Allow restarting of transmission tasks for socketcan (#1440)

* Allow restarting of transmission tasks for socketcan

* Update can/interfaces/socketcan/socketcan.py

Co-authored-by: zariiii9003 <52598363+zariiii9003@users.noreply.github.com>

can/interfaces/socketcan/socketcan.py

@@ -349,7 +349,9 @@ def __init__(
         self.task_id = task_id
         self._tx_setup(self.messages)
 
-    def _tx_setup(self, messages: Sequence[Message]) -> None:
+    def _tx_setup(
+        self, messages: Sequence[Message], raise_if_task_exists: bool = True
+    ) -> None:
         # Create a low level packed frame to pass to the kernel
         body = bytearray()
         self.flags = CAN_FD_FRAME if messages[0].is_fd else 0
@@ -363,7 +365,8 @@ def _tx_setup(self, messages: Sequence[Message]) -> None:
             ival1 = 0.0
             ival2 = self.period
 
-        self._check_bcm_task()
+        if raise_if_task_exists:
+            self._check_bcm_task()
 
         header = build_bcm_transmit_header(
             self.task_id, count, ival1, ival2, self.flags, nframes=len(messages)
@@ -375,7 +378,7 @@ def _tx_setup(self, messages: Sequence[Message]) -> None:
 
     def _check_bcm_task(self) -> None:
         # Do a TX_READ on a task ID, and check if we get EINVAL. If so,
-        # then we are referring to a CAN message with the existing ID
+        # then we are referring to a CAN message with an existing ID
         check_header = build_bcm_header(
             opcode=CAN_BCM_TX_READ,
             flags=0,
@@ -387,12 +390,19 @@ def _check_bcm_task(self) -> None:
             can_id=self.task_id,
             nframes=0,
         )
+        log.debug(
+            f"Reading properties of (cyclic) transmission task id={self.task_id}",
+        )
         try:
             self.bcm_socket.send(check_header)
         except OSError as error:
             if error.errno != errno.EINVAL:
                 raise can.CanOperationError("failed to check", error.errno) from error
+            else:
+                log.debug("Invalid argument - transmission task not known to kernel")
         else:
+            # No exception raised - transmission task with this ID exists in kernel.
+            # Existence of an existing transmission task might not be a problem!
             raise can.CanOperationError(
                 f"A periodic task for task ID {self.task_id} is already in progress "
                 "by the SocketCAN Linux layer"
@@ -438,15 +448,15 @@ def modify_data(self, messages: Union[Sequence[Message], Message]) -> None:
         send_bcm(self.bcm_socket, header + body)
 
     def start(self) -> None:
-        """Start a periodic task by sending TX_SETUP message to Linux kernel.
+        """Restart a periodic task by sending TX_SETUP message to Linux kernel.
 
         It verifies presence of the particular BCM task through sending TX_READ
         message to Linux kernel prior to scheduling.
 
         :raises ValueError:
             If the task referenced by ``task_id`` is already running.
         """
-        self._tx_setup(self.messages)
+        self._tx_setup(self.messages, raise_if_task_exists=False)
 
 
 class MultiRateCyclicSendTask(CyclicSendTask):