Merge pull request IanHarvey#169 from OnBeep/support-empty-wr-value

Support empty values in "wr" and "wrr" commands IanHarvey#169

The GATT protocol supports empty values (length=0) for variable length characteristics, and some devices require empty values in some circumstances.

Trying to set an empty value to a GATT characteristic with code like

    handle.write(bytes(''))

results in failure, with the following output when I set `Debugging = True` in `btle.py`:

    Sent:  wr 95

    Got: 'rsp=$err code=$badparam\n'

With this PR, the `handle.write(bytes(''))` succeeds with the expected behavior on the airwaves and on the GATT server.

* 'support-empty-wr-value' of https://github.com/OnBeep/bluepy:
  bluepy-helper: document "wr" and "wrr" optional quotes
  bluepy-helper: support unquoted hex strings, too
  bluepy-helper: teach "wr" and "wrr" commands to take quoted data strings
  bluepy-helper: have gatt_attr_data_from_string return -1 on error rather than 0

bluepy/bluepy-helper.c

@@ -1060,7 +1060,7 @@ static void cmd_char_write_common(int argcp, char **argvp, int with_response)
     }
 
     plen = gatt_attr_data_from_string(argvp[2], &value);
-    if (plen == 0) {
+    if (plen == (size_t)-1) {
         resp_error(err_BAD_PARAM);
         return;
     }
@@ -1665,9 +1665,9 @@ static struct {
         "Characteristics Value/Descriptor Read by handle" },
     { "rdu",        cmd_read_uuid,  "<UUID> [start hnd] [end hnd]",
         "Characteristics Value/Descriptor Read by UUID" },
-    { "wrr",        cmd_char_write_rsp, "<handle> <new value>",
+    { "wrr",        cmd_char_write_rsp, "<handle> [<new value>|'<new value>']",
         "Characteristic Value Write (Write Request)" },
-    { "wr",         cmd_char_write, "<handle> <new value>",
+    { "wr",         cmd_char_write, "<handle> [<new value>|'<new value>']",
         "Characteristic Value Write (No response)" },
     { "secu",       cmd_sec_level,  "[low | medium | high]",
         "Set security level. Default: low" },

bluepy/btle.py

@@ -498,7 +498,7 @@ def writeCharacteristic(self, handle, val, withResponse=False):
         # Without response, a value too long for one packet will be truncated,
         # but with response, it will be sent as a queued write
         cmd = "wrr" if withResponse else "wr"
-        self._writeCmd("%s %X %s\n" % (cmd, handle, binascii.b2a_hex(val).decode('utf-8')))
+        self._writeCmd("%s %X '%s'\n" % (cmd, handle, binascii.b2a_hex(val).decode('utf-8')))
         return self._getResp('wr')
 
     def setSecurityLevel(self, level):

bluez-5.47/attrib/utils.c

@@ -105,12 +105,26 @@ GIOChannel *gatt_connect(const char *src, const char *dst,
 size_t gatt_attr_data_from_string(const char *str, uint8_t **data)
 {
 	char tmp[3];
-	size_t size, i;
+	size_t size, len, i;
+	int is_quoted = 0;
 
-	size = strlen(str) / 2;
-	*data = g_try_malloc0(size);
+	if (str[0] == '\'')
+		is_quoted = 1;
+
+	len = strlen(str);
+
+	if (is_quoted) {
+		if (str[len - 1] != '\'')
+			return (size_t)-1;
+
+		str = str + 1;
+		len = len - 2;
+	}
+
+	size = len / 2;
+	*data = g_try_malloc0(len == 0 ? 1 : size);
 	if (*data == NULL)
-		return 0;
+		return (size_t)-1;
 
 	tmp[2] = '\0';
 	for (i = 0; i < size; i++) {