Crash when usbmuxd_subscribe called more than once

[qmfrederik]

If you call usbmuxd_subscribe more than once, but do not call usbmuxd_unsubscribe in between, the device_monitor thread which is created by usbmuxd_subscribe is started multiple times.

When messages such as MESSAGE_DEVICE_REMOVE are received, all of these threads will attempt to find the device, and remove the handle for that device. This leads to race conditions and ultimately to free(devinfo) being called twice.

I see three options to resolve this:

1. Make usbmuxd_subscribe check whether another callback is already registered, and if so, return -EALREADY to let the caller know this was not a valid operation
2. The same as above, but silently call usbmuxd_unsubscribe and register the new callback
3. Support handling multiple callbacks.

My preference currently goes to 1, and I can submit a PR for this, but before I do that, I'd like to check with you whether this is the project's preferred option.

[jackczhang]

I have encountered this problem, too. I preferred option 3, which can support handling multiple callbacks, because i want to do that in one process with multiple threads, but now crashed.

[nikias]

Fixed with commit a6b542b389d0536d2730c1721164a712ec2f020e. New API:

int usbmuxd_events_subscribe(usbmuxd_subscription_context_t *context, usbmuxd_event_cb_t callback, void *user_data);
int usbmuxd_events_unsubscribe(usbmuxd_subscription_context_t context);

enjoy