[PROF-10015] Fix Phusion Passenger detection when not in Gemfile/`g…

…ems.rb`

**What does this PR do?**

In #2978 because of an
incompatibility with Phusion Passenger, we added autodetection code to
enable the "no signals" workaround when this web server is in use.

This code was later extended in
#3280 once an upstream fix
was released, so that the "no signals" workaround was only applied on
old Passenger versions.

But in #3721 we got a
report that our version detection code did not work correctly in setups
where Passenger was not installed via `Gemfile`/`gems.rb`.

This PR fixes the version detection in these situations.

**Motivation:**

Make sure the "no signals" workaround does not get enabled in setups
where it does not need to be.

**Additional Notes:**

N/A

**How to test the change?**

I've added test coverage for this issue.

Additionally, here's how to manually install Passenger and validate
that the "no signals" workaround does not get enabled:

```
$ cat Gemfile
source 'https://rubygems.org'

gem 'datadog', git: 'https://github.com/DataDog/dd-trace-rb.git', branch: 'ivoanjo/prof-10015-fix-passenger-detection'
gem 'rack'
$ cat config.ru
require 'datadog/profiling/preload'

app = ->(env) {
  puts "Running on passenger #{PhusionPassenger::VERSION_STRING.inspect}"
  [200, {}, ["Hello, World!"]]
}
run app

$ docker run --network=host -ti -v `pwd`:/working ruby:3.3 /bin/bash
$ cd working/
$ apt update && apt-get install -y dirmngr gnupg apt-transport-https ca-certificates curl
$ curl https://oss-binaries.phusionpassenger.com/auto-software-signing-gpg-key.txt | gpg --dearmor | tee /etc/apt/trusted.gpg.d/phusion.gpg >/dev/null
$ sh -c 'echo deb https://oss-binaries.phusionpassenger.com/apt/passenger bookworm main > /etc/apt/sources.list.d/passenger.list'
$ apt update && apt-get install -y nginx libnginx-mod-http-passenger
$ bundle install
$ DD_PROFILING_ENABLED=true /usr/bin/passenger start
```

in master this outputs the warning message

> WARN -- datadog: [datadog] Enabling the profiling "no signals"
> workaround because an incompatible version of the passenger gem
> is installed. Profiling data will have lower quality.To fix this,
> upgrade the passenger gem to version 6.0.19 or above.

With this fix, this message is not shown, confirming the "no signals"
workaround is not enabled.

lib/datadog/profiling/component.rb

@@ -397,8 +397,12 @@ def self.build_profiler_component(settings:, agent_settings:, optional_tracer:)
 
       # See https://github.com/datadog/dd-trace-rb/issues/2976 for details.
       private_class_method def self.incompatible_passenger_version?
+        first_compatible_version = Gem::Version.new('6.0.19')
+
         if Gem.loaded_specs['passenger']
-          Gem.loaded_specs['passenger'].version < Gem::Version.new('6.0.19')
+          Gem.loaded_specs['passenger'].version < first_compatible_version
+        elsif defined?(PhusionPassenger::VERSION_STRING)
+          Gem::Version.new(PhusionPassenger::VERSION_STRING) < first_compatible_version
         else
           true
         end

spec/datadog/profiling/component_spec.rb

@@ -877,6 +877,29 @@
           end
         end
 
+        context 'when passenger gem is not available, but PhusionPassenger::VERSION_STRING is available' do
+          context 'on passenger >= 6.0.19' do
+            before { stub_const('PhusionPassenger::VERSION_STRING', '6.0.19') }
+
+            it { is_expected.to be false }
+          end
+
+          context 'on passenger < 6.0.19' do
+            before do
+              stub_const('PhusionPassenger::VERSION_STRING', '6.0.18')
+              allow(Datadog.logger).to receive(:warn)
+            end
+
+            it { is_expected.to be true }
+
+            it 'logs a warning message mentioning that the no signals workaround is going to be used' do
+              expect(Datadog.logger).to receive(:warn).with(/Enabling the profiling "no signals" workaround/)
+
+              no_signals_workaround_enabled?
+            end
+          end
+        end
+
         context 'when mysql2 / rugged gems + passenger are not available' do
           include_context('loaded gems', passenger: nil, mysql2: nil, rugged: nil)
 

vendor/rbs/passenger/0/passenger.rbs

@@ -1,2 +1,3 @@
 module PhusionPassenger
+  VERSION_STRING: ::String
 end