net/gcoap: add macro to delay initialization of gcoap

[kb2ma]

Contribution description

Sometimes we do not wish gcoap to start at auto-initialization time. For example, #12104 must initialize its credman tool for DTLS before the gcoap thread starts. We wish to facilitate this use case without breaking existing applications. This PR adds a macro, GCOAP_INIT_DELAY, to accomplish this goal. When set to 1, gcoap must be started explicitly.

Testing procedure

In the gcoap example main(), insert the snippet below before the call to gcoap_cli_init().

   if (IS_ACTIVE(GCOAP_INIT_DELAY)) {
        kernel_pid_t pid = gcoap_init();
        printf("gcoap init: %u\n", pid);
    }

Test with and without defining the macro at compile time with something like:

$ CFLAGS="${CFLAGS} -DGCOAP_INIT_DELAY=1" make clean all

When defined, you should see the following at startup, and be able to send/receive gcoap messages as usual.

main(): This is RIOT! (Version: 2020.01-devel-1032-g41e2-gcoap/init_delay)
gcoap init: 6
gcoap example app
All up, running the shell now

Issues/PRs references

Alternative to #12529.

[kb2ma]

Thanks for the thumbs up, @pokgak. Unfortunately, I can't add you as a reviewer (yet), but I know this PR is important for gcoap support of DTLS. If you'd like to review it, I bet we could talk @leandrolanzieri or @miri64 into giving a proxy-ACK. ;-)

[pokgak]

This is definitely simpler than the lazy init approach. I like how, with CFLAG, you can document what the flag does compared to pseudomodules where there is normally no documentation about it.

How can this CFLAG be included as a dependency to another module? I haven't seen any CFLAGS in Makefile.dep so not sure if there are any rules regarding CFLAGS in that file or not.

[leandrolanzieri]

I certainly find this solution simpler than lazy init mode.

Although, I wonder if we should add some inline function to check if the server is already running or not. I think, for example, about the gcoap_cli, when you call coap info it will tell you that the server is listening on GCOAP_PORT even when gcoap is not initialized yet. There may be in the future some other modules that depend on gcoap server to be running on init?

Otherwise I find this macro to be a good solution, and agree with @miri64 's proposal for the renaming.

[kb2ma]

How can this CFLAG be included as a dependency to another module? I haven't seen any CFLAGS in Makefile.dep so not sure if there are any rules regarding CFLAGS in that file or not.

sys/Makefile.dep includes addition to CFLAGS.

[kb2ma]

Although, I wonder if we should add some inline function to check if the server is already running or not.

@pokgak, would a function like bool gcoap_is_running() be useful for DTLS integration? My original idea was that initialization was only delayed, and not deferred until some arbitrary time. In other words, I didn't think there would be much question on when to call gcoap_init().

Also, we may create some more generic facility on the state of gcoap, which I kind of started with gcoap_op_state(). It might make more sense to expand on that function rather than create a standalone function. I guess this is all a way of saying I'd rather wait until we need the capability to decide how to implement it.

[kb2ma]

Comments addressed.

[leandrolanzieri]

Changes look good. I tested this and works as expected. ACK.

[leandrolanzieri]

@kb2ma please squash

[kb2ma]

Squashed and tests pass.