bus.send() fails with `No buffer space available` despite not reaching the timeout

[jazzpi]

Describe the bug

When sending a lot of packets very quickly to a physical socketcan bus, bus.send() raises a can.exceptions.CanOperationError: Failed to transmit: No buffer space available [Error Code 105] despite the timeout not being reached.

To Reproduce

Send 100 frames to a physical socketcan bus with timeout=1.0 for each frame and no delay between sending the frames.

Expected behavior

can.send() should retry transmission until buffer space is available or the timeout is exceeded.

Additional context

OS and version: Ubuntu 24.04
Python version: 3.12.3
python-can version: 4.6.1
python-can interface/s (if applicable): socketcan (for a CANable 2.0 running CANnectivity)

Traceback and logs

$ ip -d link show pcan-e
12: pcan-e: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10
    link/can  promiscuity 0  allmulti 0 minmtu 0 maxmtu 0 
    can state ERROR-ACTIVE restart-ms 0 
          bitrate 1024096 sample-point 0.746
          tq 5 prop-seg 61 phase-seg1 62 phase-seg2 42 sjw 21 brp 1
          gs_usb: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
          clock 170000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size 65536 parentbus usb parentdev 1-7.3:1.0
$ time python tests/cantest.py 
Traceback (most recent call last):
  File "/home/gos/gena-sidl/Integration-Tests/env/lib/python3.12/site-packages/can/interfaces/socketcan/socketcan.py", line 889, in _send_once
    sent = self.socket.send(data)
           ^^^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 105] No buffer space available

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/gos/gena-sidl/Integration-Tests/tests/cantest.py", line 12, in <module>
    bus.send(msg, timeout=1.0)
  File "/home/gos/gena-sidl/Integration-Tests/env/lib/python3.12/site-packages/can/interfaces/socketcan/socketcan.py", line 874, in send
    sent = self._send_once(data, channel)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/gos/gena-sidl/Integration-Tests/env/lib/python3.12/site-packages/can/interfaces/socketcan/socketcan.py", line 891, in _send_once
    raise can.CanOperationError(
can.exceptions.CanOperationError: Failed to transmit: No buffer space available [Error Code 105]
SocketcanBus was not properly shut down

real    0m0.112s
user    0m0.099s
sys     0m0.013s

with the following test script:

#!/usr/bin/env python3

import can

N_PACKETS = 100

bus = can.Bus(interface="socketcan", channel="pcan-e")

data = bytes([0xAB] * 8)
for _ in range(N_PACKETS):
    msg = can.Message(arbitration_id=1, is_extended_id=True, data=data)
    bus.send(msg, timeout=1.0)

bus.shutdown()

[jazzpi]

Modifying SocketcanBus._send_once() as follows fixes the issue:

--- a/home/jasper/Downloads/socketcan.py
+++ b/env/lib/python3.12/site-packages/can/interfaces/socketcan/socketcan.py
@@ -883,16 +883,19 @@ class SocketcanBus(BusABC):  # pylint: disable=abstract-method
     def _send_once(self, data: bytes, channel: Optional[str] = None) -> int:
         try:
             if self.channel == "" and channel:
                 # Message must be addressed to a specific channel
                 sent = self.socket.sendto(data, (channel,))
             else:
                 sent = self.socket.send(data)
         except OSError as error:
+            if error.errno == errno.ENOBUFS:
+                return 0
+
             raise can.CanOperationError(
                 f"Failed to transmit: {error.strerror}", error.errno
             ) from error
         return sent
 
     def _send_periodic_internal(
         self,
         msgs: Union[Sequence[Message], Message],

but I'm not sure if this essentially turns the while-loop in SocketcanBus.send() into a busy-waiting loop.

[jazzpi]

Today I can't reproduce this anymore...

This might be relevant.