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.

2 files changed, 24 insertions, 0 deletions

diff --git a/src/shared/hashmap.c b/src/shared/hashmap.c
index eb5c549e40..26a4eff07f 100644
--- a/src/shared/hashmap.c
+++ b/src/shared/hashmap.c
@@ -758,3 +758,25 @@ char **hashmap_get_strv(Hashmap *h) {
 
         return sv;
 }
+
+void *hashmap_next(Hashmap *h, const void *key) {
+        unsigned hash;
+        struct hashmap_entry *e;
+
+        assert(h);
+        assert(key);
+
+        if (!h)
+                return NULL;
+
+        hash = h->hash_func(key) % NBUCKETS;
+        e = hash_scan(h, hash, key);
+        if (!e)
+                return NULL;
+
+        e = e->iterate_next;
+        if (!e)
+                return NULL;
+
+        return e->value;
+}
diff --git a/src/shared/hashmap.h b/src/shared/hashmap.h
index 504f0b7637..ed41817dda 100644
--- a/src/shared/hashmap.h
+++ b/src/shared/hashmap.h
@@ -79,6 +79,8 @@ void* hashmap_first(Hashmap *h);
 void* hashmap_first_key(Hashmap *h);
 void* hashmap_last(Hashmap *h);
 
+void *hashmap_next(Hashmap *h, const void *key);
+
 char **hashmap_get_strv(Hashmap *h);
 
 #define HASHMAP_FOREACH(e, h, i) \