Add pumactl command to print thread backtraces

Completes 1 of 2 items from puma#1964

This commit adds an endpoint to the status app to print thread
backtraces, and control cli command to call that endpoint.

I tried this locally by starting a server with:

```sh
bundle exec bin/puma test/rackup/hello.ru \
  --control-url="unix://test.sock" \
  --control-token="token"
```

and then printing the backtraces with:

```sh
bundle exec bin/pumactl thread-backtraces \
  --control-url="unix://test.sock" \
  --control-token="token"
```

History.md

@@ -4,6 +4,7 @@
   * Strip whitespace at end of HTTP headers (#2010)
   * Optimize HTTP parser for JRuby (#2012)
   * Add SSL support for the control app (#2046)
+  * Add pumactl `thread-backtraces` command to print thread backtraces (#2053)
 
 * Bugfixes
   * Fix Errno::EINVAL when SSL is enabled and browser rejects cert (#1564)

lib/puma/app/status.rb

@@ -55,6 +55,11 @@ def call(env)
 
         when /\/stats$/
           rack_response(200, @cli.stats)
+
+        when /\/thread-backtraces$/
+          strings = Puma::Events.strings
+          @cli.log_thread_status(strings)
+          rack_response(200, strings.stdout.string)
         else
           rack_response 404, "Unsupported action", 'text/plain'
         end

lib/puma/control_cli.rb

@@ -11,7 +11,8 @@
 module Puma
   class ControlCLI
 
-    COMMANDS = %w{halt restart phased-restart start stats status stop reload-worker-directory gc gc-stats}
+    COMMANDS = %w{halt restart phased-restart start stats status stop reload-worker-directory gc gc-stats thread-backtraces}
+    PRINTABLE_COMMANDS = %w{gc-stats stats thread-backtraces}
 
     def initialize(argv, stdout=STDOUT, stderr=STDERR)
       @state = nil
@@ -181,7 +182,7 @@ def send_request
         end
 
         message "Command #{@command} sent success"
-        message response.last if @command == "stats" || @command == "gc-stats"
+        message response.last if PRINTABLE_COMMANDS.include?(@command)
       end
     ensure
       server.close if server && !server.closed?

lib/puma/launcher.rb

@@ -205,6 +205,21 @@ def close_binder_listeners
       @binder.close_listeners
     end
 
+    def log_thread_status(events)
+      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
+
+        if thread.backtrace
+          events.log thread.backtrace.join("\n")
+        else
+          events.log "<no backtrace available>"
+        end
+      end
+    end
+
     private
 
     # If configured, write the pid of the current process out
@@ -323,21 +338,6 @@ def graceful_stop
       log "- Goodbye!"
     end
 
-    def log_thread_status
-      Thread.list.each do |thread|
-        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)
-        log logstr
-
-        if thread.backtrace
-          log thread.backtrace.join("\n")
-        else
-          log "<no backtrace available>"
-        end
-      end
-    end
-
     def set_process_title
       Process.respond_to?(:setproctitle) ? Process.setproctitle(title) : $0 = title
     end
@@ -457,7 +457,7 @@ def setup_signals
 
       begin
         Signal.trap "SIGINFO" do
-          log_thread_status
+          log_thread_status(@events)
         end
       rescue Exception
         # Not going to log this one, as SIGINFO is *BSD only and would be pretty annoying

test/test_launcher.rb

@@ -57,7 +57,7 @@ def test_puma_wild_location_is_an_absolute_path
   end
 
   def test_prints_thread_traces
-    launcher.send(:log_thread_status)
+    launcher.log_thread_status(events)
     events.stdout.rewind
 
     assert_match "Thread TID", events.stdout.read