journal: add ability to list values a specified field can take in all entries of the journal

The new 'unique' API allows listing all unique field values that a field
specified by a field name can take in all entries of the journal. This
allows answering queries such as "What units logged to the journal?",
"What hosts have logged into the journal?", "Which boot IDs have logged
into the journal?".

Ultimately this allows implementation of tools similar to lastlog based
on journal data.

Note that listing these field values will not work for journal files
created with older journald, as the field values are not indexed in
older files.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/journal/journal-internal.h b/src/journal/journal-internal.h
index edd4f23baf..f68ca996a8 100644
--- a/src/journal/journal-internal.h
+++ b/src/journal/journal-internal.h
@@ -115,6 +115,10 @@ struct sd_journal {
         Match *level0, *level1;
 
         unsigned current_invalidate_counter, last_invalidate_counter;
+
+        char *unique_field;
+        JournalFile *unique_file;
+        uint64_t unique_offset;
 };
 
 char *journal_make_match_string(sd_journal *j);