issue #144: service: working service.Pool object.

It knows how to dispatch messages from multiple receivers (associated
with multiple services) to multiple threads, where the service
implementation is invoked on the message.

It wakes a maximum of one thread per received message.

It knows how to shut down gracefully.

Implication: due to the latch use, there are 2 file descriptors burned
for every thread. We don't need interruptibility here, so in future, it
might be nice to allow swapping a diferent queueing primitive into
Select (maybe a subclass?) just for this case.

mitogen/service.py

@@ -26,6 +26,8 @@
 # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
+import sys
+import threading
 
 import mitogen.core
 import mitogen.master
@@ -41,28 +43,21 @@ class Service(object):
     def __init__(self, router):
         self.router = router
         self.recv = mitogen.core.Receiver(router, self.handle)
+        self.recv.service = self
         self.handle = self.recv.handle
         self.running = True
 
     def validate_args(self, args):
         return True
 
-    def run_once(self):
-        try:
-            msg = self.recv.get()
-        except mitogen.core.ChannelError, e:
-            # Channel closed due to broker shutdown, exit gracefully.
-            LOG.debug('%r: channel closed: %s', self, e)
-            self.running = False
-            return
-
+    def dispatch_one(self, msg):
         if len(msg.data) > self.max_message_size:
             LOG.error('%r: larger than permitted size: %r', self, msg)
             msg.reply(mitogen.core.CallError('Message size exceeded'))
             return
 
         args = msg.unpickle(throw=False)
-        if ( args == mitogen.core._DEAD or
+        if  (args == mitogen.core._DEAD or
              isinstance(args, mitogen.core.CallError) or
              not self.validate_args(args)):
             LOG.warning('Received junk message: %r', args)
@@ -74,6 +69,17 @@ class Service(object):
             LOG.exception('While invoking %r.dispatch()', self)
             msg.reply(mitogen.core.CallError(e))
 
+    def run_once(self):
+        try:
+            msg = self.recv.get()
+        except mitogen.core.ChannelError, e:
+            # Channel closed due to broker shutdown, exit gracefully.
+            LOG.debug('%r: channel closed: %s', self, e)
+            self.running = False
+            return
+
+        self.dispatch_one(msg)
+
     def run(self):
         while self.running:
             self.run_once()
@@ -82,7 +88,41 @@ class Service(object):
 class Pool(object):
     def __init__(self, router, services, size=1):
         self.services = list(services)
-        self.select = mitogen.master.Select()
+        self.select = mitogen.master.Select(
+            receivers=[
+                service.recv
+                for service in self.services
+            ],
+            oneshot=False,
+        )
+        self.threads = []
+        for x in xrange(size):
+            thread = threading.Thread(
+                name='mitogen.service.Pool.worker-%d' % (x,),
+                target=self._worker_main,
+            )
+            thread.start()
+            self.threads.append(thread)
+
+    def stop(self):
+        self.select.close()
+        for th in self.threads:
+            th.join()
+
+    def _worker_main(self):
+        while True:
+            try:
+                msg = self.select.get()
+            except (mitogen.core.ChannelError, mitogen.core.LatchError):
+                e = sys.exc_info()[1]
+                LOG.error('%r: channel or latch closed, exitting: %s', self, e)
+                return
+
+            service = msg.receiver.service
+            try:
+                service.dispatch_one(msg)
+            except Exception:
+                LOG.exception('While handling %r using %r', msg, service)
 
 
 def call(context, handle, obj):