Allow empty reads and writes for busio.SPI

This is mostly for convenience, so that user code doesn't
need to add additional checks.

Also, bring the bitbangio into compatibility with busio wrt. empty
buffers.

shared-bindings/bitbangio/I2C.c

@@ -157,6 +157,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(bitbangio_i2c_unlock_obj, bitbangio_i2c_obj_unlock);
 //|
 //|      Read into ``buffer`` from the slave specified by ``address``.
 //|      The number of bytes read will be the length of ``buffer``.
+//|      At least one byte must be read.
 //|
 //|      If ``start`` or ``end`` is provided, then the buffer will be sliced
 //|      as if ``buffer[start:end]``. This will not cause an allocation like
@@ -186,6 +187,9 @@ STATIC mp_obj_t bitbangio_i2c_readfrom_into(size_t n_args, const mp_obj_t *pos_a
     int32_t start = args[ARG_start].u_int;
     uint32_t length = bufinfo.len;
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
+    if (length == 0) {
+        mp_raise_ValueError("Buffer must be at least length 1");
+    }
     uint8_t status = shared_module_bitbangio_i2c_read(self,
                                                       args[ARG_address].u_int,
                                                       ((uint8_t*)bufinfo.buf) + start,
@@ -206,6 +210,9 @@ MP_DEFINE_CONST_FUN_OBJ_KW(bitbangio_i2c_readfrom_into_obj, 3, bitbangio_i2c_rea
 //|      as if ``buffer[start:end]``. This will not cause an allocation like
 //|      ``buffer[start:end]`` will so it saves memory.
 //|
+//|      Writing a buffer or slice of length zero is permitted, as it can be used
+//|      to poll for the existence of a device.
+//|
 //|      :param int address: 7-bit device address
 //|      :param bytearray buffer: buffer containing the bytes to write
 //|      :param int start: Index to start writing from

shared-bindings/bitbangio/SPI.c

@@ -191,6 +191,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(bitbangio_spi_unlock_obj, bitbangio_spi_obj_unlock);
 //|   .. method:: SPI.write(buf)
 //|
 //|     Write the data contained in ``buf``. Requires the SPI being locked.
+//|     If the buffer is empty, nothing happens.
 //|
 // TODO(tannewt): Add support for start and end kwargs.
 STATIC mp_obj_t bitbangio_spi_write(mp_obj_t self_in, mp_obj_t wr_buf) {
@@ -198,6 +199,9 @@ STATIC mp_obj_t bitbangio_spi_write(mp_obj_t self_in, mp_obj_t wr_buf) {
     raise_error_if_deinited(shared_module_bitbangio_spi_deinited(self));
     mp_buffer_info_t src;
     mp_get_buffer_raise(wr_buf, &src, MP_BUFFER_READ);
+    if (src.len == 0) {
+        return mp_const_none;
+    }
     check_lock(self);
     bool ok = shared_module_bitbangio_spi_write(self, src.buf, src.len);
     if (!ok) {
@@ -210,7 +214,9 @@ MP_DEFINE_CONST_FUN_OBJ_2(bitbangio_spi_write_obj, bitbangio_spi_write);
 
 //|   .. method:: SPI.readinto(buf)
 //|
-//|     Read into the buffer specified by ``buf`` while writing zeroes. Requires the SPI being locked.
+//|     Read into the buffer specified by ``buf`` while writing zeroes.
+//|     Requires the SPI being locked.
+//|     If the number of bytes to read is 0, nothing happens.
 //|
 // TODO(tannewt): Add support for start and end kwargs.
 STATIC mp_obj_t bitbangio_spi_readinto(size_t n_args, const mp_obj_t *args) {
@@ -218,6 +224,9 @@ STATIC mp_obj_t bitbangio_spi_readinto(size_t n_args, const mp_obj_t *args) {
     raise_error_if_deinited(shared_module_bitbangio_spi_deinited(self));
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_WRITE);
+    if (bufinfo.len == 0) {
+        return mp_const_none;
+    }
     check_lock(args[0]);
     bool ok = shared_module_bitbangio_spi_read(self, bufinfo.buf, bufinfo.len);
     if (!ok) {

shared-bindings/busio/SPI.c

@@ -207,7 +207,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(busio_spi_unlock_obj, busio_spi_obj_unlock);
 //|   .. method:: SPI.write(buffer, \*, start=0, end=len(buffer))
 //|
 //|     Write the data contained in ``buf``. Requires the SPI being locked.
-//|     At least one byte must be written.
+//|     If the buffer is empty, nothing happens.
 //|
 //|     :param bytearray buffer: buffer containing the bytes to write
 //|     :param int start: Index to start writing from
@@ -233,7 +233,7 @@ STATIC mp_obj_t busio_spi_write(size_t n_args, const mp_obj_t *pos_args, mp_map_
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
     if (length == 0) {
-        mp_raise_ValueError("Buffer must be at least length 1");
+        return mp_const_none;
     }
 
     bool ok = common_hal_busio_spi_write(self, ((uint8_t*)bufinfo.buf) + start, length);
@@ -247,8 +247,9 @@ MP_DEFINE_CONST_FUN_OBJ_KW(busio_spi_write_obj, 2, busio_spi_write);
 
 //|   .. method:: SPI.readinto(buffer, \*, start=0, end=len(buffer), write_value=0)
 //|
-//|     Read into the buffer specified by ``buf`` while writing zeroes. Requires the SPI being locked.
-//|     At least one byte must be read.
+//|     Read into the buffer specified by ``buf`` while writing zeroes.
+//|     Requires the SPI being locked.
+//|     If the number of bytes to read is 0, nothing happens.
 //|
 //|     :param bytearray buffer: buffer to write into
 //|     :param int start: Index to start writing at
@@ -276,7 +277,7 @@ STATIC mp_obj_t busio_spi_readinto(size_t n_args, const mp_obj_t *pos_args, mp_m
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
     if (length == 0) {
-        mp_raise_ValueError("Buffer must be at least length 1");
+        return mp_const_none;
     }
 
     bool ok = common_hal_busio_spi_read(self, ((uint8_t*)bufinfo.buf) + start, length, args[ARG_write_value].u_int);