[PROF-10241] Use Process._fork hook in at_fork monkey patch on Ruby 3.1+

[ivoanjo]

What does this PR do?

This PR solves the long-standing "TODO" we had on the at_fork monkey patch where on Ruby 3.1+ there's a VM hook for instrumenting fork operations that avoids us needing to monkey patch Kernel and Object.

To use the new hook we need to monkey patch Process._fork instead (see ruby/ruby#5017 and
https://bugs.ruby-lang.org/issues/17795).

Thus, I've added this monkey patch and disabled patching of Kernel and Object on Ruby 3.1+.

Motivation:

Avoid monkey patching core classes that no longer need to.

Additional Notes:

I'm not quite happy with either the AtForkMonkeyPatch or its specs, but I decided to not do a full revamp for now.

TIP: I recommend reviewing this PR with no whitespace in the diff.

How to test the change?

This change includes test coverage.

Here's a tiny test app that also exercises this feature:

puts RUBY_DESCRIPTION

require 'datadog/core/utils/at_fork_monkey_patch'

Datadog::Core::Utils::AtForkMonkeyPatch.apply!

Process.datadog_at_fork(:child) { puts "Hello from child process: #{Process.pid} "}

puts "Parent pid: #{Process.pid}"

puts "fork { }"
fork { }
Process.wait

puts "Kernel.fork { }"
Kernel.fork { }
Process.wait

puts "Process.fork { }"
Process.fork { }
Process.wait

puts "Foo.new.call"
class Foo
  def call
    fork { }
  end

  def self.do_fork
    fork {  }
  end
end
Foo.new.call
Process.wait

puts "Foo.do_fork"
Foo.do_fork
Process.wait

puts "Fork from outside"
Foo.new.send(:fork) { }
Process.wait

class BasicFoo < BasicObject
  include ::Kernel

  def call
    fork { }
  end

  def self.do_fork
    fork { }
  end
end

puts "BasicFoo.new.call"
BasicFoo.new.call
Process.wait

puts "BasicObject:"
Class.new(BasicObject) { include(::Kernel); def call; fork { }; end }.new.call
Process.wait

puts "BasicFork.do_fork"
BasicFoo.do_fork
Process.wait

P.s.: You may notice when you run this example on < 3.1 that BasicFoo.new.call and BasicObject are not instrumented!

This was actually a known-gap from the old instrumentation.

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-07 16:45:12

Comparing candidate commit 9cf5444 in PR branch ivoanjo/prof-10241-use-process-fork-modern-rubies with baseline commit 4c69eb3 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 23 metrics, 2 unstable metrics.

[p-datadog]

I would check pid before key? for efficiency.

[ivoanjo]

Fixed in 9cf5444

[p-datadog]

Should the module now be called just ProcessMoneyPatch?

[ivoanjo]

Process 💰 Patch 🤣

Fixed in 0c719dd

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.71429% with 17 lines in your changes missing coverage. Please review.

Project coverage is 97.84%. Comparing base (4c69eb3) to head (9cf5444).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...ec/datadog/core/utils/at_fork_monkey_patch_spec.rb	74.07%	14 Missing ⚠️
lib/datadog/core/utils/at_fork_monkey_patch.rb	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3830      +/-   ##
==========================================
- Coverage   97.87%   97.84%   -0.03%     
==========================================
  Files        1264     1264              
  Lines       75710    75731      +21     
  Branches     3716     3719       +3     
==========================================
  Hits        74098    74098              
- Misses       1612     1633      +21     

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[p-datadog]

👍