Get tests passing with libuv on Python 3.6

This requires some changes to the way we handle errors, specifically
for signals in the monkey-patched test_selectors.py and
test_socket.py.

It's not absolutely clear that this is the right thing to do in all
cases.

.travis.yml

@@ -11,16 +11,17 @@ env:
       # These are ordered to get as much diversity in the
       # first group of parallel runs (4) as posible
       - TASK=test-py27-libuv
-      #- TASK=test-py27-noembed
-      #- TASK=test-pypy
-      #- TASK=test-py36
+      - TASK=test-py36-libuv
+      - TASK=test-py27-noembed
+      - TASK=test-pypy
+      - TASK=test-py36
       - TASK=lint-py27
-      #- TASK=test-pypy3
-      #- TASK=test-py35
-      #- TASK=test-py278
-      #- TASK=test-py27
-      #- TASK=test-py34
-      #- TASK=test-py27-cffi
+      - TASK=test-pypy3
+      - TASK=test-py35
+      - TASK=test-py278
+      - TASK=test-py27
+      - TASK=test-py34
+      - TASK=test-py27-cffi
 
 matrix:
   fast_finish: true

Makefile

@@ -185,6 +185,9 @@ test-py35: $(PY35)
 test-py36: $(PY36)
 	PYTHON=python3.6.2 PIP=pip PATH=$(BUILD_RUNTIMES)/versions/python3.6.2/bin:$(PATH) make develop toxtest
 
+test-py36-libuv: $(PY36)
+	GEVENT_CORE_CFFI_ONLY=libuv make test-py36
+
 test-pypy: $(PYPY)
 	PYTHON=$(PYPY) PIP=pip PATH=$(BUILD_RUNTIMES)/versions/pypy580/bin:$(PATH) make develop toxtest
 
@@ -195,7 +198,7 @@ test-py27-cffi: $(PY27)
 	GEVENT_CORE_CFFI_ONLY=1 PYTHON=python2.7.14 PATH=$(BUILD_RUNTIMES)/versions/python2.7.14/bin:$(PATH) make develop toxtest
 
 test-py27-libuv: $(PY27)
-	GEVENT_CORE_CFFI_ONLY=libuv PYTHON=python2.7.14 PATH=$(BUILD_RUNTIMES)/versions/python2.7.14/bin:$(PATH) make develop toxtest
+	GEVENT_CORE_CFFI_ONLY=libuv make test-py27
 
 test-py27-noembed: $(PY27)
 	cd deps/libev && ./configure --disable-dependency-tracking && make

src/gevent/_ffi/loop.py

@@ -112,7 +112,17 @@ class _Callbacks(object):
             watcher = self.ffi.from_handle(handle)
             exc_info = watcher._exc_info
             del watcher._exc_info
-            watcher.loop.handle_error(watcher, *exc_info)
+            # In the past, we passed the ``watcher`` itself as the context,
+            # which typically meant that the Hub would just print
+            # the exception. This is a problem because sometimes we can't
+            # detect signals until late in ``python_callback``; specifically,
+            # test_selectors.py:DefaultSelectorTest.test_select_interrupt_exc
+            # installs a SIGALRM handler that raises an exception. That exception can happen
+            # before we enter ``python_callback`` or at any point within it because of the way
+            # libuv swallows signals. By passing None, we get the exception prapagated into
+            # the main greenlet (which is probably *also* not what we always want, but
+            # I see no way to distinguish the cases).
+            watcher.loop.handle_error(None, *exc_info)
         finally:
             # XXX Since we're here on an error condition, and we
             # made sure that the watcher object was put in loop._keepaliveset,
@@ -126,6 +136,15 @@ class _Callbacks(object):
                 watcher.loop.handle_error(watcher, *sys.exc_info())
             return # pylint:disable=lost-exception
 
+    def unhandled_onerror(self, t, v, tb):
+        # This is supposed to be called for signals, etc.
+        if tb is not None:
+            handle = tb.tb_frame.f_locals['handle']
+            watcher = self.ffi.from_handle(handle)
+            watcher.loop._check_callback_handle_error(t, v, tb)
+        else:
+            raise v
+
     def python_stop(self, handle):
         watcher = self.ffi.from_handle(handle)
         watcher.stop()
@@ -137,7 +156,7 @@ def assign_standard_callbacks(ffi, lib):
     for sig, func in (("int(void* handle, int revents)", callbacks.python_callback),
                       ("void(void* handle, int revents)", callbacks.python_handle_error),
                       ("void(void* handle)", callbacks.python_stop)):
-        callback = ffi.callback(sig)(func)
+        callback = ffi.callback(sig, onerror=callbacks.unhandled_onerror)(func)
         # keep alive the cdata
         callbacks.callbacks.append(callback)
         # pass to the library C variable

src/gevent/libuv/watcher.py

@@ -306,7 +306,7 @@ class io(_base.IoMixin, watcher):
             # the reader, we get a LoopExit. So we can't return here and arguably shouldn't print it
             # either. The negative events mask will match the watcher's mask.
             # See test__fileobject.py:Test.test_newlines for an example.
-            #return
+            # return
 
         for watcher_ref in self._multiplex_watchers:
             watcher = watcher_ref()