[libgpiod][PATCH] bindings: python: examples: Add graceful exit to async_watch_line_value

[Kent Gibson <warthog618@xxxxxxxxx>]

The purpose of the example is demonstrate using a request with poll().
It provides a hint as to how the poll can be combined with other fds but,
as Python comes with batteries included, the Python version of the example
can be readily extended to actually demonstrate this, as well as how it
can be used in multi-threaded environments.

Extend the example to use an eventfd to allow the poll() to be run in
a background thread and be gracefully terminated by the main thread.

Signed-off-by: Kent Gibson <warthog618@xxxxxxxxx>
---
 .../python/examples/async_watch_line_value.py | 51 ++++++++++++++-----
 1 file changed, 38 insertions(+), 13 deletions(-)

diff --git a/bindings/python/examples/async_watch_line_value.py b/bindings/python/examples/async_watch_line_value.py
index 1d6a184..055d54f 100755
--- a/bindings/python/examples/async_watch_line_value.py
+++ b/bindings/python/examples/async_watch_line_value.py
@@ -19,7 +19,7 @@ def edge_type_str(event):
     return "Unknown"
 
 
-def async_watch_line_value(chip_path, line_offset):
+def async_watch_line_value(chip_path, line_offset, done_fd):
     # Assume a button connecting the pin to ground,
     # so pull it up and provide some debounce.
     with gpiod.request_lines(
@@ -35,20 +35,45 @@ def async_watch_line_value(chip_path, line_offset):
     ) as request:
         poll = select.poll()
         poll.register(request.fd, select.POLLIN)
+        # Other fds could be registered with the poll and be handled
+        # separately using the return value (fd, event) from poll():
+        poll.register(done_fd, select.POLLIN)
         while True:
-            # Other fds could be registered with the poll and be handled
-            # separately using the return value (fd, event) from poll()
-            poll.poll()
-            for event in request.read_edge_events():
-                print(
-                    "offset: {}  type: {:<7}  event #{}".format(
-                        event.line_offset, edge_type_str(event), event.line_seqno
+            for fd, _event in poll.poll():
+                if fd == done_fd:
+                    # perform any cleanup before exiting...
+                    return
+                # handle any edge events
+                for event in request.read_edge_events():
+                    print(
+                        "offset: {}  type: {:<7}  event #{}".format(
+                            event.line_offset, edge_type_str(
+                                event), event.line_seqno
+                        )
                     )
-                )
 
 
 if __name__ == "__main__":
-    try:
-        async_watch_line_value("/dev/gpiochip0", 5)
-    except OSError as ex:
-        print(ex, "\nCustomise the example configuration to suit your situation")
+    import os
+    import threading
+
+    # run the async executor (select.poll) in a thread to demonstrate a graceful exit.
+    done_fd = os.eventfd(0)
+
+    def bg_thread():
+        try:
+            async_watch_line_value("/dev/gpiochip0", 5, done_fd)
+        except OSError as ex:
+            print(ex, "\nCustomise the example configuration to suit your situation")
+        print("background thread exiting...")
+
+    t = threading.Thread(target=bg_thread)
+    t.start()
+
+    # Wait for two minutes, unless bg_thread exits earlier, then graceful exit.
+    t.join(timeout=120)
+    if t.is_alive():
+        os.eventfd_write(done_fd, 1)
+        t.join()
+    os.close(done_fd)
+    print("main thread exiting...")
-- 
2.39.5