[riot-notifications] [RIOT-OS/RIOT] gcoap: don't call random_uint32_range() when COAP_ACK_VARIANCE=0 (#11784)

Martine Lenders notifications at github.com
Wed Jul 3 15:52:02 CEST 2019

The RIOT community cares a lot about code quality.
Therefore, before describing what your contribution is about, we would like
you to make sure that your modifications are compliant with the RIOT
coding conventions, see https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions.

### Contribution description
`COAP_ACK_VARIANCE` is a configurable macro, so when it is 0, the [precondition for `random_uint32_range()`][1] is not held.

Background is that for some experimental setups you want to disable the randomness in retransmissions to have better comparibility, as we did e.g. for [this paper](https://conferences.sigcomm.org/acm-icn/2018/proceedings/icn18-final46.pdf).

[1]: https://doc.riot-os.org/group__sys__random.html#gab6ee09e1e56df3cc78acd1fbf97bfb24

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- if it's a bug fix, describe the bug that it solves and how it is solved
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### Testing procedure
Compiling `examples/gcoap` with `COAP_ACK_VARIANCE=0` and the default value should still work. The randomness should be reduced with `COAP_ACK_VARIANCE` (but not completely turned of due to a race condition in `gcoap`, see [issue will follow]).
Details steps to test your contribution:
- which test/example to compile for which board and is there a 'test' command
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- the expected success test output

### Issues/PRs references
Examples: Fixes #1234. See also #5678. Depends on PR #9876.

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-- Commit Summary --

  * gcoap: don't call random_uint32_range() when COAP_ACK_VARIANCE=0

-- File Changes --

    M sys/net/application_layer/gcoap/gcoap.c (4)

-- Patch Links --


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