git.distorted.org.uk Git - u/mdw/putty/blob - timing.c

   1 /*
   2  * timing.c
   3  *
   4  * This module tracks any timers set up by schedule_timer(). It
   5  * keeps all the currently active timers in a list; it informs the
   6  * front end of when the next timer is due to go off if that
   7  * changes; and, very importantly, it tracks the context pointers
   8  * passed to schedule_timer(), so that if a context is freed all
   9  * the timers associated with it can be immediately annulled.
  10  *
  11  *
  12  * The problem is that computer clocks aren't perfectly accurate.
  13  * The GETTICKCOUNT function returns a 32bit number that normally
  14  * increases by about 1000 every second. On windows this uses the PC's
  15  * interrupt timer and so is only accurate to around 20ppm.  On unix it's
  16  * a value that's calculated from the current UTC time and so is in theory
  17  * accurate in the long term but may jitter and jump in the short term.
  18  *
  19  * What PuTTY needs from these timers is simply a way of delaying the
  20  * calling of a function for a little while, if it's occasionally called a
  21  * little early or late that's not a problem. So to protect against clock
  22  * jumps schedule_timer records the time that it was called in the timer
  23  * structure. With this information the run_timers function can see when
  24  * the current GETTICKCOUNT value is after the time the event should be
  25  * fired OR before the time it was set. In the latter case the clock must
  26  * have jumped, the former is (probably) just the normal passage of time.
  27  *
  28  */
  29
  30 #include <assert.h>
  31 #include <stdio.h>
  32
  33 #include "putty.h"
  34 #include "tree234.h"
  35
  36 struct timer {
  37     timer_fn_t fn;
  38     void *ctx;
  39     long now;
  40     long when_set;
  41 };
  42
  43 static tree234 *timers = NULL;
  44 static tree234 *timer_contexts = NULL;
  45 static long now = 0L;
  46
  47 static int compare_timers(void *av, void *bv)
  48 {
  49     struct timer *a = (struct timer *)av;
  50     struct timer *b = (struct timer *)bv;
  51     long at = a->now - now;
  52     long bt = b->now - now;
  53
  54     if (at < bt)
  55         return -1;
  56     else if (at > bt)
  57         return +1;
  58
  59     /*
  60      * Failing that, compare on the other two fields, just so that
  61      * we don't get unwanted equality.
  62      */
  63 #if defined(__LCC__) || defined(__clang__)
  64     /* lcc won't let us compare function pointers. Legal, but annoying. */
  65     return memcmp(&a->fn, &b->fn, sizeof(a->fn));
  66 #else
  67     if (a->fn < b->fn)
  68         return -1;
  69     else if (a->fn > b->fn)
  70         return +1;
  71 #endif
  72
  73     if (a->ctx < b->ctx)
  74         return -1;
  75     else if (a->ctx > b->ctx)
  76         return +1;
  77
  78     /*
  79      * Failing _that_, the two entries genuinely are equal, and we
  80      * never have a need to store them separately in the tree.
  81      */
  82     return 0;
  83 }
  84
  85 static int compare_timer_contexts(void *av, void *bv)
  86 {
  87     char *a = (char *)av;
  88     char *b = (char *)bv;
  89     if (a < b)
  90         return -1;
  91     else if (a > b)
  92         return +1;
  93     return 0;
  94 }
  95
  96 static void init_timers(void)
  97 {
  98     if (!timers) {
  99         timers = newtree234(compare_timers);
 100         timer_contexts = newtree234(compare_timer_contexts);
 101         now = GETTICKCOUNT();
 102     }
 103 }
 104
 105 long schedule_timer(int ticks, timer_fn_t fn, void *ctx)
 106 {
 107     long when;
 108     struct timer *t, *first;
 109
 110     init_timers();
 111
 112     now = GETTICKCOUNT();
 113     when = ticks + now;
 114
 115     /*
 116      * Just in case our various defences against timing skew fail
 117      * us: if we try to schedule a timer that's already in the
 118      * past, we instead schedule it for the immediate future.
 119      */
 120     if (when - now <= 0)
 121         when = now + 1;
 122
 123     t = snew(struct timer);
 124     t->fn = fn;
 125     t->ctx = ctx;
 126     t->now = when;
 127     t->when_set = now;
 128
 129     if (t != add234(timers, t)) {
 130         sfree(t);                      /* identical timer already exists */
 131     } else {
 132         add234(timer_contexts, t->ctx);/* don't care if this fails */
 133     }
 134
 135     first = (struct timer *)index234(timers, 0);
 136     if (first == t) {
 137         /*
 138          * This timer is the very first on the list, so we must
 139          * notify the front end.
 140          */
 141         timer_change_notify(first->now);
 142     }
 143
 144     return when;
 145 }
 146
 147 /*
 148  * Call to run any timers whose time has reached the present.
 149  * Returns the time (in ticks) expected until the next timer after
 150  * that triggers.
 151  */
 152 int run_timers(long anow, long *next)
 153 {
 154     struct timer *first;
 155
 156     init_timers();
 157
 158     now = GETTICKCOUNT();
 159
 160     while (1) {
 161         first = (struct timer *)index234(timers, 0);
 162
 163         if (!first)
 164             return FALSE;              /* no timers remaining */
 165
 166         if (find234(timer_contexts, first->ctx, NULL) == NULL) {
 167             /*
 168              * This timer belongs to a context that has been
 169              * expired. Delete it without running.
 170              */
 171             delpos234(timers, 0);
 172             sfree(first);
 173         } else if (first->now - now <= 0 ||
 174                    now - (first->when_set - 10) < 0) {
 175             /*
 176              * This timer is active and has reached its running
 177              * time. Run it.
 178              */
 179             delpos234(timers, 0);
 180             first->fn(first->ctx, first->now);
 181             sfree(first);
 182         } else {
 183             /*
 184              * This is the first still-active timer that is in the
 185              * future. Return how long it has yet to go.
 186              */
 187             *next = first->now;
 188             return TRUE;
 189         }
 190     }
 191 }
 192
 193 /*
 194  * Call to expire all timers associated with a given context.
 195  */
 196 void expire_timer_context(void *ctx)
 197 {
 198     init_timers();
 199
 200     /*
 201      * We don't bother to check the return value; if the context
 202      * already wasn't in the tree (presumably because no timers
 203      * ever actually got scheduled for it) then that's fine and we
 204      * simply don't need to do anything.
 205      */
 206     del234(timer_contexts, ctx);
 207 }