examples: add Select lifetimes to example.

examples/select_loop.py

@@ -66,16 +66,23 @@ def main(router):
     # Select will wake it up -- be it a message arriving on the status
     # receiver, or any message arriving on any of the function call result
     # receivers.
-    both_sel = mitogen.select.Select([status_recv, calls_sel], oneshot=False)
 
     # Once last call is completed, calls_sel will be empty since it's
     # oneshot=True (the default), causing __bool__ to be False
+    both_sel = mitogen.select.Select([status_recv, calls_sel], oneshot=False)
+
+    # Internally selects store a strong reference from Receiver->Select that
+    # will keep the Select alive as long as the receiver is alive. If a
+    # receiver or select otherwise 'outlives' some parent select, attempting to
+    # re-add it to a new select will raise an error. In all cases it's
+    # desirable to call Select.close(). This can be done as a context manager.
+    with calls_sel, both_sel:
         while calls_sel:
             try:
                 msg = both_sel.get(timeout=60.0)
             except mitogen.core.TimeoutError:
                 print("No update in 60 seconds, something's broke")
-            raise Crash()
+                break
 
             hostname = hostname_by_context_id[msg.src_id]
 
@@ -90,4 +97,7 @@ def main(router):
                 except mitogen.core.CallError as e:
                     print('Task failed on host %s: %s' % (hostname, e))
 
+        if calls_sel:
+            print('Some tasks did not complete.')
+        else:
             print('All tasks completed.')