Callback into mJS from another thread: is it safe?

robmeades · May 24, 2019, 11:54am

I have a C API which takes a callback from the mJS world (including the userdata void * parameter) and calls that callback some time later from another RTOS thread (this is ESP32, so FreeRTOS). The thread that is doing the calling-back is allocated 8 kbytes of stack.

If I do something simple, like print("Hello.\n"), in the mJS callback then all works fine but when I do anything more adventurous: perform some processing on received data, access other mJS global, variables, I get random errors, e.g. mJS complains that the global variable is not defined or even that print is not defined.

My question is: is it safe to call back into mJS from another RTOS thread? If so then I guess I must just be suffering from stack overflow or a waggly pointer.

cpq · May 24, 2019, 12:24pm

Yeah, mjs is expecting to be executed by the mongoose os thread.

You might try this:

github.com

cesanta/mongoose-os/blob/master/include/mgos_system.h#L92-L96


/*
 * Invoke a callback in the main MGOS event loop.
 * Returns true if the callback has been scheduled for execution.
 */
bool mgos_invoke_cb(mgos_cb_t cb, void *arg, bool from_isr);

robmeades · May 24, 2019, 12:28pm

Ah, unfortunately I don’t seem to have such a function, I’m using standalone mJS rather than Mongoose OS as a whole.

cpq · May 26, 2019, 8:42pm

Ah I see.
Then, make your own!
The idea is: mjs VM should execute by only one task / thread. Thus, is another thread should execute the callback, do not execute it but instead signal the mjs task to do it. Use whatever IPC API is available in your environment.

robmeades · May 28, 2019, 1:00pm

Understood, will do.