`alt_clock_gettime()` does not check return value of `host_get_clock_service()` (macOS)

[zqw86713]

Summary

The implementation of alt_clock_gettime() in clock.cpp does not check the return value of host_get_clock_service() or clock_get_time(). If either of these system calls fail, the function proceeds to use an uninitialized mach_timespec_t, resulting in undefined behavior or potential crashes.

Affected Code

https://github.com/zeromq/libzmq/blob/master/src/clock.cpp

int alt_clock_gettime (int clock_id, timespec *ts)
{
    clock_serv_t cclock;
    mach_timespec_t mts;
    host_get_clock_service (mach_host_self (), clock_id, &cclock);
    clock_get_time (cclock, &mts);
    mach_port_deallocate (mach_task_self (), cclock);
    ts->tv_sec = mts.tv_sec;
    ts->tv_nsec = mts.tv_nsec;
    return 0;
}

Problem

• host_get_clock_service() and clock_get_time() can fail, returning a non-zero kern_return_t
• No return code is checked before using the result
• If either call fails, mts.tv_sec and mts.tv_nsec are undefined
• This can result in corrupted timestamps or segmentation faults

Expected Behavior

• Both system calls should be checked for KERN_SUCCESS before using mts
• If any of them fails, alt_clock_gettime() should return -1 as an error signal, consistent with clock_gettime behavior

Platform

• macOS < 10.12 (this code path is used as fallback)
• Affects users relying on alt_clock_gettime() for monotonic time

Impact

Low frequency, but high risk when triggered — undefined time values or potential crash.

[zqw86713]

I submit a PR: #4803