Log threads as JSON in control app

With this commit the status app sends the thread backtraces as an array
of objects with `name` and `backtrace` keys, rather than as a string
matching the SIGINFO output.

While working on this I noticed that we logged the thread TID twice.
This commit simplifies that so we only log the thread TID once, with
both the label (I don't know when the label would get set) and name if
they are available.

lib/puma/app/status.rb

@@ -57,9 +57,12 @@ def call(env)
           rack_response(200, @cli.stats)
 
         when /\/thread-backtraces$/
-          strings = Puma::Events.strings
-          @cli.log_thread_status(strings)
-          rack_response(200, strings.stdout.string)
+          backtraces = []
+          @cli.thread_status do |name, backtrace|
+            backtraces << { name: name, backtrace: backtrace }
+          end
+
+          rack_response(200, backtraces.to_json)
         else
           rack_response 404, "Unsupported action", 'text/plain'
         end

lib/puma/launcher.rb

@@ -205,18 +205,14 @@ def close_binder_listeners
       @binder.close_listeners
     end
 
-    def log_thread_status(events)
+    def thread_status
       Thread.list.each do |thread|
-        events.log "Thread TID-#{thread.object_id.to_s(36)} #{thread['label']}"
-        logstr = "Thread: TID-#{thread.object_id.to_s(36)}"
-        logstr += " #{thread.name}" if thread.respond_to?(:name)
-        events.log logstr
+        name = "Thread: TID-#{thread.object_id.to_s(36)}"
+        name += " #{thread['label']}" if thread['label']
+        name += " #{thread.name}" if thread.respond_to?(:name) && thread.name
+        backtrace = thread.backtrace || ["<no backtrace available>"]
 
-        if thread.backtrace
-          events.log thread.backtrace.join("\n")
-        else
-          events.log "<no backtrace available>"
-        end
+        yield name, backtrace
       end
     end
 
@@ -457,7 +453,10 @@ def setup_signals
 
       begin
         Signal.trap "SIGINFO" do
-          log_thread_status(@events)
+          thread_status do |name, backtrace|
+            @events.log name
+            @events.log backtrace
+          end
         end
       rescue Exception
         # Not going to log this one, as SIGINFO is *BSD only and would be pretty annoying

test/test_integration_cluster.rb

@@ -44,7 +44,7 @@ def test_siginfo_thread_print
     Process.kill :INT , @pid
     t.join
 
-    assert_match "Thread TID", output.join
+    assert_match "Thread: TID", output.join
   end
 
   def test_usr2_restart

test/test_integration_single.rb

@@ -99,6 +99,6 @@ def test_siginfo_thread_print
     Process.kill :INT , @pid
     t.join
 
-    assert_match "Thread TID", output.join
+    assert_match "Thread: TID", output.join
   end
 end

test/test_launcher.rb

@@ -57,10 +57,9 @@ def test_puma_wild_location_is_an_absolute_path
   end
 
   def test_prints_thread_traces
-    launcher.log_thread_status(events)
-    events.stdout.rewind
-
-    assert_match "Thread TID", events.stdout.read
+    launcher.thread_status do |name, _backtrace|
+      assert_match "Thread: TID", name
+    end
   end
 
   def test_pid_file