error checks for usb device string properties

Improve the error handling for usb.core.Device string properties:
.serial_number, .product, .manufacturer

Previously, the property getters didn't check the device descriptor
to see if the device actually had the requested string. Instead,
they relied on TinyUSB to return a failure result if the string was
not available. That made it impossible to distinguish missing
strings from other more serious USB errors (e.g. unplugged device).

These changes make it possible to return None for a missing string
or raise a USBError exception in case of a more serious problem.

Author: samblenny

shared-module/usb/core/Device.c

@@ -101,9 +101,28 @@ static bool _wait_for_callback(void) {
         // we provided above. In other words, the callback isn't in an interrupt.
         RUN_BACKGROUND_TASKS;
     }
+    if (mp_hal_is_interrupted()) {
+        // Handle case of VM being interrupted by Ctrl-C or autoreload
+        return false;
+    }
+    // Handle callback result code from TinyUSB
     xfer_result_t result = _xfer_result;
     _xfer_result = XFER_RESULT_INVALID;
-    return result == XFER_RESULT_SUCCESS;
+    switch (result) {
+        case XFER_RESULT_SUCCESS:
+            return true;
+        case XFER_RESULT_FAILED:
+            mp_raise_usb_core_USBError(NULL);
+            break;
+        case XFER_RESULT_STALLED:
+            mp_raise_usb_core_USBError(MP_ERROR_TEXT("Pipe error"));
+            break;
+        case XFER_RESULT_TIMEOUT:
+        case XFER_RESULT_INVALID:
+            mp_raise_usb_core_USBTimeoutError();
+            break;
+    }
+    return false;
 }
 
 static void _prepare_for_transfer(void) {
@@ -173,41 +192,75 @@ static void _get_langid(usb_core_device_obj_t *self) {
     }
     // Two control bytes and one uint16_t language code.
     uint16_t temp_buf[2];
-    if (!tuh_descriptor_get_string(self->device_address, 0, 0, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0) ||
-        !_wait_for_callback()) {
-        return;
+    _prepare_for_transfer();
+    if (!tuh_descriptor_get_string(self->device_address, 0, 0, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0)) {
+        mp_raise_usb_core_USBError(NULL);
+    } else if (_wait_for_callback()) {
+        self->first_langid = temp_buf[1];
     }
-    self->first_langid = temp_buf[1];
 }
 
 mp_obj_t common_hal_usb_core_device_get_serial_number(usb_core_device_obj_t *self) {
     uint16_t temp_buf[127];
-    _get_langid(self);
-    if (!tuh_descriptor_get_serial_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0) ||
-        !_wait_for_callback()) {
+    tusb_desc_device_t descriptor;
+    // First, be sure not to ask TinyUSB for a non-existent string (avoid error)
+    if (!tuh_descriptor_get_device_local(self->device_address, &descriptor)) {
+        return mp_const_none;
+    }
+    if (descriptor.iSerialNumber == 0) {
         return mp_const_none;
     }
-    return _get_string(temp_buf);
+    // Device does provide this string, so continue
+    _get_langid(self);
+    _prepare_for_transfer();
+    if (!tuh_descriptor_get_serial_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0)) {
+        mp_raise_usb_core_USBError(NULL);
+    } else if (_wait_for_callback()) {
+        return _get_string(temp_buf);
+    }
+    return mp_const_none;
 }
 
 mp_obj_t common_hal_usb_core_device_get_product(usb_core_device_obj_t *self) {
     uint16_t temp_buf[127];
-    _get_langid(self);
-    if (!tuh_descriptor_get_product_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0) ||
-        !_wait_for_callback()) {
+    tusb_desc_device_t descriptor;
+    // First, be sure not to ask TinyUSB for a non-existent string (avoid error)
+    if (!tuh_descriptor_get_device_local(self->device_address, &descriptor)) {
         return mp_const_none;
     }
-    return _get_string(temp_buf);
+    if (descriptor.iProduct == 0) {
+        return mp_const_none;
+    }
+    // Device does provide this string, so continue
+    _get_langid(self);
+    _prepare_for_transfer();
+    if (!tuh_descriptor_get_product_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0)) {
+        mp_raise_usb_core_USBError(NULL);
+    } else if (_wait_for_callback()) {
+        return _get_string(temp_buf);
+    }
+    return mp_const_none;
 }
 
 mp_obj_t common_hal_usb_core_device_get_manufacturer(usb_core_device_obj_t *self) {
     uint16_t temp_buf[127];
-    _get_langid(self);
-    if (!tuh_descriptor_get_manufacturer_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0) ||
-        !_wait_for_callback()) {
+    tusb_desc_device_t descriptor;
+    // First, be sure not to ask TinyUSB for a non-existent string (avoid error)
+    if (!tuh_descriptor_get_device_local(self->device_address, &descriptor)) {
         return mp_const_none;
     }
-    return _get_string(temp_buf);
+    if (descriptor.iManufacturer == 0) {
+        return mp_const_none;
+    }
+    // Device does provide this string, so continue
+    _get_langid(self);
+    _prepare_for_transfer();
+    if (!tuh_descriptor_get_manufacturer_string(self->device_address, self->first_langid, temp_buf, sizeof(temp_buf), _transfer_done_cb, 0)) {
+        mp_raise_usb_core_USBError(NULL);
+    } else if (_wait_for_callback()) {
+        return _get_string(temp_buf);
+    }
+    return mp_const_none;
 }