Do not advance decay epoch when time goes backwards.

Instead, move the epoch backward in time.  Additionally, add
nstime_monotonic() and use it in debug builds to assert that time only
goes backward if nstime_update() is using a non-monotonic time source.

test/unit/decay.c

@@ -2,12 +2,20 @@
 
 const char *malloc_conf = "purge:decay,decay_time:1,lg_tcache_max:0";
 
+static nstime_monotonic_t *nstime_monotonic_orig;
 static nstime_update_t *nstime_update_orig;
 
 static unsigned nupdates_mock;
 static nstime_t time_mock;
 static bool nonmonotonic_mock;
 
+static bool
+nstime_monotonic_mock(void)
+{
+
+	return (false);
+}
+
 static bool
 nstime_update_mock(nstime_t *time)
 {
@@ -315,7 +323,9 @@ TEST_BEGIN(test_decay_nonmonotonic)
 	nstime_update(&time_mock);
 	nonmonotonic_mock = true;
 
+	nstime_monotonic_orig = nstime_monotonic;
 	nstime_update_orig = nstime_update;
+	nstime_monotonic = nstime_monotonic_mock;
 	nstime_update = nstime_update_mock;
 
 	for (i = 0; i < NPS; i++) {
@@ -339,8 +349,9 @@ TEST_BEGIN(test_decay_nonmonotonic)
 	    config_stats ? 0 : ENOENT, "Unexpected mallctl result");
 
 	if (config_stats)
-		assert_u64_gt(npurge1, npurge0, "Expected purging to occur");
+		assert_u64_eq(npurge0, npurge1, "Unexpected purging occurred");
 
+	nstime_monotonic = nstime_monotonic_orig;
 	nstime_update = nstime_update_orig;
 #undef NPS
 }

test/unit/nstime.c

@@ -176,6 +176,13 @@ TEST_BEGIN(test_nstime_divide)
 }
 TEST_END
 
+TEST_BEGIN(test_nstime_monotonic)
+{
+
+	nstime_monotonic();
+}
+TEST_END
+
 TEST_BEGIN(test_nstime_update)
 {
 	nstime_t nst;
@@ -198,7 +205,6 @@ TEST_BEGIN(test_nstime_update)
 		assert_d_eq(nstime_compare(&nst, &nst0), 0,
 		    "Time should not have been modified");
 	}
-
 }
 TEST_END
 
@@ -216,5 +222,6 @@ main(void)
 	    test_nstime_imultiply,
 	    test_nstime_idivide,
 	    test_nstime_divide,
+	    test_nstime_monotonic,
 	    test_nstime_update));
 }