init_timers();
when = ticks + GETTICKCOUNT();
- assert(when - now > 0);
+
+ /*
+ * Just in case our various defences against timing skew fail
+ * us: if we try to schedule a timer that's already in the
+ * past, we instead schedule it for the immediate future.
+ */
+ if (when - now <= 0)
+ when = now + 1;
t = snew(struct timer);
t->fn = fn;
init_timers();
+#ifdef TIMING_SYNC
+ /*
+ * In this ifdef I put some code which deals with the
+ * possibility that `anow' disagrees with GETTICKCOUNT by a
+ * significant margin. Our strategy for dealing with it differs
+ * depending on platform, because on some platforms
+ * GETTICKCOUNT is more likely to be right whereas on others
+ * `anow' is a better gold standard.
+ */
+ {
+ long tnow = GETTICKCOUNT();
+
+ if (tnow + TICKSPERSEC/50 - anow < 0 ||
+ anow + TICKSPERSEC/50 - tnow < 0
+ ) {
+#if defined TIMING_SYNC_ANOW
+ /*
+ * If anow is accurate and the tick count is wrong,
+ * this is likely to be because the tick count is
+ * derived from the system clock which has changed (as
+ * can occur on Unix). Therefore, we resolve this by
+ * inventing an offset which is used to adjust all
+ * future output from GETTICKCOUNT.
+ *
+ * A platform which defines TIMING_SYNC_ANOW is
+ * expected to have also defined this offset variable
+ * in (its platform-specific adjunct to) putty.h.
+ * Therefore we can simply reference it here and assume
+ * that it will exist.
+ */
+ tickcount_offset += anow - tnow;
+#elif defined TIMING_SYNC_TICKCOUNT
+ /*
+ * If the tick count is more likely to be accurate, we
+ * simply use that as our time value, which may mean we
+ * run no timers in this call (because we got called
+ * early), or alternatively it may mean we run lots of
+ * timers in a hurry because we were called late.
+ */
+ anow = tnow;
+#else
+/*
+ * Any platform which defines TIMING_SYNC must also define one of the two
+ * auxiliary symbols TIMING_SYNC_ANOW and TIMING_SYNC_TICKCOUNT, to
+ * indicate which measurement to trust when the two disagree.
+ */
+#error TIMING_SYNC definition incomplete
+#endif
+ }
+ }
+#endif
+
now = anow;
while (1) {