Wait for timers on the monotonic clock; events still see the wall clock

The terminal, the pixel surfaces and the web page reckoned when a timer
was due on the wall clock: set back, a timer waited until the clock came
again to the tick it had waited for (Pure's review 004). vega_clock is
the monotonic clock; timers are due on it, and a tick sets the time its
event sees from the wall clock (vega_now), as before.

check.py's clock_set_back runs a timer of 20 ms in the terminal with the
wall clock set back an hour 0.2 s in (Pure's build/clock-back.so, an
LD_PRELOAD of clock_gettime): 40 ticks in 0.8 s, where Vega before this
stops at 9. make check passes it --clock-back.

4 files changed, 30 insertions, 20 deletions

diff --git a/surfaces/main.c b/surfaces/main.c
index 6c7e368..70958e1 100644
--- a/surfaces/main.c
+++ b/surfaces/main.c
@@ -483,7 +483,7 @@ static int interactive(void) {
     VegaGrid grid = {0};
     drawn = &grid;
     VegaTimers timers;
-    vega_timers_start(&timers, app.module, vega_now());
+    vega_timers_start(&timers, app.module, vega_clock());
     for (;;) {
         if (resized) {
             struct winsize size;
@@ -494,7 +494,7 @@ static int interactive(void) {
         }
         /* Tick what is due, draw, then wait for input, a frame or the next tick. */
         bool ticked = false;
-        long due = vega_timers_run(&timers, &app, vega_now(), &ticked);
+        long due = vega_timers_run(&timers, &app, vega_clock(), &ticked);
         char line[VEGA_TEXT + 4];
         vega_draw(&grid, &app, &screen, line, shown_entry(line), typing);
         paint_terminal(&grid);
@@ -505,7 +505,7 @@ static int interactive(void) {
         app.now = vega_now();
         if (ready == 0) {
             if (screen.ticks) pure_app_frame(&app);
-            if (module_changed()) vega_timers_start(&timers, app.module, app.now);
+            if (module_changed()) vega_timers_start(&timers, app.module, vega_clock());
             continue;
         }
         if (ready < 0) continue; /* a signal, such as a resize */
@@ -647,7 +647,7 @@ static int live(VegaInput *input, unsigned columns, unsigned rows, bool recorded
         signal(SIGTERM, on_stop);
     }
     VegaTimers timers;
-    vega_timers_start(&timers, app.module, vega_now());
+    vega_timers_start(&timers, app.module, vega_clock());
     char path[512] = "";
     unsigned long differing = 0;
     bool open_input = true, printed = false;
@@ -664,7 +664,7 @@ static int live(VegaInput *input, unsigned columns, unsigned rows, bool recorded
     bool dirty = true;
     while (!stopping && !(node && node->stopping(node->host))) {
         bool ticked = false;
-        long due = node ? -1 : vega_timers_run(&timers, &app, vega_now(), &ticked);
+        long due = node ? -1 : vega_timers_run(&timers, &app, vega_clock(), &ticked);
         PureSnapshot *snapshot = node ? node->read(node->host, &app) : NULL;
         dirty = dirty || ticked || snapshot || access_acted;
         access_acted = false;
diff --git a/surfaces/screen.c b/surfaces/screen.c
index fdecaf4..f079bd5 100644
--- a/surfaces/screen.c
+++ b/surfaces/screen.c
@@ -11,23 +11,29 @@ double vega_now(void) {
     return floor((double)t.tv_sec * 1000 + (double)t.tv_nsec / 1e6);
 }
 
-void vega_timers_start(VegaTimers *timers, const PureModule *m, double now) {
+double vega_clock(void) {
+    struct timespec t;
+    clock_gettime(CLOCK_MONOTONIC, &t);
+    return floor((double)t.tv_sec * 1000 + (double)t.tv_nsec / 1e6);
+}
+
+void vega_timers_start(VegaTimers *timers, const PureModule *m, double clock) {
     for (uint32_t i = 0; i < m->inputs; ++i)
-        timers->next[i] = m->input[i].kind == PURE_KIND_TIMER ? now + m->input[i].period : 0;
+        timers->next[i] = m->input[i].kind == PURE_KIND_TIMER ? clock + m->input[i].period : 0;
 }
 
