sched: Check for zero sleep time and yield CPU if

necessary

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

sched/signal/sig_nanosleep.c

@@ -103,6 +103,24 @@ int nxsig_nanosleep(FAR const struct timespec *rqtp,
       return -EINVAL;
     }
 
+  /* If rqtp is zero, yield CPU and return
+   * Notice: The behavior of sleep(0) is not defined in POSIX, so there are
+   * different implementations:
+   * 1. In Linux, nanosleep(0) will call schedule() to yield CPU:
+   *    https://elixir.bootlin.com/linux/latest/source/kernel/time/
+   *    hrtimer.c#L2038
+   * 2. In BSD, nanosleep(0) will return immediately:
+   *    https://github.com/freebsd/freebsd-src/blob/
+   *    475fa89800086718bd9249fd4dc3f862549f1f78/crypto/openssh/
+   *    openbsd-compat/bsd-misc.c#L243
+   */
+
+  if (rqtp->tv_sec == 0 && rqtp->tv_nsec == 0)
+    {
+      sched_yield();
+      return OK;
+    }
+
   /* Get the start time of the wait.  Interrupts are disabled to prevent
    * timer interrupts while we do tick-related calculations before and
    * after the wait.