SIGINFO prints thread backtraces

lib/puma/cluster.rb

@@ -269,6 +269,10 @@ def worker(index, master)
         server.stop
       end
 
+      Signal.trap "SIGINFO" do
+        server.log_thread_status
+      end
+
       begin
         @worker_write << "b#{Process.pid}\n"
       rescue SystemCallError, IOError

lib/puma/server.rb

@@ -1023,5 +1023,16 @@ def self.current
     def shutting_down?
       @status == :stop || @status == :restart
     end
+
+    def log_thread_status
+      Thread.list.each do |thread|
+        @events.log "Thread TID-#{thread.object_id.to_s(36)} #{thread['label']}"
+        if thread.backtrace
+          @events.log thread.backtrace.join("\n")
+        else
+          @events.log "<no backtrace available>"
+        end
+      end
+    end
   end
 end

lib/puma/single.rb

@@ -113,6 +113,10 @@ def run
 
       @launcher.events.fire_on_booted!
 
+      Signal.trap "SIGINFO" do
+        server.log_thread_status
+      end
+
       begin
         server.run.join
       rescue Interrupt