--- a/src/lib/test-guid.c	Mon Jun 05 12:55:56 2017 +0300
+++ b/src/lib/test-guid.c	Mon Jun 05 14:21:23 2017 +0300
@@ -2,6 +2,170 @@
 
 #include "test-lib.h"
 #include "guid.h"
+#include "ioloop.h"
+
+/*
+ * We want earlier timestamps to compare as < with later timestamps, but
+ * guid_128_cmp() doesn't do that because the timestamps in the guid are
+ * stored in little-endian byte order.
+ */
+static int reverse_guid_128_cmp(const guid_128_t a, const guid_128_t b)
+{
+	int i;
+
+	for (i = GUID_128_SIZE - 1; i >= 0; i--)
+		if (a[i] != b[i])
+			return (int)a[i] - (int)b[i];
+
+	return 0;
+}
+
+static bool guid_128_has_sane_nsecs(const guid_128_t g)
+{
+	unsigned long nsecs;
+
+	nsecs = (g[3] << 24) | (g[2] << 16) | (g[1] << 8) | g[0];
+
+	return nsecs < 1000000000UL;
+}
+
+static inline void set_fake_time(time_t sec, long usec)
+{
+	ioloop_timeval.tv_sec = sec;
+	ioloop_timeval.tv_usec = usec;
+}
+
+/*
+ * We muck with the ioloop_timeval in various ways and make sure that the
+ * guids that get generated make sense.  To make sure that the guid
+ * generation code takes up our faked timestamp, we use a far-away time (Jan
+ * 1 2038) as the base time.  We don't want to go beyond 32-bit signed
+ * time_t for the base time to avoid issues on systems with 32-bit signed
+ * time_t.
+ *
+ * While guids really only need to be unique, here we actually enforce that
+ * they are increasing (as defined by reverse_guid_128_cmp()).  If guids are
+ * always increasing, they will always be unique.
+ */
+static void test_ioloop_guid_128_generate(void)
+{
+	const time_t basetime = 2145909600; /* Jan 1 2038 */
+	struct timeval saved_ioloop_timeval;
+	guid_128_t guids[2];
+	int i;
+
+	/* save the ioloop_timeval before we start messing with it */
+	saved_ioloop_timeval = ioloop_timeval;
+
+	/*
+	 * Generating multiple guids within a microsecond should keep
+	 * incrementing them.
+	 */
+	test_begin("guid_128_generate() increasing guid within a usec");
+	set_fake_time(basetime, 0);
+	guid_128_generate(guids[1]);
+	for (i = 0; i < 10; i++) {
+		const int this = i % 2;
+		const int prev = 1 - this;
+
+		guid_128_generate(guids[this]);
+
+		test_assert(reverse_guid_128_cmp(guids[prev], guids[this]) < 0);
+		test_assert(guid_128_has_sane_nsecs(guids[this]));
+	}
+	test_end();
+
+	/*
+	 * If the current time changes by +1 usec, so should the guids.
+	 */
+	test_begin("guid_128_generate() increasing guid with usec fast-forward");
+	for (i = 0; i < 10; i++) {
+		const int this = i % 2;
+		const int prev = 1 - this;
+
+		set_fake_time(basetime, 1 + i);
+		guid_128_generate(guids[this]);
+
+		test_assert(reverse_guid_128_cmp(guids[prev], guids[this]) < 0);
+		test_assert(guid_128_has_sane_nsecs(guids[this]));
+	}
+	test_end();
+
+	/*
+	 * If the current time changes by +1 sec, so should the guids.
+	 */
+	test_begin("guid_128_generate() increasing guid with sec fast-forward");
+	for (i = 0; i < 10; i++) {
+		const int this = i % 2;
+		const int prev = 1 - this;
+
+		set_fake_time(basetime + 1 + i, 0);
+		guid_128_generate(guids[this]);
+
+		test_assert(reverse_guid_128_cmp(guids[prev], guids[this]) < 0);
+		test_assert(guid_128_has_sane_nsecs(guids[this]));
+	}
+	test_end();
+
+	/*
+	 * Requesting enough guids should increment the seconds but always
+	 * produce valid nsecs.
+	 *
+	 * (Set a time that leaves us 1000 guids before seconds overflow and
+	 * then ask for 2500 guids.)
+	 */
+	test_begin("guid_128_generate() proper guid nsec overflow");
+	set_fake_time(basetime + 11, 999999L);
+	for (i = 0; i < 2500; i++) {
+		const int this = i % 2;
+		const int prev = 1 - this;
+
+		guid_128_generate(guids[this]);
+		test_assert(reverse_guid_128_cmp(guids[prev], guids[this]) < 0);
+		test_assert(guid_128_has_sane_nsecs(guids[this]));
+	}
+	test_end();
+
+	/*
+	 * When ahead by 1500 guids (see previous test), +1 usec shouldn't
+	 * have any effect.
+	 */
+	test_begin("guid_128_generate() no effect with increasing time when ahead");
+	set_fake_time(basetime + 12, 0);
+	guid_128_generate(guids[0]);
+	test_assert(reverse_guid_128_cmp(guids[1], guids[0]) < 0);
+	test_assert(guid_128_has_sane_nsecs(guids[0]));
+	test_end();
+
+	/* not a test - just set a more convenient time */
+	set_fake_time(basetime + 15, 500);
+	guid_128_generate(guids[1]);
+	test_assert(reverse_guid_128_cmp(guids[0], guids[1]) < 0);
+	test_assert(guid_128_has_sane_nsecs(guids[1]));
+
+	/*
+	 * Time going backwards by 1 usec should have no effect on guids.
+	 */
+	test_begin("guid_128_generate() usec time-travel still increasing");
+	set_fake_time(basetime + 15, 499);
+	guid_128_generate(guids[0]);
+	test_assert(reverse_guid_128_cmp(guids[1], guids[0]) < 0);
+	test_assert(guid_128_has_sane_nsecs(guids[0]));
+	test_end();
+
+	/*
+	 * Time going backwards by 1 sec should have no effect on guids.
+	 */
+	test_begin("guid_128_generate() sec time-travel still increasing");
+	set_fake_time(basetime + 14, 499);
+	guid_128_generate(guids[1]);
+	test_assert(reverse_guid_128_cmp(guids[0], guids[1]) < 0);
+	test_assert(guid_128_has_sane_nsecs(guids[1]));
+	test_end();
+
+	/* restore the previously saved value just in case */
+	ioloop_timeval = saved_ioloop_timeval;
+}
 
 void test_guid(void)
 {
@@ -90,4 +254,6 @@
 	test_assert(guid_128_from_uuid_string("fe-e0ceac-0327-11e7-ad39-52540078f374", guid3) < 0);
 
 	test_end();
+
+	test_ioloop_guid_128_generate();
 }