sd-journal: introduce sd_journal_step_one()

After the commit 7a4ee86161,
sd_journal_next() following sd_journal_seek_tail() takes no-op,
and we need to call sd_journal_previous(). This may be useful in
some cases, e.g. to fix the issue explained in the previous commit.

src/libsystemd/libsystemd.sym

@@ -825,4 +825,5 @@ global:
         sd_event_trim_memory;
         sd_pid_notify_barrier;
         sd_event_source_leave_ratelimit;
+        sd_journal_step_one;
 } LIBSYSTEMD_253;

src/libsystemd/sd-journal/sd-journal.c

@@ -979,6 +979,16 @@ _public_ int sd_journal_previous(sd_journal *j) {
         return real_journal_next(j, DIRECTION_UP);
 }
 
+_public_ int sd_journal_step_one(sd_journal *j, int advanced) {
+        assert_return(j, -EINVAL);
+
+        if (j->current_location.type == LOCATION_HEAD)
+                return sd_journal_next(j);
+        if (j->current_location.type == LOCATION_TAIL)
+                return sd_journal_previous(j);
+        return real_journal_next(j, advanced ? DIRECTION_DOWN : DIRECTION_UP);
+}
+
 static int real_journal_next_skip(sd_journal *j, direction_t direction, uint64_t skip) {
         int c = 0, r;
 

src/systemd/sd-journal.h

@@ -93,6 +93,7 @@ void sd_journal_close(sd_journal *j);
 
 int sd_journal_previous(sd_journal *j);
 int sd_journal_next(sd_journal *j);
+int sd_journal_step_one(sd_journal *j, int advanced);
 
 int sd_journal_previous_skip(sd_journal *j, uint64_t skip);
 int sd_journal_next_skip(sd_journal *j, uint64_t skip);