Correct the rounding behavior on tv_mdiff.

Fix for bug 19428.

changes/bug19428

@@ -0,0 +1,5 @@
+  - Minor bugfixes (timing):
+    o When computing the difference between two times in milliseconds,
+      we now round to the nearest millisecond correctly. Previously,
+      we could sometimes round in the wrong direction. Fixes bug 19428;
+      bugfix on 0.2.2.2-alpha.

src/common/util.c

@@ -1386,7 +1386,7 @@ tv_udiff(const struct timeval *start, const struct timeval *end)
   long udiff;
   long secdiff = end->tv_sec - start->tv_sec;
 
-  if (labs(secdiff+1) > LONG_MAX/1000000) {
+  if (labs(secdiff)+1 > LONG_MAX/1000000) {
     log_warn(LD_GENERAL, "comparing times on microsecond detail too far "
              "apart: %ld seconds", secdiff);
     return LONG_MAX;
@@ -1404,7 +1404,7 @@ tv_mdiff(const struct timeval *start, const struct timeval *end)
   long mdiff;
   long secdiff = end->tv_sec - start->tv_sec;
 
-  if (labs(secdiff+1) > LONG_MAX/1000) {
+  if (labs(secdiff)+1 > LONG_MAX/1000) {
     log_warn(LD_GENERAL, "comparing times on millisecond detail too far "
              "apart: %ld seconds", secdiff);
     return LONG_MAX;
@@ -1412,7 +1412,13 @@ tv_mdiff(const struct timeval *start, const struct timeval *end)
 
   /* Subtract and round */
   mdiff = secdiff*1000L +
-      ((long)end->tv_usec - (long)start->tv_usec + 500L) / 1000L;
+      /* We add a million usec here to ensure that the result is positive,
+       * so that the round-towards-zero behavior of the division will give
+       * the right result for rounding to the nearest msec. Later we subtract
+       * 1000 in order to get the correct result.
+       */
+      ((long)end->tv_usec - (long)start->tv_usec + 500L + 1000000L) / 1000L
+      - 1000;
   return mdiff;
 }
 

src/test/test_util.c

@@ -286,12 +286,12 @@ test_util_time(void *arg)
   end.tv_usec = 0;
 
   tt_int_op(995000L,OP_EQ, tv_udiff(&start, &end));
-  // tt_int_op(996L,OP_EQ, tv_mdiff(&start, &end)); // XXXX fails
+  tt_int_op(995L,OP_EQ, tv_mdiff(&start, &end));
 
   end.tv_sec = 4;
 
   tt_int_op(-1005000L,OP_EQ, tv_udiff(&start, &end));
-  // tt_int_op(-1005L,OP_EQ, tv_udiff(&start, &end)); // XXXX Fails
+  tt_int_op(-1005L,OP_EQ, tv_mdiff(&start, &end));
 
   end.tv_sec = TIME_MAX;
   tt_int_op(LONG_MAX, OP_EQ, tv_udiff(&start, &end));