From b5831a0d766bba240d525ae10dac60dd399dfaf4 Mon Sep 17 00:00:00 2001
From: David Wilson <dw@botanicus.net>
Date: Tue, 11 Dec 2018 23:17:20 +0000
Subject: [PATCH] examples: import select_loop.py.

---
 examples/select_loop.py | 88 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 examples/select_loop.py

diff --git a/examples/select_loop.py b/examples/select_loop.py
new file mode 100644
index 00000000..414bb358
--- /dev/null
+++ b/examples/select_loop.py
@@ -0,0 +1,88 @@
+
+#
+# This demonstrates using a nested select.Select() to simultaneously watch for
+# in-progress events generated by a bunch of function calls, and the completion
+# of those function calls.
+#
+# We start 5 children and run a function in each of them in parallel. The
+# function writes the numbers 1..5 to a Sender before returning. The master
+# reads the numbers from each child as they are generated, and exits the loop
+# when the last function returns.
+#
+
+from __future__ import absolute_import
+from __future__ import print_function
+
+import time
+
+import mitogen
+import mitogen.select
+
+
+def count_to(sender, n, wait=0.333):
+    for x in range(n):
+        sender.send(x)
+        time.sleep(wait)
+
+
+@mitogen.main()
+def main(router):
+    # Start 5 subprocesses and give them made up names.
+    contexts = {
+        'host%d' % (i,): router.local()
+        for i in range(5)
+    }
+
+    # Used later to recover hostname from failed call.
+    hostname_by_context_id = {
+        context.context_id: hostname
+        for hostname, context in contexts.items()
+    }
+
+    # I am a select that holds the receivers that will receive the function
+    # call results. Selects are one-shot by default, which means each receiver
+    # is removed from them as a result arrives. Therefore it means the last
+    # function has completed when bool(calls_sel) is False.
+    calls_sel = mitogen.select.Select()
+
+    # I receive the numbers as they are counted.
+    status_recv = mitogen.core.Receiver(router)
+
+    # Start the function calls
+    for hostname, context in contexts.items():
+        calls_sel.add(
+            context.call_async(
+                count_to,
+                sender=status_recv.to_sender(),
+                n=5,
+                wait=0.333
+            )
+        )
+
+    # Create a select subscribed to the function call result Select, and to the
+    # number-counting receiver.
+    both_sel = mitogen.select.Select([status_recv, calls_sel], oneshot=False)
+
+    # Once last call is completed, calls will be empty since it's oneshot=True
+    # (the default), causing __bool__ to be False
+    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()
+
+        hostname = hostname_by_context_id[msg.src_id]
+
+        if msg.receiver is status_recv:   # https://mitogen.readthedocs.io/en/stable/api.html#mitogen.core.Message.receiver
+            # handle a status update
+            print('Got status update from %s: %s' % (hostname, msg.unpickle()))
+        elif msg.receiver is calls_sel:  # subselect
+            # handle a function call result.
+            try:
+                assert None == msg.unpickle()
+                print('Task succeeded on %s' % (hostname,))
+            except mitogen.core.CallError as e:
+                print('Task failed on host %s: %s' % (hostname, e))
+
+    print('All tasks completed.')