-long vega_timers_run(VegaTimers *timers, PureApp *app, double now, bool *ticked) {
+long vega_timers_run(VegaTimers *timers, PureApp *app, double clock, bool *ticked) {
     const PureModule *m = app->module;
     double soonest = -1;
     for (uint32_t i = 0; i < m->inputs; ++i) {
         if (m->input[i].kind != PURE_KIND_TIMER) continue;
-        if (timers->next[i] <= now) {
-            app->now = now;
+        if (timers->next[i] <= clock) {
+            app->now = vega_now();
             pure_app_tick(app, i); /* a failed tick changes nothing, as any failed event */
             *ticked = true;
-            timers->next[i] = now + m->input[i].period;
+            timers->next[i] = clock + m->input[i].period;
         }
-        if (soonest < 0 || timers->next[i] - now < soonest) soonest = timers->next[i] - now;
+        if (soonest < 0 || timers->next[i] - clock < soonest) soonest = timers->next[i] - clock;
     }
     return soonest < 0 ? -1 : (long)ceil(soonest);
 }
diff --git a/surfaces/vega.h b/surfaces/vega.h
index ff880c5..0ab87dd 100644
--- a/surfaces/vega.h
+++ b/surfaces/vega.h
@@ -43,12 +43,16 @@ void vega_screen(VegaScreen *, const PureModule *);
 /* The wall-clock time events see on a live surface: milliseconds since 1970
  * (UTC). Scripts set their own (time MS). */
 double vega_now(void);
-/* When each timer input ticks next. run ticks every one due at now (a tick
- * sees now) and gives the milliseconds until the next, or -1 without timers;
- * ticked says whether any did. Missed ticks are not made up. */
+/* A clock that only goes on (milliseconds, from boot): when timers are due,
+ * which setting the wall clock back must not hold up. */
+double vega_clock(void);
+/* When each timer input ticks next, on vega_clock. run ticks every one due
+ * at clock (a tick sees the wall clock, vega_now) and gives the
+ * milliseconds until the next, or -1 without timers; ticked says whether
+ * any did. Missed ticks are not made up. */
 typedef struct { double next[PURE_MODULE_INPUTS]; } VegaTimers;
-void vega_timers_start(VegaTimers *, const PureModule *, double now);
-long vega_timers_run(VegaTimers *, PureApp *, double now, bool *ticked);
+void vega_timers_start(VegaTimers *, const PureModule *, double clock);
+long vega_timers_run(VegaTimers *, PureApp *, double clock, bool *ticked);
 /* What a module seems to mean but does not meet: a list label that is not
  * text, a _focus view without its table, a column a list never shows, a
  * second text input. Each finding names the view or input it is about;
diff --git a/surfaces/web.c b/surfaces/web.c
index 7103c8f..a4759c4 100644
--- a/surfaces/web.c
+++ b/surfaces/web.c
@@ -418,11 +418,11 @@ int vega_web(PureApp *app, const VegaScreen *screen, const char *listen_on, unsi
     static Client clients[CLIENTS];
     for (unsigned i = 0; i < CLIENTS; ++i) clients[i].fd = -1;
     VegaTimers timers;
-    vega_timers_start(&timers, app->module, vega_now());
+    vega_timers_start(&timers, app->module, vega_clock());
     while (!stopping) {
         /* Tick what is due; every page hears of it this round, without waiting. */
         bool ticked = false;
-        long due = vega_timers_run(&timers, app, vega_now(), &ticked);
+        long due = vega_timers_run(&timers, app, vega_clock(), &ticked);
         struct pollfd fds[CLIENTS + 1];
         unsigned slot[CLIENTS + 1], count = 1;
         fds[0] = (struct pollfd){listener, POLLIN, 0};
@@ -441,7 +441,7 @@ int vega_web(PureApp *app, const VegaScreen *screen, const char *listen_on, unsi
         if (ready == 0 && screen->ticks) changed = pure_app_frame(app) == PURE_OK;
         if (reload && reload()) { /* a new module: every page draws it */
             changed = true;
-            vega_timers_start(&timers, app->module, app->now);
+            vega_timers_start(&timers, app->module, vega_clock());
         }
         if (fds[0].revents & POLLIN)
             for (int fd; (fd = accept4(listener, NULL, NULL, SOCK_NONBLOCK | SOCK_CLOEXEC)) >= 0;) {