Mark ddtrace threads as fork-safe

**What does this PR do?**

This PR applies to all relevant ddtrace threads a pattern pioneered by
rails: set the `:fork_safe` thread variable to indicate that it's OK
if ddtrace threads are started before a Ruby app calls fork.

**Motivation:**

This PR fixes the issue reported in DataDog#3203. The puma webserver in
clustering mode has a check where it will warn when it detects that
threads were started before the webserver has fork'd the child
processes, see:

https://github.com/puma/puma/blob/32e011ab9e029c757823efb068358ed255fb7ef4/lib/puma/cluster.rb#L353-L359

This is not a problem for ddtrace (as discussed in DataDog#3203), but the
warning is annoying.

While looking into the issue again today, I spotted that actually
there's a way to tell puma that a thread is fine and is it's not a
problem if it's started before the webserver has fork'd: puma
checks if the thread has a `:fork_safe` thread-variable set to `true`.

**Additional Notes:**

We have a bunch of places in ddtrace where we create threads...
I briefly played with the idea of creating a superclass of all
ddtrace Threads, but decided to not get into a bigger refactoring
for such a small issue. Maybe next time...?

**How to test the change?**

Test coverage included. Furthermore, you can try running
a Ruby webapp in puma before/after the change to confirm the
warning is gone.

Fixes DataDog#3203

lib/datadog/core/remote/worker.rb

@@ -30,6 +30,7 @@ def start
 
           thread = Thread.new { poll(@interval) }
           thread.name = self.class.name unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3')
+          thread.thread_variable_set(:fork_safe, true)
           @thr = thread
 
           @started = true

lib/datadog/core/workers/async.rb

@@ -147,6 +147,7 @@ def start_worker
               # rubocop:enable Lint/RescueException
             end
             @worker.name = self.class.name unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3')
+            @worker.thread_variable_set(:fork_safe, true)
 
             nil
           end

lib/datadog/profiling/collectors/cpu_and_wall_time_worker.rb

@@ -78,6 +78,7 @@ def start(on_failure_proc: nil)
               end
             end
             @worker_thread.name = self.class.name # Repeated from above to make sure thread gets named asap
+            @worker_thread.thread_variable_set(:fork_safe, true)
           end
 
           true

lib/datadog/profiling/collectors/idle_sampling_helper.rb

@@ -43,6 +43,7 @@ def start
               end
             end
             @worker_thread.name = self.class.name # Repeated from above to make sure thread gets named asap
+            @worker_thread.thread_variable_set(:fork_safe, true)
           end
 
           true

lib/datadog/tracing/workers.rb

@@ -69,6 +69,7 @@ def start
             Datadog.logger.debug { "Starting thread for: #{self}" }
             @worker = Thread.new { perform }
             @worker.name = self.class.name unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3')
+            @worker.thread_variable_set(:fork_safe, true)
 
             nil
           end

spec/datadog/core/remote/worker_spec.rb

@@ -60,6 +60,12 @@
         expect(Thread.list.map(&:name)).to include(described_class.to_s)
       end
     end
+
+    it 'marks the worker thread as fork-safe (to avoid fork-safety warnings in webservers)' do
+      worker.start
+
+      expect(worker.instance_variable_get(:@thr).thread_variable_get(:fork_safe)).to be true
+    end
   end
 
   describe '#stop' do

spec/datadog/core/workers/async_spec.rb

@@ -451,7 +451,7 @@
       end
     end
 
-    describe 'thread naming' do
+    describe 'thread naming and fork-safety marker' do
       after { worker.terminate }
 
       context 'on Ruby < 2.3' do
@@ -488,6 +488,12 @@
           expect(worker.send(:worker).name).to eq worker_class.to_s
         end
       end
+
+      it 'marks the worker thread as fork-safe (to avoid fork-safety warnings in webservers)' do
+        worker.perform
+
+        expect(worker.send(:worker).thread_variable_get(:fork_safe)).to be true
+      end
     end
   end
 end

spec/datadog/profiling/collectors/cpu_and_wall_time_worker_spec.rb

@@ -71,6 +71,12 @@
       expect(Thread.list.map(&:name)).to include(described_class.name)
     end
 
+    it 'marks the new thread as fork-safe' do
+      start
+
+      expect(cpu_and_wall_time_worker.instance_variable_get(:@worker_thread).thread_variable_get(:fork_safe)).to be true
+    end
+
     it 'does not create a second thread if start is called again' do
       start
 

spec/datadog/profiling/collectors/idle_sampling_helper_spec.rb

@@ -26,6 +26,12 @@
       start
     end
 
+    it 'marks the new thread as fork-safe' do
+      start
+
+      expect(idle_sampling_helper.instance_variable_get(:@worker_thread).thread_variable_get(:fork_safe)).to be true
+    end
+
     it 'does not create a second thread if start is called again' do
       start
 

spec/datadog/tracing/workers_spec.rb

@@ -39,7 +39,7 @@
     end
   end
 
-  describe 'thread naming' do
+  describe 'thread naming and fork-safety marker' do
     context 'on Ruby < 2.3' do
       before do
         skip 'Only applies to old Rubies' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.3')
@@ -65,6 +65,12 @@
         expect(worker.instance_variable_get(:@worker).name).to eq described_class.name
       end
     end
+
+    it 'marks the worker thread as fork-safe (to avoid fork-safety warnings in webservers)' do
+      worker.start
+
+      expect(worker.instance_variable_get(:@worker).thread_variable_get(:fork_safe)).to be true
+    end
   end
 
   describe '#start' do