basic: implement the IteratedCache

Adds the basics of the IteratedCache and constructor support for the
Hashmap and OrderedHashmap types.

iterated_cache_get() is responsible for synchronizing the cache with
the associated Hashmap and making it available to the caller at the
supplied result pointers.  Since iterated_cache_get() may need to
allocate memory, it may fail, so callers must check the return value.

On success, pointer arrays containing pointers to the associated
Hashmap's keys and values, in as-iterated order, are returned in
res_keys and res_values, respectively.  Either may be supplied as NULL
to inhibit caching of the keys or values, respectively.

Note that if the cached Hashmap hasn't changed since the previous call
to iterated_cache_get(), and it's not a call activating caching of the
values or keys, the cost is effectively zero as the resulting pointers
will simply refer to the previously returned arrays as-is.

A cleanup function has also been added, iterated_cache_free().

This only frees the IteratedCache container and related arrays.  The
associated Hashmap, its keys, and values are not affected.  Also note
that the associated Hashmap does not automatically free its associated
IteratedCache when freed.

One could, in theory, safely access the arrays returned by a
successful iterated_cache_get() call after its associated Hashmap has
been freed, including the referenced values and keys.  Provided the
iterated_cache_get() was performed prior to the hashmap free, and that
the type of hashmap free performed didn't free keys and/or values as
well.

src/basic/hashmap.c

@@ -229,7 +229,8 @@ struct HashmapBase {
         unsigned n_direct_entries:3; /* Number of entries in direct storage.
                                       * Only valid if !has_indirect. */
         bool from_pool:1;            /* whether was allocated from mempool */
-        bool dirty:1;                /* whether dirtied since cache sync */
+        bool dirty:1;                /* whether dirtied since last iterated_cache_get() */
+        bool cached:1;               /* whether this hashmap is being cached */
         HASHMAP_DEBUG_FIELDS         /* optional hashmap_debug_info */
 };
 
@@ -249,6 +250,17 @@ struct Set {
         struct HashmapBase b;
 };
 
+typedef struct CacheMem {
+        const void **ptr;
+        size_t n_populated, n_allocated;
+        bool active:1;
+} CacheMem;
+
+struct IteratedCache {
+        HashmapBase *hashmap;
+        CacheMem keys, values;
+};
+
 DEFINE_MEMPOOL(hashmap_pool,         Hashmap,        8);
 DEFINE_MEMPOOL(ordered_hashmap_pool, OrderedHashmap, 8);
 /* No need for a separate Set pool */
@@ -744,6 +756,25 @@ bool set_iterate(Set *s, Iterator *i, void **value) {
              (idx != IDX_NIL); \
              (idx) = hashmap_iterate_entry((h), &(i)))
 
+IteratedCache *internal_hashmap_iterated_cache_new(HashmapBase *h) {
+        IteratedCache *cache;
+
+        assert(h);
+        assert(!h->cached);
+
+        if (h->cached)
+                return NULL;
+
+        cache = new0(IteratedCache, 1);
+        if (!cache)
+                return NULL;
+
+        cache->hashmap = h;
+        h->cached = true;
+
+        return cache;
+}
+
 static void reset_direct_storage(HashmapBase *h) {
         const struct hashmap_type_info *hi = &hashmap_type_info[h->type];
         void *p;
@@ -1866,3 +1897,95 @@ int set_put_strsplit(Set *s, const char *v, const char *separators, ExtractFlags
                         return r;
         }
 }
+
+/* expand the cachemem if needed, return true if newly (re)activated. */
+static int cachemem_maintain(CacheMem *mem, unsigned size) {
+        int r = false;
+
+        assert(mem);
+
+        if (!GREEDY_REALLOC(mem->ptr, mem->n_allocated, size)) {
+                if (size > 0)
+                        return -ENOMEM;
+        }
+
+        if (!mem->active)
+                mem->active = r = true;
+
+        return r;
+}
+
+int iterated_cache_get(IteratedCache *cache, const void ***res_keys, const void ***res_values, unsigned *res_n_entries) {
+        bool sync_keys = false, sync_values = false;
+        unsigned size;
+        int r;
+
+        assert(cache);
+        assert(cache->hashmap);
+
+        size = n_entries(cache->hashmap);
+
+        if (res_keys) {
+                r = cachemem_maintain(&cache->keys, size);
+                if (r < 0)
+                        return r;
+
+                sync_keys = r;
+        } else
+                cache->keys.active = false;
+
+        if (res_values) {
+                r = cachemem_maintain(&cache->values, size);
+                if (r < 0)
+                        return r;
+
+                sync_values = r;
+        } else
+                cache->values.active = false;
+
+        if (cache->hashmap->dirty) {
+                if (cache->keys.active)
+                        sync_keys = true;
+                if (cache->values.active)
+                        sync_values = true;
+
+                cache->hashmap->dirty = false;
+        }
+
+        if (sync_keys || sync_values) {
+                unsigned i, idx;
+                Iterator iter;
+
+                i = 0;
+                HASHMAP_FOREACH_IDX(idx, cache->hashmap, iter) {
+                        struct hashmap_base_entry *e;
+
+                        e = bucket_at(cache->hashmap, idx);
+
+                        if (sync_keys)
+                                cache->keys.ptr[i] = e->key;
+                        if (sync_values)
+                                cache->values.ptr[i] = entry_value(cache->hashmap, e);
+                        i++;
+                }
+        }
+
+        if (res_keys)
+                *res_keys = cache->keys.ptr;
+        if (res_values)
+                *res_values = cache->values.ptr;
+        if (res_n_entries)
+                *res_n_entries = size;
+
+        return 0;
+}
+
+IteratedCache *iterated_cache_free(IteratedCache *cache) {
+        if (cache) {
+                free(cache->keys.ptr);
+                free(cache->values.ptr);
+                free(cache);
+        }
+
+        return NULL;
+}

