osMessageQueuePut blocks indefinitely when used within an ISR

[drtarr]

STM32CubeIDE 1.17.0, using FreeRTOS (CMSISRTOS-2) on a NUCLEO-F446RE board.  I am trying to call `osMessageQueuePut(...)` from an ISR (CAN RX0 Interrupt) and find my program hangs indefinitely at that call.

// Globals
const osMessageQueueAttr_t canMsgQueueAttr = {
    .name = "CanMsgQueue"
};
osMessageQueueId_t canMsgQueue = nullptr;

// Create the queue
canMsgQueue = osMessageQueueNew(10, 256, &canMsgQueueAttr);

// CAN RX0 callback
void HAL_CAN_RxFifo0MsgPendingCallback(CAN_HandleTypeDef *hcan) {
    char buf[128];
    snprintf((char*)buf, sizeof(buf), "Got message");
    osMessageQueuePut(canMsgQueue, buf, 0U, 0U); // Stuck forever
}

I'm following this documentation:

https://www.keil.com/pack/doc/CMSIS_Dev/RTOS2/html/group__CMSIS__RTOS__Message.html#gad90d4959466a7a65105061da8256ab9e

There is a note:
> May be called from Interrupt Service Routines if the parameter timeout is set to 0.

And that is what I have done.  Otherwise it returns `-4`, which is consistent with the documentation:
> osErrorParameter: parameter mq_id is NULL or invalid, non-zero timeout specified in an ISR.

Feels like I'm missing something very basic - With a value of `0` I would expect it to return instantly even if it failed.

[drtarr]

This is where its sticking.

[drtarr · Accepted solution]

My interrupt priority (4) was less than the max sys call (5).  Fix:

HAL_NVIC_SetPriority(CAN1_RX0_IRQn, 5, 0);