posix: clock: current method of capturing elapsed time leads to loss in seconds

[cfriedt]

Describe the bug
Although it is a boundary, before the 64-bit tick counter rolls-over, the value of the monotonic clock (seconds field) should equal UINT64_MAX / CONFIG_SYS_CLOCK_TICKS_PER_SEC (integer division).

Currently, it seems to be off by an order of magnitude (test case provided in PR).

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? all
• What have you tried to diagnose or workaround this issue? written a test case
• Is this a regression? If yes, have you been able to "git bisect" it to a
  specific commit? No

To Reproduce
Steps to reproduce the behavior:

1. apply changes in referenced PR
2. un-apply changes to lib/posix/clock.c
3. west build -p auto -b qemu_x86_64 -t run tests/posix/common
4. See error

Expected behavior

*** Booting Zephyr OS build zephyr-v3.2.0-2459-gdffb02fd134e ***
Running TESTSUITE posix_apis
===================================================================
START - test_clock_gettime_rollover
CONFIG_SYS_CLOCK_TICKS_PER_SEC: 100
rollover_s: 184467440737095516
t-1: {184467440737095516, 140000000}
t+0: {184467440737095516, 150000000}
t+1: {0, 0}
 PASS - test_clock_gettime_rollover in 0.063 seconds
 ===================================================================

Impact
That really depends on the application. It can be an annoyance for some but a showstopper for others.

Logs and console output

*** Booting Zephyr OS build zephyr-v3.2.0-2459-gdffb02fd134e ***
Running TESTSUITE posix_apis
===================================================================
START - test_clock_gettime_rollover
CONFIG_SYS_CLOCK_TICKS_PER_SEC: 100
rollover_s: 184467440737095516
t-1: {18446744073709551, 596000000}

    Assertion failed at WEST_TOPDIR/zephyr/tests/posix/common/src/clock.c:169: posix_apis_test_clock_gettime_rollover: rollover_s not equal to ts[0].tv_sec

 FAIL - test_clock_gettime_rollover in 0.025 seconds
 ===================================================================

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows): any
• Toolchain (e.g Zephyr SDK, ...): Zephyr SDK 0.15.2
• Commit SHA or Version used: bba9fc9

Additional context
This issue was encountered while writing tests to verify the fixes in

• sys: time_units: Increase range of z_tmcvt #41602
• lib: posix: clock: Prevent early overflows #41814
  and while working on
• Overflow in settime posix function #52825

[cfriedt]

Technically, I believe we are seeing a bug in z_tmcvt() which does not have a comprehensive testsuite, AFAIK. So #52977 is more of a workaround for the POSIX subsystem.

Also, I should mention, the error factor is 1 order of magnitude with ticks/s = 100. With ticks/s = 10, it's 2 orders of magnitude. With ticks/s >= 1000 the error vanishes.

I'm fairly concerned about the test coverage for z_tmcvt().

generator.cpp.txt