util-lib: make sure usec_sub() doesn't degrade USEC_INFINITY

As suggested in: https://github.com/systemd/systemd/pull/2542#issuecomment-181877820

2 files changed, 21 insertions, 6 deletions

diff --git a/src/basic/time-util.h b/src/basic/time-util.h
index 87440faeee..080c1110f5 100644
--- a/src/basic/time-util.h
+++ b/src/basic/time-util.h
@@ -143,11 +143,13 @@ static inline usec_t usec_add(usec_t a, usec_t b) {
 
 static inline usec_t usec_sub(usec_t timestamp, int64_t delta) {
         if (delta < 0)
-                timestamp = usec_add(timestamp, (usec_t) (-delta));
-        else if (timestamp > (usec_t) delta)
-                timestamp -= delta;
-        else
-                timestamp = 0;
+                return usec_add(timestamp, (usec_t) (-delta));
 
-        return timestamp;
+        if (timestamp == USEC_INFINITY) /* Make sure infinity doesn't degrade */
+                return USEC_INFINITY;
+
+        if (timestamp < (usec_t) delta)
+                return 0;
+
+        return timestamp - delta;
 }
diff --git a/src/test/test-time.c b/src/test/test-time.c
index 254a8d0e52..fdbf9fa881 100644
--- a/src/test/test-time.c
+++ b/src/test/test-time.c
@@ -191,6 +191,18 @@ static void test_usec_add(void) {
         assert_se(usec_add(USEC_INFINITY, 2) == USEC_INFINITY);
 }
 
+static void test_usec_sub(void) {
+        assert_se(usec_sub(0, 0) == 0);
+        assert_se(usec_sub(4, 1) == 3);
+        assert_se(usec_sub(4, 4) == 0);
+        assert_se(usec_sub(4, 5) == 0);
+        assert_se(usec_sub(USEC_INFINITY-3, -3) == USEC_INFINITY);
+        assert_se(usec_sub(USEC_INFINITY-3, -3) == USEC_INFINITY);
+        assert_se(usec_sub(USEC_INFINITY-3, -4) == USEC_INFINITY);
+        assert_se(usec_sub(USEC_INFINITY-3, -5) == USEC_INFINITY);
+        assert_se(usec_sub(USEC_INFINITY, 5) == USEC_INFINITY);
+}
+
 int main(int argc, char *argv[]) {
         uintmax_t x;
 
@@ -203,6 +215,7 @@ int main(int argc, char *argv[]) {
         test_timezone_is_valid();
         test_get_timezones();
         test_usec_add();
+        test_usec_sub();
 
         /* Ensure time_t is signed */
         assert_cc((time_t) -1 < (time_t) 1);