Decay: Add a (mostly stub) test case.

Makefile.in

@@ -197,6 +197,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/cache_bin.c \
 	$(srcroot)test/unit/ckh.c \
 	$(srcroot)test/unit/counter.c \
+	$(srcroot)test/unit/decay.c \
 	$(srcroot)test/unit/div.c \
 	$(srcroot)test/unit/edata_cache.c \
 	$(srcroot)test/unit/emitter.c \

test/unit/decay.c

@@ -0,0 +1,45 @@
+#include "test/jemalloc_test.h"
+
+#include "jemalloc/internal/decay.h"
+
+/*
+ * Honestly, this is mostly a stub for now.  Eventually, we should beef up
+ * testing here.
+ */
+
+TEST_BEGIN(test_decay_empty) {
+	/* If we never have any decaying pages, npages_limit should be 0. */
+	decay_t decay;
+	memset(&decay, 0, sizeof(decay));
+
+	nstime_t curtime;
+	nstime_init(&curtime, 0);
+
+	uint64_t decay_ms = 1000;
+	uint64_t decay_ns = decay_ms * 1000 * 1000;
+
+	bool err = decay_init(&decay, &curtime, (ssize_t)decay_ms);
+	assert_false(err, "");
+
+	uint64_t time_between_calls = decay_epoch_duration_ns(&decay) / 5;
+	int nepochs = 0;
+	for (uint64_t i = 0; i < decay_ns / time_between_calls * 10; i++) {
+		size_t dirty_pages = 0;
+		nstime_init(&curtime, i * time_between_calls);
+		bool epoch_advanced = decay_maybe_advance_epoch(&decay,
+		    &curtime, dirty_pages);
+		if (epoch_advanced) {
+			nepochs++;
+			assert_zu_eq(decay_npages_limit_get(&decay), 0,
+			    "Should not increase the limit arbitrarily");
+		}
+	}
+	assert_d_gt(nepochs, 0, "Should have advanced epochs");
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_decay_empty);
+}