Use SIGWINCH to print thread backtraces (#2195)

#1320 added support for using SIGINFO,
but this is only available on BSD-based systems. SIGWINCH is on Linux.

This is useful for debugging infinite loops or slow performance.

History.md

@@ -7,6 +7,7 @@
   * `GC.compact` is called before fork if available (#2093)
   * Add `requests_count` to workers stats. (#2106)
   * Increases maximum URI path length from 2048 to 8196 bytes (#2167)
+  * Sending SIGWINCH to any Puma worker now prints currently active threads and their backtraces (#2195)
 
 * Deprecations, Removals and Breaking API Changes
   * `Puma.stats` now returns a Hash instead of a JSON string (#2086)

docs/signals.md

@@ -40,7 +40,7 @@ Puma cluster responds to these signals:
 - `USR1` restart workers in phases, a rolling restart. This will not reload configuration file.
 - `HUP`  reopen log files defined in stdout_redirect configuration parameter. If there is no stdout_redirect option provided it will behave like `INT`
 - `INT` equivalent of sending Ctrl-C to cluster. Will attempt to finish then exit.
-- `CHLD`
+- `WINCH` (`INFO` on BSD-based systems) prints a thread backtrace. This is useful for debugging infinite loops or slow performance.
 
 ## Callbacks order in case of different signals
 

lib/puma/launcher.rb

@@ -452,13 +452,18 @@ def setup_signals
         log "*** SIGHUP not implemented, signal based logs reopening unavailable!"
       end
 
+      begin
+        Signal.trap "SIGWINCH" do
+          log_backtrace
+        end
+      rescue Exception
+        log "*** SIGWINCH not implemented, signal based thread backtraces unavailable!"
+      end
+
       begin
         unless Puma.jruby? # INFO in use by JVM already
           Signal.trap "SIGINFO" do
-            thread_status do |name, backtrace|
-              @events.log name
-              @events.log backtrace.map { |bt| "  #{bt}" }
-            end
+            log_backtrace
           end
         end
       rescue Exception
@@ -467,6 +472,13 @@ def setup_signals
       end
     end
 
+    def log_backtrace
+      thread_status do |name, backtrace|
+        @events.log name
+        @events.log backtrace.map { |bt| "  #{bt}" }
+      end
+    end
+
     def require_rubygems_min_version!(min_version, feature)
       return if min_version <= Gem::Version.new(Gem::VERSION)
 

test/test_integration_cluster.rb

@@ -33,18 +33,16 @@ def test_pre_existing_unix
     end
   end
 
+  def test_sigwinch_thread_print
+    skip_unless_signal_exist? :WINCH
+
+    signal_thread_backtrace :WINCH
+  end
+
   def test_siginfo_thread_print
     skip_unless_signal_exist? :INFO
 
-    cli_server "-w #{WORKERS} -q test/rackup/hello.ru"
-    worker_pids = get_worker_pids
-    output = []
-    t = Thread.new { output << @server.readlines }
-    Process.kill :INFO, worker_pids.first
-    Process.kill :INT , @pid
-    t.join
-
-    assert_match "Thread: TID", output.join
+    signal_thread_backtrace :INFO
   end
 
   def test_usr2_restart
@@ -135,6 +133,18 @@ def test_stuck_phased_restart
 
   private
 
+  def signal_thread_backtrace(signal)
+    cli_server "-w #{WORKERS} -q test/rackup/hello.ru"
+    worker_pids = get_worker_pids
+    output = []
+    t = Thread.new { output << @server.readlines }
+    Process.kill signal, worker_pids.first
+    Process.kill :INT , @pid
+    t.join
+
+    assert_match "Thread: TID", output.join
+  end
+
   # Send requests 10 per second.  Send 10, then :TERM server, then send another 30.
   # No more than 10 should throw Errno::ECONNRESET.
   def term_closes_listeners(unix: false)