From 421ff8f27ed04b7c962519a7bde70a74946f7128 Mon Sep 17 00:00:00 2001
From: David Feldman <david.feldman@amount.com>
Date: Wed, 3 Jun 2020 11:33:31 -0500
Subject: [PATCH 1/2] Writer thread safety

---
 lib/ddtrace/writer.rb | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/lib/ddtrace/writer.rb b/lib/ddtrace/writer.rb
index abbe4d0f5bd..2b45bf2605c 100644
--- a/lib/ddtrace/writer.rb
+++ b/lib/ddtrace/writer.rb
@@ -43,9 +43,17 @@ def initialize(options = {})
       @worker = nil
     end
 
-    # spawns a worker for spans; they share the same transport which is thread-safe
     def start
-      @pid = Process.pid
+      @mutex_after_fork.synchronize do
+        pid = Process.pid
+        return if @worker && pid == @pid
+        @pid = pid
+        start_worker
+      end
+    end
+
+    # spawns a worker for spans; they share the same transport which is thread-safe
+    def start_worker
       @trace_handler = ->(items, transport) { send_spans(items, transport) }
       @worker = Datadog::Workers::AsyncTransport.new(
         transport: @transport,
@@ -57,14 +65,19 @@ def start
       @worker.start
     end
 
-    # stops worker for spans.
     def stop
-      return if worker.nil?
+      @mutex_after_fork.synchronize { stop_worker }
+    end
+
+    def stop_worker
+      return if @worker.nil?
       @worker.stop
       @worker = nil
       true
     end
 
+    private :start_worker, :stop_worker
+
     # flush spans to the trace-agent, handles spans only
     def send_spans(traces, transport)
       return true if traces.empty?
@@ -106,13 +119,7 @@ def write(trace, services = nil)
       #
       # This check ensures that if a process doesn't own the current +Writer+, async workers
       # will be initialized again (but only once for each process).
-      pid = Process.pid
-      if pid != @pid # avoid using Mutex when pids are equal
-        @mutex_after_fork.synchronize do
-          # we should start threads because the worker doesn't own this
-          start if pid != @pid
-        end
-      end
+      start if @worker.nil? || @pid != Process.pid
 
       # TODO: Remove this, and have the tracer pump traces directly to runtime metrics
       #       instead of working through the trace writer.

From a88bda3bd4c348c1f5026ab84d5ec59dbec9f41b Mon Sep 17 00:00:00 2001
From: David Feldman <david.feldman@amount.com>
Date: Wed, 3 Jun 2020 12:04:46 -0500
Subject: [PATCH 2/2] nicer error message when one thread stops a writer
 another thread is in the middle of writing to

---
 lib/ddtrace/writer.rb | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/ddtrace/writer.rb b/lib/ddtrace/writer.rb
index 2b45bf2605c..b0c07492f28 100644
--- a/lib/ddtrace/writer.rb
+++ b/lib/ddtrace/writer.rb
@@ -49,6 +49,7 @@ def start
         return if @worker && pid == @pid
         @pid = pid
         start_worker
+        true
       end
     end
 
@@ -128,7 +129,13 @@ def write(trace, services = nil)
         Datadog.runtime_metrics.associate_with_span(trace.first)
       end
 
-      @worker.enqueue_trace(trace)
+      worker_local = @worker
+
+      if worker_local
+        worker_local.enqueue_trace(trace)
+      else
+        Datadog.logger.debug('Writer either failed to start or was stopped before #write could complete')
+      end
     end
 
     # stats returns a dictionary of stats about the writer.