journal: make skipping of exhausted journal files effective again

Commit 668c965af "journal: skipping of exhausted journal files is bad if
direction changed" fixed a correctness issue, but it also significantly
limited the cases where the optimization that skips exhausted journal
files could apply.
As a result, some journalctl queries are much slower in v219 than in v218.
(e.g. queries where a "--since" cutoff should have quickly eliminated
older journal files from consideration, but didn't.)

If already in the initial iteration find_location_with_matches() finds
no entry, the journal file's location is not updated. This is fine,
except that:
 - We must update at least f->last_direction. The optimization relies on
   it. Let's separate that from journal_file_save_location() and update
   it immediately after the direction checks.
 - The optimization was conditional on "f->current_offset > 0", but it
   would always be 0 in this scenario. This check is unnecessary for the
   optimization.

1 files changed, 15 insertions, 9 deletions

diff --git a/src/journal/sd-journal.c b/src/journal/sd-journal.c
index 94891cdf35..9b57e5945d 100644
--- a/src/journal/sd-journal.c
+++ b/src/journal/sd-journal.c
@@ -723,13 +723,17 @@ static int next_beyond_location(sd_journal *j, JournalFile *f, direction_t direc
         assert(j);
         assert(f);
 
-        if (f->last_direction == direction && f->current_offset > 0) {
-                /* If we hit EOF before, recheck if any new entries arrived. */
-                n_entries = le64toh(f->header->n_entries);
-                if (f->location_type == LOCATION_TAIL && n_entries == f->last_n_entries)
-                        return 0;
-                f->last_n_entries = n_entries;
+        n_entries = le64toh(f->header->n_entries);
+
+        /* If we hit EOF before, we don't need to look into this file again
+         * unless direction changed or new entries appeared. */
+        if (f->last_direction == direction && f->location_type == LOCATION_TAIL &&
+            n_entries == f->last_n_entries)
+                return 0;
 
+        f->last_n_entries = n_entries;
+
+        if (f->last_direction == direction && f->current_offset > 0) {
                 /* LOCATION_SEEK here means we did the work in a previous
                  * iteration and the current location already points to a
                  * candidate entry. */
@@ -738,14 +742,16 @@ static int next_beyond_location(sd_journal *j, JournalFile *f, direction_t direc
                         if (r <= 0)
                                 return r;
 
-                        journal_file_save_location(f, direction, c, cp);
+                        journal_file_save_location(f, c, cp);
                 }
         } else {
+                f->last_direction = direction;
+
                 r = find_location_with_matches(j, f, direction, &c, &cp);
                 if (r <= 0)
                         return r;
 
-                journal_file_save_location(f, direction, c, cp);
+                journal_file_save_location(f, c, cp);
         }
 
         /* OK, we found the spot, now let's advance until an entry
@@ -773,7 +779,7 @@ static int next_beyond_location(sd_journal *j, JournalFile *f, direction_t direc
                 if (r <= 0)
                         return r;
 
-                journal_file_save_location(f, direction, c, cp);
+                journal_file_save_location(f, c, cp);
         }
 }