Issue #25. Detect deadlocks and raise if detected during select.

goless/backends.py

@@ -1,4 +1,5 @@
 import contextlib as _contextlib
+import gc as _gc
 import os as _os
 import platform as _platform
 import sys as _sys
@@ -51,6 +52,11 @@ def propagate_exc(self, errtype, *args):
         so the program hears it and it doesn't die lonely in a tasklet."""
         raise NotImplementedError()
 
+    def would_deadlock(self):
+        """Return True if a send or receive would deadlock
+        (current tasklet/greenlet is the last one running)."""
+        raise NotImplementedError()
+
 
 # We can't easily use stackless on our CI,
 # so don't worry about covering it.
@@ -90,13 +96,17 @@ def resume(self, tasklet):
         def propagate_exc(self, errtype, *args):
             stackless.getmain().throw(errtype, *args)
 
+        def would_deadlock(self):
+            return stackless.runcount == 1
+
     return StacklessBackend()
 
 
 def _make_gevent():
     import gevent
     import gevent.hub
     import gevent.queue
+    import greenlet
     deadlock_errtype = SystemError if _os.name == 'nt' else gevent.hub.LoopExit
 
     class Channel(gevent.queue.Channel):
@@ -133,6 +143,17 @@ def resume(self, tasklet):
         def propagate_exc(self, errtype, *args):
             raise errtype
 
+        def would_deadlock(self):
+            # The Hub and main greenlet are always running,
+            # if there are more than those alive, we aren't going to deadlock.
+            count = 0
+            for obj in _gc.get_objects():
+                if isinstance(obj, greenlet.greenlet) and not obj.dead:
+                    count += 1
+                if count > 2:
+                    return False
+            return True
+
     return GeventBackend()
 
 

goless/selecting.py

@@ -1,4 +1,4 @@
-from .backends import current as _be
+from .backends import current as _be, Deadlock as _Deadlock
 
 
 # noinspection PyPep8Naming,PyShadowingNames
@@ -77,6 +77,14 @@ def select(*cases):
         # noinspection PyCallingNonCallable
         return default, None
 
+    # We need to check for deadlocks before selecting.
+    # We can't rely on the underlying backend to do it,
+    # as we do for channels, since we don't do an actual send or recv here.
+    # It's possible to still have a deadlock unless we move the check into
+    # the loop, but since the check is slow
+    # (gevent doesn't provide a fast way), let's leave it out here.
+    if _be.would_deadlock():
+        raise _Deadlock('No other tasklets running, cannot select.')
     while True:
         for c in cases:
             if c.ready():

tests/test_select.py

@@ -137,4 +137,8 @@ def test_select_raises_for_list_and_args(self):
 
     def test_select_with_no_args_should_do_nothing(self):
         goless.select()
-        goless.select([])
+        goless.select([])
+
+    def test_raises_deadlock_if_no_goroutines(self):
+        with self.assertRaises(goless.Deadlock):
+            goless.select(goless.rcase(goless.chan()))