work around quirky behavior of Solaris adjtime

I could not find a lot of precedence for this, because most time
daemons do not actually look at the value of olddelta. Account for
olddelta getting stuck at 1ms, and for a NULL value of delta being
treated as an error condition.

include/sys/time.h

@@ -3,8 +3,41 @@
  * sys/time.h compatibility shim
  */
 
+#ifndef LIBCOMPAT_SYS_TIME_H
+#define LIBCOMPAT_SYS_TIME_H
+
 #include_next <sys/time.h>
 
 #include <stdint.h>
 
 int adjfreq(const int64_t *freq, int64_t *oldfreq);
+
+#ifdef __sun
+static inline int sun_adjtime(struct timeval *delta, struct timeval *olddelta)
+{
+	struct timeval zero = {0};
+	int rc;
+
+	/*
+	 * adjtime on Solaris appears to handle a NULL delta differently than
+	 * other OSes. Fill in a dummy value as necessary.
+	 */
+	if (delta)
+		rc = adjtime(delta, olddelta);
+	else
+		rc = adjtime(&zero, olddelta);
+
+	/*
+	 * Old delta on Solaris frequently gets stuck with 1 ms left.
+	 * Round down to 0 in this case so we do not get flapping clock sync.
+	 */
+	if (rc == 0 && olddelta &&
+	    olddelta->tv_sec == 0 && olddelta->tv_usec == 1)
+		olddelta->tv_usec = 0;
+
+	return rc;
+}
+#define adjtime(d, o) sun_adjtime(d, o)
+#endif
+
+#endif