����! I work for Datadog [on the `ddtrace` gem](https://github.com/DataDog/dd-trace-rb) and I found an issue with Ractors + TracePoint.



### Background



For tracking time spent in Garbage Collection on our Ruby profiler, we use a tracepoint that listens to the internal GC events. While doing other experiments around Ractors and profiling, I noticed that our tracepoint would suddenly stop working after Ractors were used in the application. I was able to reduce this down to the testcase below.



### How to reproduce (Ruby version & script)



I was able to reproduce this on any Ruby 3 version, including master (`ruby 3.2.0dev (2022-11-08T08:08:48Z master cdb3ec3af8) [x86_64-linux]`).



```ruby

puts RUBY_DESCRIPTION



if ENV['RUN_RACTOR'] == 'true'

  Ractor.new { 10 }.take

  puts "Ractor ran!"

else

  puts "Ractor DID NOT ran!"

end



puts "Enabling tracepoint"

tracepoint = TracePoint.new(:line) { |tp| p [tp.path, tp.lineno, tp.event] }.tap(&:enable)



5.times { puts "Triggering gc!"; GC.start }



puts "Tracepoint state is enabled? #{tracepoint.enabled?}"

puts "Re-enabling tracepoint"

tracepoint.enable



puts "Finished!"

```



### Expectation and result



Without the Ractor, the tracepoint works as intended:



```

# RUN_RACTOR=false ./ruby ../ruby-3.1.2/triggergc.rb

ruby 3.2.0dev (2022-11-08T08:08:48Z master cdb3ec3af8) [x86_64-linux]

Ractor DID NOT ran!

Enabling tracepoint

["<internal:kernel>", 91, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

["../ruby-3.1.2/triggergc.rb", 16, :line]

["<internal:trace_point>", 257, :line]

Tracepoint state is enabled? true

["../ruby-3.1.2/triggergc.rb", 17, :line]

Re-enabling tracepoint

["../ruby-3.1.2/triggergc.rb", 18, :line]

["<internal:trace_point>", 213, :line]

["../ruby-3.1.2/triggergc.rb", 20, :line]

["../ruby-3.1.2/triggergc.rb", 20, :line]

Finished!

```



...but once the Ractor runs and is garbage collected, we see that the tracepoint breaks:





```

# RUN_RACTOR=true ./ruby ../ruby-3.1.2/triggergc.rb

ruby 3.2.0dev (2022-11-08T08:08:48Z master cdb3ec3af8) [x86_64-linux]

../ruby-3.1.2/triggergc.rb:5: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.

Ractor ran!

Enabling tracepoint

["<internal:kernel>", 91, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

["../ruby-3.1.2/triggergc.rb", 14, :line]

Triggering gc!

["<internal:gc>", 34, :line]

Triggering gc!

Triggering gc!

Triggering gc!

Triggering gc!

Tracepoint state is enabled? true

Re-enabling tracepoint

["../ruby-3.1.2/triggergc.rb", 20, :line]

["../ruby-3.1.2/triggergc.rb", 20, :line]

Finished

```



### Additional notes



This is a somewhat annoying bug, because it can break all gems that use tracepoints, including `ddtrace`. In particular for `ddtrace`, our plan is to disable the feature that uses tracepoints for Ruby 3 users, to avoid any issues from our tracepoints getting disabled.







-- 

https://bugs.ruby-lang.org/

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