Support custom attribute for well-known/core query

[bethmal]

Is your feature request related to a problem? Please describe.

There is no easy way to support custom attribute to query via "well-known/core?custom_attr=...".

Describe the solution you would like

In coap_resource.c, declare _rt_attributes like below:

static const coap_str_const_t _rt_attributes[] = {
{2, (const uint8_t *)"rt"},
{2, (const uint8_t *)"if"},
{3, (const uint8_t *)"rel"},
#ifdef WELLKNOWN_CORE_CUSTOM_ATTRIBUTES
WELLKNOWN_CORE_CUSTOM_ATTRIBUTES,
#endif
{0, NULL}
};

Describe alternatives you have considered

Implement custom well-known/core handler. However, this seems meaningless to not easily reuse the existing well implemented well-known/core handler.

[mrdeep1]

Have you looked at coap_attribute(3) where attributes can be set when resources are set up?

Adding in a custom attribute into the examples/coap-server as per

$ git diff
diff --git a/examples/coap-server.c b/examples/coap-server.c
index 0cae5150..2415d43b 100644
--- a/examples/coap-server.c
+++ b/examples/coap-server.c
@@ -1209,6 +1209,7 @@ init_resources(coap_context_t *ctx) {
     coap_add_attr(r, coap_make_str_const("title"), coap_make_str_const("\"Internal Clock\""), 0);
     coap_add_attr(r, coap_make_str_const("rt"), coap_make_str_const("\"ticks\""), 0);
     coap_add_attr(r, coap_make_str_const("if"), coap_make_str_const("\"clock\""), 0);
+    coap_add_attr(r, coap_make_str_const("custom"), coap_make_str_const("\"a-test\""), 0);

     coap_add_resource(ctx, r);
     time_resource = r;

I get

$ examples/coap-client "coap://127.0.0.1/.well-known/core"
</>;title="General Info";ct=0,</time>;custom="a-test";if="clock";rt="ticks";title="Internal Clock";ct=0;obs,</async>;ct=0,</example_data>;title="Example Data";ct=0;obs

$ examples/coap-client "coap://127.0.0.1/.well-known/core?custom=*"
</time>;custom="a-test";if="clock";rt="ticks";title="Internal Clock";ct=0;obs

$ examples/coap-client "coap://127.0.0.1/.well-known/core?custom=a*"
</time>;custom="a-test";if="clock";rt="ticks";title="Internal Clock";ct=0;obs

$ examples/coap-client "coap://127.0.0.1/.well-known/core?custom=b*"

which works as I think you want.

[bethmal]

I think we also need something more - query against custom attribute by MATCH_SUBSTRING. To enable MATCH_SUBSTRING, I see only the approach to extend struture _rt_attributes. Any other approaches to support this? Thanks.

libcoap/src/coap_resource.c

Line 162 in 9f14450

for (rt_attributes = _rt_attributes; rt_attributes->s; rt_attributes++) {