Detect USB device disconnection

The device disconnection is detected when the video socket closes.

In order to introduce an OTG mode (HID events) without mirroring (and
without server), we must be able to detect USB device disconnection.

This feature will only be used in OTG mode.

PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>

app/src/usb/usb.c

@@ -135,13 +135,111 @@ sc_usb_destroy(struct sc_usb *usb) {
     libusb_exit(usb->context);
 }
 
+static int
+sc_usb_libusb_callback(libusb_context *ctx, libusb_device *device,
+                       libusb_hotplug_event event, void *userdata) {
+    (void) ctx;
+    (void) device;
+    (void) event;
+
+    struct sc_usb *usb = userdata;
+
+    libusb_device *dev = libusb_get_device(usb->handle);
+    assert(dev);
+    if (dev != device) {
+        // Not the connected device
+        return 0;
+    }
+
+    assert(usb->cbs && usb->cbs->on_disconnected);
+    usb->cbs->on_disconnected(usb, usb->cbs_userdata);
+
+    // Do not automatically deregister the callback by returning 1. Instead,
+    // manually deregister to interrupt libusb_handle_events() from the libusb
+    // event thread: <https://stackoverflow.com/a/60119225/1987178>
+    return 0;
+}
+
+static int
+run_libusb_event_handler(void *data) {
+    struct sc_usb *usb = data;
+    while (!atomic_load(&usb->stopped)) {
+        // Interrupted by events or by libusb_hotplug_deregister_callback()
+        libusb_handle_events(usb->context);
+    }
+    return 0;
+}
+
+static bool
+sc_usb_register_callback(struct sc_usb *usb) {
+    if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG)) {
+        LOGW("libusb does not have hotplug capability");
+        return false;
+    }
+
+    libusb_device *device = libusb_get_device(usb->handle);
+    assert(device);
+
+    struct libusb_device_descriptor desc;
+    int result = libusb_get_device_descriptor(device, &desc);
+    if (result < 0) {
+        log_libusb_error((enum libusb_error) result);
+        LOGW("Could not read USB device descriptor");
+        return false;
+    }
+
+    int events = LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT;
+    int flags = LIBUSB_HOTPLUG_NO_FLAGS;
+    int vendor_id = desc.idVendor;
+    int product_id = desc.idProduct;
+    int dev_class = LIBUSB_HOTPLUG_MATCH_ANY;
+    result = libusb_hotplug_register_callback(usb->context, events, flags,
+                                              vendor_id, product_id, dev_class,
+                                              sc_usb_libusb_callback, usb,
+                                              &usb->callback_handle);
+    if (result < 0) {
+        log_libusb_error((enum libusb_error) result);
+        LOGW("Could not register USB callback");
+        return false;
+    }
+
+    usb->has_callback_handle = true;
+    return true;
+}
+
 bool
-sc_usb_connect(struct sc_usb *usb, libusb_device *device) {
+sc_usb_connect(struct sc_usb *usb, libusb_device *device,
+              const struct sc_usb_callbacks *cbs, void *cbs_userdata) {
     usb->handle = sc_usb_open_handle(device);
     if (!usb->handle) {
         return false;
     }
 
+    usb->has_callback_handle = false;
+    usb->has_libusb_event_thread = false;
+
+    // If cbs is set, then cbs->on_disconnected must be set
+    assert(!cbs || cbs->on_disconnected);
+    usb->cbs = cbs;
+    usb->cbs_userdata = cbs_userdata;
+
+    if (cbs) {
+        atomic_init(&usb->stopped, false);
+        if (sc_usb_register_callback(usb)) {
+            // Create a thread to process libusb events, so that device
+            // disconnection could be detected immediately
+            usb->has_libusb_event_thread =
+                sc_thread_create(&usb->libusb_event_thread,
+                                 run_libusb_event_handler, "scrcpy-usbev", usb);
+            if (!usb->has_libusb_event_thread) {
+                LOGW("Libusb event thread handler could not be created, USB "
+                     "device disconnection might not be detected immediately");
+            }
+        } else {
+            LOGW("Could not register USB device disconnection callback");
+        }
+    }
+
     return true;
 }
 
@@ -149,3 +247,18 @@ void
 sc_usb_disconnect(struct sc_usb *usb) {
     libusb_close(usb->handle);
 }
+
+void
+sc_usb_stop(struct sc_usb *usb) {
+    if (usb->has_callback_handle) {
+        atomic_store(&usb->stopped, true);
+        libusb_hotplug_deregister_callback(usb->context, usb->callback_handle);
+    }
+}
+
+void
+sc_usb_join(struct sc_usb *usb) {
+    if (usb->has_libusb_event_thread) {
+        sc_thread_join(&usb->libusb_event_thread, NULL);
+    }
+}