[libgpiod] [RFC PATCH] bindings: python: allow specifying infinite timeout

[Nicolas Frattaroli <frattaroli.nicolas@gmail.com>]

So far, libgpiod's Python bindings had no way to state that a
user wishes to wait for events indefinitely, as a timeout of
None would intentionally be converted to 0 seconds, i.e. return
from the select call in poll_fd immediately.

The usual Python convention and even the select convention is
to block indefinitely on a timeout=None. However, changing the
poll_fd function to do this now would change an (intentional)
API design choice by libgpiod 2.0 that API users presumably
rely on.

By allowing float("inf") (or in fact math.inf, or your favourite
other way to get an infinite float) to mean waiting infinitely
solves this by extending the API rather than changing it.

On gpiod Python bindings without this change, passing inf results
in an OverflowError being raised in select. API users who wish to
support older versions of the bindings can catch this exception and
act on it.

Signed-off-by: Nicolas Frattaroli <frattaroli.nicolas@gmail.com>
---
 bindings/python/gpiod/chip.py         | 3 ++-
 bindings/python/gpiod/internal.py     | 4 ++++
 bindings/python/gpiod/line_request.py | 3 ++-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/bindings/python/gpiod/chip.py b/bindings/python/gpiod/chip.py
index 97ff340..95c5757 100644
--- a/bindings/python/gpiod/chip.py
+++ b/bindings/python/gpiod/chip.py
@@ -195,7 +195,8 @@ class Chip:
         Args:
           timeout:
             Wait time limit represented as either a datetime.timedelta object
-            or the number of seconds stored in a float.
+            or the number of seconds stored in a float. A timeout of None
+            returns immediately, use float("inf") to wait indefinitely.
 
         Returns:
           True if an info event is ready to be read from the chip, False if the
diff --git a/bindings/python/gpiod/internal.py b/bindings/python/gpiod/internal.py
index 37e8b62..141cfe9 100644
--- a/bindings/python/gpiod/internal.py
+++ b/bindings/python/gpiod/internal.py
@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: 2022 Bartosz Golaszewski <brgl@bgdev.pl>
 
 from datetime import timedelta
+from math import inf
 from select import select
 from typing import Optional, Union
 
@@ -15,5 +16,8 @@ def poll_fd(fd: int, timeout: Optional[Union[timedelta, float]] = None) -> bool:
     else:
         sec = timeout
 
+    if sec == inf:
+        sec = None
+
     readable, _, _ = select([fd], [], [], sec)
     return True if fd in readable else False
diff --git a/bindings/python/gpiod/line_request.py b/bindings/python/gpiod/line_request.py
index a0f97b7..ae21835 100644
--- a/bindings/python/gpiod/line_request.py
+++ b/bindings/python/gpiod/line_request.py
@@ -178,7 +178,8 @@ class LineRequest:
         Args:
           timeout:
             Wait time limit expressed as either a datetime.timedelta object
-            or the number of seconds stored in a float.
+            or the number of seconds stored in a float. None returns
+            immediately. Use float("inf") to wait indefinitely.
 
         Returns:
           True if events are ready to be read. False on timeout.
-- 
2.40.1