[PROF-7307] Enable allocation counting feature by default for some Ru…

…by 3 versions

**What does this PR do?**

This PR enables the Profiler's `allocation_counting_enabled` feature
by default for some Ruby 3 versions.

(This feature depends on the profiler itself being enabled).

TL;DR we were already enabling this feature by default on Ruby 2, but
did not do it on Ruby 3 because of a VM bug
( https://bugs.ruby-lang.org/issues/18464 ).

Now that this VM bug has been fixed for some Ruby versions
(3.1.4, 3.2.3, 3.3.0), we can enable it by default again.

**Motivation:**

The allocation counting feature is required to enable other
features in the future (including allocation profiling) and thus
we want to allow as many customers as possible to have it.

**Additional Notes:**

There's a second annoying VM bug that can also break
`allocation_counting_enabled` (https://bugs.ruby-lang.org/issues/19112)
but since that bug doesn't cause a VM crash and only makes it so that
data collection stops for this feature, I think we can live with it
for now.

I'm also reaching out through some contacts to see if we can get the
other bug fixed soon (and possibly backported).

**How to test the change?**

Change includes test coverage.

lib/datadog/core/configuration/settings.rb

@@ -314,15 +314,28 @@ def initialize(*_)
 
             # Can be used to enable/disable the Datadog::Profiling.allocation_count feature.
             #
-            # This feature is safe and enabled by default on Ruby 2.x, but
-            # on Ruby 3.x it can break in applications that make use of Ractors due to two Ruby VM bugs:
-            # https://bugs.ruby-lang.org/issues/19112 AND https://bugs.ruby-lang.org/issues/18464.
+            # This feature is safe and enabled by default on Ruby 2.x, but has a few caveats on Ruby 3.x.
             #
-            # If you use Ruby 3.x and your application does not use Ractors (or if your Ruby has been patched), the
-            # feature is fully safe to enable and this toggle can be used to do so.
+            # Caveat 1 (severe):
+            # On Ruby versions 3.0 (all), 3.1.0 to 3.1.3, and 3.2.0 to 3.2.2 this is disabled by default because it
+            # can trigger a VM bug that causes a segmentation fault during garbage collection of Ractors
+            # (https://bugs.ruby-lang.org/issues/18464). We don't recommend using this feature on such Rubies.
+            # This bug is fixed on Ruby versions 3.1.4, 3.2.3 and 3.3.0.
             #
-            # @default `true` on Ruby 2.x, `false` on Ruby 3.x
-            option :allocation_counting_enabled, default: RUBY_VERSION.start_with?('2.')
+            # Caveat 2 (annoyance):
+            # On all known versions of Ruby 3.x, due to https://bugs.ruby-lang.org/issues/19112, when a ractor gets
+            # garbage collected, Ruby will disable all active tracepoints, which this feature internally relies on.
+            # Thus this feature is only usable if you're not using Ractors.
+            #
+            # @default `true` on Ruby 2.x and 3.1.4+, 3.2.3+ and 3.3.0+; `false` for Ruby 3.0 and unpatched Rubies.
+            option :allocation_counting_enabled do |o|
+              o.default do
+                RUBY_VERSION.start_with?('2.') ||
+                  (RUBY_VERSION.start_with?('3.1.') && RUBY_VERSION >= '3.1.4') ||
+                  (RUBY_VERSION.start_with?('3.2.') && RUBY_VERSION >= '3.2.3') ||
+                  RUBY_VERSION >= '3.3.'
+              end
+            end
 
             # Can be used to disable checking which version of `libmysqlclient` is being used by the `mysql2` gem.
             #

spec/datadog/core/configuration/settings_spec.rb

@@ -500,16 +500,25 @@
       describe '#allocation_counting_enabled' do
         subject(:allocation_counting_enabled) { settings.profiling.advanced.allocation_counting_enabled }
 
-        context 'on Ruby 2.x' do
-          before { skip("Spec doesn't run on Ruby 3.x") unless RUBY_VERSION.start_with?('2.') }
+        before { stub_const('RUBY_VERSION', testing_version) }
 
+        context 'on Ruby 2.x' do
+          let(:testing_version) { '2.3.0 ' }
           it { is_expected.to be true }
         end
 
-        context 'on Ruby 3.x' do
-          before { skip("Spec doesn't run on Ruby 2.x") if RUBY_VERSION.start_with?('2.') }
+        ['3.0.0', '3.1.0', '3.1.3', '3.2.0', '3.2.2'].each do |broken_ruby|
+          context "on a Ruby 3 version affected by https://bugs.ruby-lang.org/issues/18464 (#{broken_ruby})" do
+            let(:testing_version) { broken_ruby }
+            it { is_expected.to be false }
+          end
+        end
 
-          it { is_expected.to be false }
+        ['3.1.4', '3.2.3', '3.3.0'].each do |fixed_ruby|
+          context "on a Ruby 3 version where https://bugs.ruby-lang.org/issues/18464 is fixed (#{fixed_ruby})" do
+            let(:testing_version) { fixed_ruby }
+            it { is_expected.to be true }
+          end
         end
       end