Added the "stacktrace" command (#120)

* Added the "stacktrace" command

This will be useful for debugging when the event loop thread is stuck.

* Fixed mypy errors

README.rst

@@ -113,6 +113,7 @@ Now you can type commands, for instance ``help``::
                  where taskid     : Show stack frames for a task
                  cancel taskid    : Cancel an indicated task
                  signal signame   : Send a Unix signal
+                 stacktrace       : Print a stack trace from the event loop thread
                  console          : Switch to async Python REPL
                  quit             : Leave the monitor
 

aiomonitor/monitor.py

@@ -3,7 +3,9 @@
 import os
 import signal
 import socket
+import sys
 import threading
+import traceback
 from textwrap import wrap
 from typing import IO, Dict, Any, Optional  # noqa
 from concurrent.futures import Future  # noqa
@@ -38,6 +40,8 @@ def start_monitor(loop: Loop, *,
 
 
 class Monitor:
+    _event_loop_thread_id = None  # type: int
+
     def __init__(self,
                  loop: asyncio.AbstractEventLoop, *,
                  host: str=MONITOR_HOST,
@@ -72,6 +76,7 @@ def start(self) -> None:
 
         self._started = True
         h, p = self._host, self._port
+        self._event_loop_thread_id = threading.get_ident()
         self._ui_thread.start()
         if self._console_enabled:
             log.info('Starting console at %s:%d', h, p)
@@ -138,6 +143,9 @@ def _monitor_commans(self, sin: IO[str], sout: IO[str], resp: str) -> None:
             _, signame = resp.split()
             self._command_signal(sout, signame)
 
+        elif resp.startswith('stacktrace'):
+            self._command_stacktrace(sout)
+
         elif resp.startswith('w'):
             _, taskid_s = resp.split()
             self._command_where(sout, int(taskid_s))
@@ -174,6 +182,7 @@ def _command_help(self, sout: IO[str]):
              where taskid     : Show stack frames for a task
              cancel taskid    : Cancel an indicated task
              signal signame   : Send a Unix signal
+             stacktrace       : Print a stack trace from the event loop thread
              console          : Switch to async Python REPL
              quit             : Leave the monitor
             """)
@@ -206,6 +215,10 @@ def _command_signal(self, sout: IO[str], signame: str) -> None:
         else:
             sout.write('Unknown signal %s\n' % signame)
 
+    def _command_stacktrace(self, sout: IO[str]) -> None:
+        frame = sys._current_frames()[self._event_loop_thread_id]
+        traceback.print_stack(frame, file=sout)
+
     def _command_cancel(self, sout: IO[str], taskid) -> None:
         task = task_by_id(taskid, self._loop)
         if task:

tests/test_monitor.py

@@ -99,6 +99,9 @@ def test_basic_monitor(monitor, tn_client, loop):
     resp = execute(tn, 'signal name\n')
     assert 'Unknown signal name' in resp
 
+    resp = execute(tn, 'stacktrace\n')
+    assert 'loop.run_forever()' in resp
+
     resp = execute(tn, 'wehere 123\n')
     assert 'No task 123' in resp