poll-loop: Track the poll timeout as an absolute, not a relative time.

This is a necessary prerequisite for allowing time to be "fast forwarded"
in unit tests, to keep tests that depend on the passage of time from
running in real time.  Without this change, a code sequence like this:

	poll_timer_wait(1000);
	...fast forward time 5 seconds...
	poll_block();

would still sleep for a second, because the poll_loop module would still
have a relative timeout of 1000 ms.

Signed-off-by: Ben Pfaff <blp@nicira.com>

lib/poll-loop.c

@@ -55,9 +55,10 @@ struct poll_waiter {
 /* All active poll waiters. */
 static struct list waiters = LIST_INITIALIZER(&waiters);
 
-/* Max time to wait in next call to poll_block(), in milliseconds, or -1 to
+/* Time at which to wake up the next call to poll_block(), in milliseconds as
+ * returned by time_msec(), LLONG_MIN to wake up immediately, or LLONG_MAX to
  * wait forever. */
-static int timeout = -1;
+static long long int timeout_when = LLONG_MAX;
 
 /* Location where waiter created. */
 static const char *timeout_where;
@@ -82,16 +83,6 @@ poll_fd_wait(int fd, short int events, const char *where)
     return new_waiter(fd, events, where);
 }
 
-/* The caller must ensure that 'msec' is not negative. */
-static void
-poll_timer_wait__(int msec, const char *where)
-{
-    if (timeout < 0 || msec < timeout) {
-        timeout = msec;
-        timeout_where = where;
-    }
-}
-
 /* Causes the following call to poll_block() to block for no more than 'msec'
  * milliseconds.  If 'msec' is nonpositive, the following call to poll_block()
  * will not block at all.
@@ -105,14 +96,25 @@ poll_timer_wait__(int msec, const char *where)
 void
 poll_timer_wait(long long int msec, const char *where)
 {
-    poll_timer_wait__((msec < 0 ? 0
-                       : msec > INT_MAX ? INT_MAX
-                       : msec),
-                      where);
+    long long int now = time_msec();
+    long long int when;
+
+    if (msec <= 0) {
+        /* Wake up immediately. */
+        when = LLONG_MIN;
+    } else if ((unsigned long long int) now + msec <= LLONG_MAX) {
+        /* Normal case. */
+        when = now + msec;
+    } else {
+        /* now + msec would overflow. */
+        when = LLONG_MAX;
+    }
+
+    poll_timer_wait_until(when, where);
 }
 
 /* Causes the following call to poll_block() to wake up when the current time,
- * as returned by time_msec(), reaches 'msec' or later.  If 'msec' is earlier
+ * as returned by time_msec(), reaches 'when' or later.  If 'when' is earlier
  * than the current time, the following call to poll_block() will not block at
  * all.
  *
@@ -123,13 +125,12 @@ poll_timer_wait(long long int msec, const char *where)
  * Ordinarily the 'where' argument is supplied automatically; see poll-loop.h
  * for more information. */
 void
-poll_timer_wait_until(long long int msec, const char *where)
+poll_timer_wait_until(long long int when, const char *where)
 {
-    long long int now = time_msec();
-    poll_timer_wait__((msec <= now ? 0
-                       : msec < now + INT_MAX ? msec - now
-                       : INT_MAX),
-                      where);
+    if (when < timeout_when) {
+        timeout_when = when;
+        timeout_where = where;
+    }
 }
 
 /* Causes the following call to poll_block() to wake up immediately, without
@@ -215,6 +216,7 @@ poll_block(void)
 
     struct poll_waiter *pw, *next;
     int n_waiters, n_pollfds;
+    int elapsed;
     int retval;
 
     /* Register fatal signal events before actually doing any real work for
@@ -236,15 +238,15 @@ poll_block(void)
         n_pollfds++;
     }
 
-    if (!timeout) {
+    if (timeout_when == LLONG_MIN) {
         COVERAGE_INC(poll_zero_timeout);
     }
-    retval = time_poll(pollfds, n_pollfds, timeout);
+    retval = time_poll(pollfds, n_pollfds, timeout_when, &elapsed);
     if (retval < 0) {
         static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 5);
         VLOG_ERR_RL(&rl, "poll: %s", strerror(-retval));
     } else if (!retval) {
-        log_wakeup(timeout_where, NULL, timeout);
+        log_wakeup(timeout_where, NULL, elapsed);
     }
 
     LIST_FOR_EACH_SAFE (pw, next, node, &waiters) {
@@ -254,7 +256,7 @@ poll_block(void)
         poll_cancel(pw);
     }
 
-    timeout = -1;
+    timeout_when = LLONG_MAX;
     timeout_where = NULL;
 
     /* Handle any pending signals before doing anything else. */

lib/timeval.c

@@ -268,17 +268,22 @@ time_alarm(unsigned int secs)
 }
 
 /* Like poll(), except:
+ *
+ *      - The timeout is specified as an absolute time, as defined by
+ *        time_msec(), instead of a duration.
  *
  *      - On error, returns a negative error code (instead of setting errno).
  *
  *      - If interrupted by a signal, retries automatically until the original
- *        'timeout' expires.  (Because of this property, this function will
+ *        timeout is reached.  (Because of this property, this function will
  *        never return -EINTR.)
  *
  *      - As a side effect, refreshes the current time (like time_refresh()).
- */
+ *
+ * Stores the number of milliseconds elapsed during poll in '*elapsed'. */
 int
-time_poll(struct pollfd *pollfds, int n_pollfds, int timeout)
+time_poll(struct pollfd *pollfds, int n_pollfds, long long int timeout_when,
+          int *elapsed)
 {
     static long long int last_wakeup;
     long long int start;
@@ -292,12 +297,15 @@ time_poll(struct pollfd *pollfds, int n_pollfds, int timeout)
     start = time_msec();
     blocked = false;
     for (;;) {
+        long long int now = time_msec();
         int time_left;
-        if (timeout > 0) {
-            long long int elapsed = time_msec() - start;
-            time_left = timeout >= elapsed ? timeout - elapsed : 0;
+
+        if (now >= timeout_when) {
+            time_left = 0;
+        } else if ((unsigned long long int) timeout_when - now > INT_MAX) {
+            time_left = INT_MAX;
         } else {
-            time_left = timeout;
+            time_left = timeout_when - now;
         }
 
         retval = poll(pollfds, n_pollfds, time_left);
@@ -319,6 +327,7 @@ time_poll(struct pollfd *pollfds, int n_pollfds, int timeout)
     }
     last_wakeup = time_msec();
     refresh_rusage();
+    *elapsed = last_wakeup - start;
     return retval;
 }
 

lib/timeval.h

@@ -55,7 +55,8 @@ long long int time_wall_msec(void);
 void time_timespec(struct timespec *);
 void time_wall_timespec(struct timespec *);
 void time_alarm(unsigned int secs);
-int time_poll(struct pollfd *, int n_pollfds, int timeout);
+int time_poll(struct pollfd *, int n_pollfds, long long int timeout_when,
+              int *elapsed);
 
 long long int timespec_to_msec(const struct timespec *);
 long long int timeval_to_msec(const struct timeval *);