Keep alive when i3bar hidden

By default i3bar sends a -SIGSTOP to a process sending it data.  This causes
some modules interacting with i3 ipc to be suspended, which lead to locking
issues inside i3.  This patch switches to using SIGUSR2 as the suspend
signal.

The async modules eg window_title_async were affected.

see ultrabug#253

py3status/core.py

@@ -7,7 +7,7 @@
 
 from json import dumps
 from signal import signal
-from signal import SIGTERM, SIGUSR1
+from signal import SIGTERM, SIGUSR1, SIGUSR2, SIGCONT
 from subprocess import Popen
 from subprocess import call
 from threading import Event
@@ -37,6 +37,7 @@ def __init__(self):
         Useful variables we'll need.
         """
         self.config = {}
+        self.i3bar_running = True
         self.last_refresh_ts = time()
         self.lock = Event()
         self.modules = {}
@@ -235,6 +236,14 @@ def setup(self):
         # set the Event lock
         self.lock.set()
 
+        # SIGUSR2 will be recieced from i3bar indicating that all output should
+        # stop and we should consider py3status suspended.  It is however
+        # important that any processes using i3 ipc should continue to recieve
+        # those events otherwise it can lead to a stall in i3.
+        signal(SIGUSR2, self.i3bar_stop)
+        # SIGCONT indicates output should be resumed.
+        signal(SIGCONT, self.i3bar_start)
+
         # setup configuration
         self.config = self.get_config()
 
@@ -443,6 +452,12 @@ def create_output_modules(self):
 
         self.output_modules = output_modules
 
+    def i3bar_stop(self, signum, frame):
+        self.i3bar_running = False
+
+    def i3bar_start(self, signum, frame):
+        self.i3bar_running = True
+
     @profile
     def run(self):
         """
@@ -471,6 +486,9 @@ def run(self):
 
         # main loop
         while True:
+            while not self.i3bar_running:
+                sleep(0.1)
+
             sec = int(time())
 
             # only check everything is good each second

py3status/i3status.py

@@ -7,6 +7,7 @@
 from subprocess import Popen
 from subprocess import PIPE
 from syslog import syslog, LOG_INFO
+from signal import SIGUSR2
 from tempfile import NamedTemporaryFile
 from threading import Thread
 from time import time
@@ -540,6 +541,8 @@ def run(self):
                                 if 'version' in line:
                                     header = loads(line)
                                     header.update({'click_events': True})
+                                    # set custom stop signal
+                                    header['stop_signal'] = SIGUSR2
                                     line = dumps(header)
                                 print_line(line)
                             else: