X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/8136216bdb02b9f8cc3af159fced247084714536..HEAD:/timing.c

diff --git a/timing.c b/timing.c
index 6dd8aa5e..ccd76cd6 100644
--- a/timing.c
+++ b/timing.c
@@ -7,6 +7,24 @@
  * changes; and, very importantly, it tracks the context pointers
  * passed to schedule_timer(), so that if a context is freed all
  * the timers associated with it can be immediately annulled.
+ *
+ *
+ * The problem is that computer clocks aren't perfectly accurate.
+ * The GETTICKCOUNT function returns a 32bit number that normally
+ * increases by about 1000 every second. On windows this uses the PC's
+ * interrupt timer and so is only accurate to around 20ppm.  On unix it's
+ * a value that's calculated from the current UTC time and so is in theory
+ * accurate in the long term but may jitter and jump in the short term.
+ *
+ * What PuTTY needs from these timers is simply a way of delaying the
+ * calling of a function for a little while, if it's occasionally called a
+ * little early or late that's not a problem. So to protect against clock
+ * jumps schedule_timer records the time that it was called in the timer
+ * structure. With this information the run_timers function can see when
+ * the current GETTICKCOUNT value is after the time the event should be
+ * fired OR before the time it was set. In the latter case the clock must
+ * have jumped, the former is (probably) just the normal passage of time.
+ *
  */
 
 #include <assert.h>
@@ -18,12 +36,13 @@
 struct timer {
     timer_fn_t fn;
     void *ctx;
-    long now;
+    unsigned long now;
+    unsigned long when_set;
 };
 
 static tree234 *timers = NULL;
 static tree234 *timer_contexts = NULL;
-static long now = 0L;
+static unsigned long now = 0L;
 
 static int compare_timers(void *av, void *bv)
 {
@@ -41,14 +60,12 @@ static int compare_timers(void *av, void *bv)
      * Failing that, compare on the other two fields, just so that
      * we don't get unwanted equality.
      */
-#ifdef __LCC__
+#if defined(__LCC__) || defined(__clang__)
     /* lcc won't let us compare function pointers. Legal, but annoying. */
     {
 	int c = memcmp(&a->fn, &b->fn, sizeof(a->fn));
-	if (c < 0)
-	    return -1;
-	else if (c > 0)
-	    return +1;
+	if (c)
+	    return c;
     }
 #else    
     if (a->fn < b->fn)
@@ -89,20 +106,29 @@ static void init_timers(void)
     }
 }
 
-long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
+unsigned long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
 {
-    long when;
+    unsigned long when;
     struct timer *t, *first;
 
     init_timers();
 
-    when = ticks + GETTICKCOUNT();
-    assert(when - now > 0);
+    now = GETTICKCOUNT();
+    when = ticks + now;
+
+    /*
+     * 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;
     t->ctx = ctx;
     t->now = when;
+    t->when_set = now;
 
     if (t != add234(timers, t)) {
 	sfree(t);		       /* identical timer already exists */
@@ -127,13 +153,13 @@ long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
  * Returns the time (in ticks) expected until the next timer after
  * that triggers.
  */
-int run_timers(long anow, long *next)
+int run_timers(unsigned long anow, unsigned long *next)
 {
     struct timer *first;
 
     init_timers();
 
-    now = anow;
+    now = GETTICKCOUNT();
 
     while (1) {
 	first = (struct timer *)index234(timers, 0);
@@ -148,7 +174,8 @@ int run_timers(long anow, long *next)
 	     */
 	    delpos234(timers, 0);
 	    sfree(first);
-	} else if (first->now - now <= 0) {
+	} else if (now - (first->when_set - 10) >
+		   first->now - (first->when_set - 10)) {
 	    /*
 	     * This timer is active and has reached its running
 	     * time. Run it.