Fix SDO writes of empty strings (#551)

Previously, when attempting a write of "" to a VISIBLE_STRING, the
expedited SDO request would not be sent and the transaction would
timeout.  To resolve this, if the transaction size is known to be 0, a
segmented transfer is used.

canopen/sdo/client.py

@@ -353,7 +353,7 @@ def __init__(self, sdo_client, index, subindex=0, size=None, force_segment=False
         self._exp_header = None
         self._done = False
 
-        if size is None or size > 4 or force_segment:
+        if size is None or size < 1 or size > 4 or force_segment:
             # Initiate segmented download
             request = bytearray(8)
             command = REQUEST_DOWNLOAD

test/test_sdo.py

@@ -68,6 +68,8 @@ def _send_message(self, can_id, data, remote=False):
         while self.data and self.data[0][0] == RX:
             self.network.notify(0x582, self.data.pop(0)[1], 0.0)
 
+        self.message_sent = True
+
     def setUp(self):
         network = canopen.Network()
         network.NOTIFIER_SHUTDOWN_TIMEOUT = 0.0
@@ -76,6 +78,8 @@ def setUp(self):
         node.sdo.RESPONSE_TIMEOUT = 0.01
         self.network = network
 
+        self.message_sent = False
+
     def test_expedited_upload(self):
         self.data = [
             (TX, b'\x40\x18\x10\x01\x00\x00\x00\x00'),
@@ -91,6 +95,7 @@ def test_expedited_upload(self):
         ]
         trans_type = self.network[2].sdo[0x1400]['Transmission type RPDO 1'].raw
         self.assertEqual(trans_type, 254)
+        self.assertTrue(self.message_sent)
 
     def test_size_not_specified(self):
         self.data = [
@@ -100,13 +105,15 @@ def test_size_not_specified(self):
         # Make sure the size of the data is 1 byte
         data = self.network[2].sdo.upload(0x1400, 2)
         self.assertEqual(data, b'\xfe')
+        self.assertTrue(self.message_sent)
 
     def test_expedited_download(self):
         self.data = [
             (TX, b'\x2b\x17\x10\x00\xa0\x0f\x00\x00'),
             (RX, b'\x60\x17\x10\x00\x00\x00\x00\x00')
         ]
         self.network[2].sdo[0x1017].raw = 4000
+        self.assertTrue(self.message_sent)
 
     def test_segmented_upload(self):
         self.data = [
@@ -153,6 +160,16 @@ def test_block_download(self):
             'wb', size=len(data), block_transfer=True) as fp:
             fp.write(data)
 
+    def test_segmented_download_zero_length(self):
+        self.data = [
+            (TX, b'\x21\x00\x20\x00\x00\x00\x00\x00'),
+            (RX, b'\x60\x00\x20\x00\x00\x00\x00\x00'),
+            (TX, b'\x0F\x00\x00\x00\x00\x00\x00\x00'),
+            (RX, b'\x20\x00\x00\x00\x00\x00\x00\x00'),
+        ]
+        self.network[2].sdo[0x2000].raw = ""
+        self.assertTrue(self.message_sent)
+
     def test_block_upload(self):
         self.data = [
             (TX, b'\xa4\x08\x10\x00\x7f\x00\x00\x00'),