Add support for setting a custom monotonic time function in mono_time

[zugz]

(derived from #1095)

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This change is 

[codecov]

Codecov Report

Merging #1123 into master will increase coverage by 0.1%.
The diff coverage is 92%.

@@           Coverage Diff            @@
##           master   #1123     +/-   ##
========================================
+ Coverage    82.5%   82.6%   +0.1%     
========================================
  Files          81      81             
  Lines       14414   14437     +23     
========================================
+ Hits        11895   11929     +34     
+ Misses       2519    2508     -11

Impacted Files	Coverage Δ
toxcore/mono_time_test.cc	100% <100%> (ø)	⬆️
toxcore/mono_time.c	92.8% <81.8%> (-7.2%)	⬇️
toxcore/LAN_discovery.c	82% <0%> (-2.9%)	⬇️
toxcore/onion_client.c	94.2% <0%> (-0.8%)	⬇️
toxcore/network.c	83.6% <0%> (+0.2%)	⬆️
toxav/toxav.c	68.6% <0%> (+0.4%)	⬆️
toxcore/DHT.c	77.5% <0%> (+0.8%)	⬆️
toxav/msi.c	66.7% <0%> (+2.2%)	⬆️

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[iphydf]

Reviewable status: 0 of 1 approvals obtained (waiting on @zugz)

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toxcore/mono_time.h, line 62 at r1 (raw file):

 * to increase monotonically.
 */
void set_time_monotonic_callback(Mono_Time *mono_time,

Let's name this something starting with "mono_time", so this file is more api-standard compliant. We try to place functions belonging to a module in the same "namespace". (Same for the cb above)

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toxcore/mono_time.c, line 78 at r1 (raw file):

{
    if (time_monotonic_callback == nullptr) {
        time_monotonic_callback = current_time_monotonic_default;

nit: assigning a parameter is not great. Assignment is generally icky, but parameter assignment is often considered bad style (in JavaScript, it's a really bad idea, in other languages it's just a little frowned upon). Let's put if (..) { mono_time->tmc = ..; } else { mono_time->tmc = ctmd; } instead.

[iphydf]

@zugz can you add a test for this functionality to mono_time_test.cc?

[iphydf]

Reviewed 2 of 3 files at r2.
Reviewable status: 1 change requests, 0 of 1 approvals obtained (waiting on @zugz)

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toxcore/mono_time.c, line 47 at r2 (raw file):

    mono_time->time = 0;
    mono_time->current_time_callback = current_time_monotonic_default;

Move this one line down, so initialisation of the fields happens in declaration order and so the callback-related fields are initialised together.

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toxcore/mono_time.c, line 85 at r2 (raw file):

    }

    mono_time->user_data = user_data;

Move this to the second branch, and add an assignment of nullptr to the first branch. This way we avoid pointing at user data when nothing will ever be able to read it.

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toxcore/mono_time_test.cc, line 48 at r2 (raw file):

  uint64_t test_time = 42137;

  mono_time_set_current_time_callback(mono_time, test_current_time_callback, &test_time);

test_current_time_callback is probably better as a lambda: [](void *user_data) { return *(uint64_t *)user_data; }.

[zugz]

Move this one line down

The initialisation of base_time involves calling current_time_monotonic, which expects current_time_callback to be set. We could have current_time_monotonic use default if current_time_callback is NULL, but current_time_monotonic may be called frequently so it might be best to avoid branches?

`test_current_time_callback` is probably better as a lambda:

Maybe better to stick to pure C, since this doubles as soft documentation on how to use the callback functionality?

[iphydf]

Reviewed 2 of 2 files at r3.
Reviewable status: 1 change requests, 0 of 1 approvals obtained

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toxcore/mono_time.c, line 47 at r2 (raw file):

Previously, iphydf wrote…

Move this one line down, so initialisation of the fields happens in declaration order and so the callback-related fields are initialised together.

Ok, can you move the callback initialisation code up then?

[iphydf]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @zugz)

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toxcore/mono_time.c, line 47 at r2 (raw file):

Previously, iphydf wrote…

Ok, can you move the callback initialisation code up then?

Unresolved.

[iphydf]

Reviewed 2 of 2 files at r4.
Reviewable status: 1 change requests, 0 of 1 approvals obtained (waiting on @zugz)

[zugz]

Ok, can you move the callback initialisation code up then?

Done