From cca9d6e7191f25d5cd717e7be917465b643b7456 Mon Sep 17 00:00:00 2001
From: jafrivacation <jaqs7053@gmail.com>
Date: Mon, 29 Jun 2026 13:18:47 -0400
Subject: [PATCH] Fix SocketCAN send timeout semantics

---
 can/interfaces/socketcan/socketcan.py | 10 ++++------
 test/test_socketcan.py                | 16 +++++++++++++++-
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/can/interfaces/socketcan/socketcan.py b/can/interfaces/socketcan/socketcan.py
index 6dc856cbf..aaeb152fa 100644
--- a/can/interfaces/socketcan/socketcan.py
+++ b/can/interfaces/socketcan/socketcan.py
@@ -841,7 +841,7 @@ def send(self, msg: Message, timeout: float | None = None) -> None:
         :param msg: A message object.
         :param timeout:
             Wait up to this many seconds for the transmit queue to be ready.
-            If not given, the call may fail immediately.
+            If not given, wait indefinitely.
 
         :raises ~can.exceptions.CanError:
             if the message could not be written.
@@ -851,13 +851,10 @@ def send(self, msg: Message, timeout: float | None = None) -> None:
         logger_tx.debug("sending: %s", msg)
 
         started = time.time()
-        # If no timeout is given, poll for availability
-        if timeout is None:
-            timeout = 0
         time_left = timeout
         data = build_can_frame(msg)
 
-        while time_left >= 0:
+        while time_left is None or time_left >= 0:
             # Wait for write availability
             ready = select.select([], [self.socket], [], time_left)[1]
             if not ready:
@@ -869,7 +866,8 @@ def send(self, msg: Message, timeout: float | None = None) -> None:
                 return
             # Not all data were sent, try again with remaining data
             data = data[sent:]
-            time_left = timeout - (time.time() - started)
+            if timeout is not None:
+                time_left = timeout - (time.time() - started)
 
         raise can.CanOperationError("Transmit buffer full")
 
diff --git a/test/test_socketcan.py b/test/test_socketcan.py
index 9d042f425..a5e33e02e 100644
--- a/test/test_socketcan.py
+++ b/test/test_socketcan.py
@@ -9,7 +9,7 @@
 import sys
 import unittest
 import warnings
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 import can
 from can.interfaces.socketcan.constants import (
@@ -21,6 +21,7 @@
 )
 from can.interfaces.socketcan.socketcan import (
     BcmMsgHead,
+    SocketcanBus,
     bcm_header_factory,
     build_bcm_header,
     build_bcm_transmit_header,
@@ -36,6 +37,19 @@ def setUp(self):
         self._ctypes_sizeof = ctypes.sizeof
         self._ctypes_alignment = ctypes.alignment
 
+    @patch("can.interfaces.socketcan.socketcan.select.select")
+    def test_send_without_timeout_blocks_indefinitely(self, select):
+        bus = SocketcanBus.__new__(SocketcanBus)
+        bus.channel = "can0"
+        bus.socket = MagicMock()
+        bus._send_once = MagicMock(side_effect=[1, 15])
+        select.return_value = ([], [bus.socket], [])
+
+        bus.send(can.Message(arbitration_id=0x123, data=[1, 2, 3, 4]))
+
+        self.assertEqual(2, select.call_count)
+        self.assertTrue(all(call.args[3] is None for call in select.call_args_list))
+
     @unittest.skipIf(sys.version_info >= (3, 14), "Fails on Python 3.14 or newer")
     @patch("ctypes.sizeof")
     @patch("ctypes.alignment")