net: lwm2m: update internal API select_writer() and select_reader()

As per review of PR #5893, this is a follow up patch to update
select_writer() API to match the behavior of select_reader() API.

Quote from OMA-TS-LightweightM2M-V1_0_1-20170704-A. 8.2.5
"An Object Instance or Resource is Read by sending a CoAP GET to the
 corresponding path. The response includes the value in the
 corresponding Plain Text, Opaque, TLV or JSON format according to
 the specified Content-Format (see section 6.4).The request MAY
 specify an Accept option containing the preferred Content-Format
 to receive. When the specified Content-Format is not supported by
 the LwM2M Client, the request MUST be rejected."

Therefore, we do not attempt to assign a content-format when the
requested one is not supported. Instead, we return an error code.

0 is returned when reader or writer has been successfully selected by
select_reader() or select_writer()

Signed-off-by: Robert Chou <[email protected]>

Author: rbtchc

subsys/net/lib/lwm2m/lwm2m_engine.c

@@ -1133,7 +1133,7 @@ u16_t lwm2m_get_rd_data(u8_t *client_data, u16_t size)
 
 /* input / output selection */
 
-static u16_t select_writer(struct lwm2m_output_context *out, u16_t accept)
+static int select_writer(struct lwm2m_output_context *out, u16_t accept)
 {
 	switch (accept) {
 
@@ -1159,15 +1159,12 @@ static u16_t select_writer(struct lwm2m_output_context *out, u16_t accept)
 #endif
 
 	default:
-		SYS_LOG_ERR("Unknown Accept type %u, using LWM2M plain text",
-			    accept);
-		out->writer = &plain_text_writer;
-		accept = LWM2M_FORMAT_PLAIN_TEXT;
-		break;
+		SYS_LOG_WRN("Unknown content type %u", accept);
+		return -ENOMSG;
 
 	}
 
-	return accept;
+	return 0;
 }
 
 static int select_reader(struct lwm2m_input_context *in, u16_t format)
@@ -1190,7 +1187,7 @@ static int select_reader(struct lwm2m_input_context *in, u16_t format)
 		return -ENOMSG;
 	}
 
-	return format;
+	return 0;
 }
 
 /* user data setter functions */
@@ -3045,7 +3042,7 @@ static int handle_request(struct coap_packet *request,
 		}
 	}
 
-	/* read Content Format */
+	/* read Content Format / setup in.reader */
 	r = coap_find_options(in.in_cpkt, COAP_OPTION_CONTENT_FORMAT,
 			      options, 1);
 	if (r > 0) {
@@ -3056,7 +3053,7 @@ static int handle_request(struct coap_packet *request,
 		}
 	}
 
-	/* read Accept */
+	/* read Accept / setup out.writer */
 	r = coap_find_options(in.in_cpkt, COAP_OPTION_ACCEPT, options, 1);
 	if (r > 0) {
 		accept = coap_option_value_to_int(&options[0]);
@@ -3065,6 +3062,11 @@ static int handle_request(struct coap_packet *request,
 		accept = LWM2M_FORMAT_OMA_TLV;
 	}
 
+	r = select_writer(&out, accept);
+	if (r < 0) {
+		goto error;
+	}
+
 	if (!well_known) {
 		/* find registered obj */
 		obj = get_engine_obj(path.obj_id);
@@ -3075,8 +3077,6 @@ static int handle_request(struct coap_packet *request,
 		}
 	}
 
-	accept = select_writer(&out, accept);
-
 	/* set the operation */
 	switch (code & COAP_REQUEST_MASK) {