Add single method to fetch profiler

The MRI probe always initialized the profiler needed to actually profile
the garbage collection. If `enable_gc_instrumentation` is set to
`false`, which it is by default, it has some overhead that's not needed.

Instead it should use the `NilProfiler` like we had in the Transaction
class.

Move this logic to a new `GarbageCollection` wrapper module in which the
`GarbageCollection.profiler` will check the config option value and
return the suitable profiler.

Part of #868

.changesets/fix-garbage-collection-profiler.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "fix"
+---
+
+Fix the MRI probe using the Garbage Collection profiler instead of the NilProfiler when garbage collection instrumentation is not enabled for MRI probe. This caused unnecessary overhead.

lib/appsignal.rb

@@ -298,7 +298,7 @@ def collect_environment_metadata
 require "appsignal/probes"
 require "appsignal/marker"
 require "appsignal/minutely"
-require "appsignal/garbage_collection_profiler"
+require "appsignal/garbage_collection"
 require "appsignal/integrations/railtie" if defined?(::Rails)
 require "appsignal/transaction"
 require "appsignal/version"

lib/appsignal/garbage_collection.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+module Appsignal
+  # @api private
+  module GarbageCollection
+    # Return the GC profiler wrapper.
+    #
+    # Returns {Profiler} if `enable_gc_instrumentation` is enabled and
+    # {NilProfiler} if it is disabled.
+    #
+    # GC profiling is disabled by default due to the overhead it causes. Do not
+    # enable this in production for long periods of time.
+    def self.profiler(appsignal = Appsignal)
+      @profiler ||=
+        if appsignal.config[:enable_gc_instrumentation]
+          Profiler.new
+        else
+          NilProfiler.new
+        end
+    end
+
+    # Unset the currently cached profiler
+    #
+    # @return [void]
+    def self.clear_profiler!
+      @profiler = nil
+    end
+
+    # A wrapper around Ruby's `GC::Profiler` that tracks garbage collection
+    # time, while clearing `GC::Profiler`'s total_time to make sure it doesn't
+    # leak memory by keeping garbage collection run samples in memory.
+    class Profiler
+      def self.lock
+        @lock ||= Mutex.new
+      end
+
+      def initialize
+        @total_time = 0
+      end
+
+      # Whenever {#total_time} is called, the current `GC::Profiler#total_time`
+      # gets added to `@total_time`, after which `GC::Profiler.clear` is called
+      # to prevent it from leaking memory. A class-level lock is used to make
+      # sure garbage collection time is never counted more than once.
+      #
+      # Whenever `@total_time` gets above two billion milliseconds (about 23
+      # days), it's reset to make sure the result fits in a signed 32-bit
+      # integer.
+      #
+      # @return [Integer]
+      def total_time
+        lock.synchronize do
+          @total_time += (internal_profiler.total_time * 1000).round
+          internal_profiler.clear
+        end
+
+        @total_time = 0 if @total_time > 2_000_000_000
+
+        @total_time
+      end
+
+      private
+
+      def internal_profiler
+        GC::Profiler
+      end
+
+      def lock
+        self.class.lock
+      end
+    end
+
+    # A dummy profiler that always returns 0 as the total time. Used when GC
+    # profiler is disabled.
+    class NilProfiler
+      def total_time
+        0
+      end
+    end
+  end
+end

lib/appsignal/probes/mri.rb

@@ -8,7 +8,7 @@ def self.dependencies_present?
         defined?(::RubyVM) && ::RubyVM.respond_to?(:stat)
       end
 
-      def initialize(appsignal: Appsignal, gc_profiler: Appsignal::GarbageCollectionProfiler.new)
+      def initialize(appsignal: Appsignal, gc_profiler: Appsignal::GarbageCollection.profiler)
         Appsignal.logger.debug("Initializing VM probe")
         @appsignal = appsignal
         @gc_profiler = gc_profiler

lib/appsignal/transaction.rb

@@ -68,8 +68,7 @@ def clear_current_transaction!
       end
 
       def garbage_collection_profiler
-        @garbage_collection_profiler ||=
-          Appsignal.config[:enable_gc_instrumentation] ? Appsignal::GarbageCollectionProfiler.new : NilGarbageCollectionProfiler.new
+        Appsignal::GarbageCollection.profiler
       end
     end
 

spec/lib/appsignal/garbage_collection_profiler_spec.rb → spec/lib/appsignal/garbage_collection_spec.rb

@@ -1,4 +1,34 @@
-describe Appsignal::GarbageCollectionProfiler do
+describe Appsignal::GarbageCollection do
+  describe ".profiler" do
+    let(:appsignal) { class_double("Appsignal") }
+    before do
+      # Unset the internal memoized variable to avoid state leaking
+      described_class.clear_profiler!
+    end
+
+    context "when GC instrumentation is disabled" do
+      let(:config) { Appsignal::Config.new("production", Dir.pwd, :enable_gc_instrumentation => false) }
+
+      it "returns the NilProfiler" do
+        allow(appsignal).to receive(:config).and_return(config)
+
+        expect(described_class.profiler(appsignal)).to be_a(Appsignal::GarbageCollection::NilProfiler)
+      end
+    end
+
+    context "when GC profiling is enabled" do
+      let(:config) { Appsignal::Config.new("production", Dir.pwd, :enable_gc_instrumentation => true) }
+
+      it "returns the Profiler" do
+        allow(appsignal).to receive(:config).and_return(config)
+
+        expect(described_class.profiler(appsignal)).to be_a(Appsignal::GarbageCollection::Profiler)
+      end
+    end
+  end
+end
+
+describe Appsignal::GarbageCollection::Profiler do
   let(:internal_profiler) { FakeGCProfiler.new }
   let(:profiler) { described_class.new }
 
@@ -52,7 +82,7 @@
 
       2.times do
         threads << Thread.new do
-          profiler = Appsignal::GarbageCollectionProfiler.new
+          profiler = Appsignal::GarbageCollection::Profiler.new
           results << profiler.total_time
         end
       end
@@ -63,7 +93,7 @@
   end
 end
 
-describe Appsignal::NilGarbageCollectionProfiler do
+describe Appsignal::GarbageCollection::NilProfiler do
   let(:profiler) { described_class.new }
 
   describe "#total_time" do

spec/lib/appsignal/transaction_spec.rb

@@ -792,23 +792,6 @@ def to_s
       end
     end
 
-    describe "#garbage_collection_profiler" do
-      before { Appsignal::Transaction.instance_variable_set(:@garbage_collection_profiler, nil) }
-
-      it "returns the NilGarbageCollectionProfiler" do
-        expect(Appsignal::Transaction.garbage_collection_profiler).to be_a(Appsignal::NilGarbageCollectionProfiler)
-      end
-
-      context "when gc profiling is enabled" do
-        before { Appsignal.config.config_hash[:enable_gc_instrumentation] = true }
-        after { Appsignal.config.config_hash[:enable_gc_instrumentation] = false }
-
-        it "returns the GarbageCollectionProfiler" do
-          expect(Appsignal::Transaction.garbage_collection_profiler).to be_a(Appsignal::GarbageCollectionProfiler)
-        end
-      end
-    end
-
     describe "#start_event" do
       it "starts the event in the extension" do
         expect(transaction.ext).to receive(:start_event).with(0).and_call_original