src/basic/hashmap.h

@@ -53,6 +53,8 @@ typedef struct Hashmap Hashmap;               /* Maps keys to values */
 typedef struct OrderedHashmap OrderedHashmap; /* Like Hashmap, but also remembers entry insertion order */
 typedef struct Set Set;                       /* Stores just keys */
 
+typedef struct IteratedCache IteratedCache;   /* Caches the iterated order of one of the above */
+
 /* Ideally the Iterator would be an opaque struct, but it is instantiated
  * by hashmap users, so the definition has to be here. Do not use its fields
  * directly. */
@@ -126,6 +128,9 @@ static inline OrderedHashmap *ordered_hashmap_free_free_free(OrderedHashmap *h)
         return (void*)hashmap_free_free_free(PLAIN_HASHMAP(h));
 }
 
+IteratedCache *iterated_cache_free(IteratedCache *cache);
+int iterated_cache_get(IteratedCache *cache, const void ***res_keys, const void ***res_values, unsigned *res_n_entries);
+
 HashmapBase *internal_hashmap_copy(HashmapBase *h);
 static inline Hashmap *hashmap_copy(Hashmap *h) {
         return (Hashmap*) internal_hashmap_copy(HASHMAP_BASE(h));
@@ -139,6 +144,14 @@ int internal_ordered_hashmap_ensure_allocated(OrderedHashmap **h, const struct h
 #define hashmap_ensure_allocated(h, ops) internal_hashmap_ensure_allocated(h, ops  HASHMAP_DEBUG_SRC_ARGS)
 #define ordered_hashmap_ensure_allocated(h, ops) internal_ordered_hashmap_ensure_allocated(h, ops  HASHMAP_DEBUG_SRC_ARGS)
 
+IteratedCache *internal_hashmap_iterated_cache_new(HashmapBase *h);
+static inline IteratedCache *hashmap_iterated_cache_new(Hashmap *h) {
+        return (IteratedCache*) internal_hashmap_iterated_cache_new(HASHMAP_BASE(h));
+}
+static inline IteratedCache *ordered_hashmap_iterated_cache_new(OrderedHashmap *h) {
+        return (IteratedCache*) internal_hashmap_iterated_cache_new(HASHMAP_BASE(h));
+}
+
 int hashmap_put(Hashmap *h, const void *key, void *value);
 static inline int ordered_hashmap_put(OrderedHashmap *h, const void *key, void *value) {
         return hashmap_put(PLAIN_HASHMAP(h), key, value);
@@ -394,3 +407,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(OrderedHashmap*, ordered_hashmap_free_free_free);
 #define _cleanup_ordered_hashmap_free_ _cleanup_(ordered_hashmap_freep)
 #define _cleanup_ordered_hashmap_free_free_ _cleanup_(ordered_hashmap_free_freep)
 #define _cleanup_ordered_hashmap_free_free_free_ _cleanup_(ordered_hashmap_free_free_freep)
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(IteratedCache*, iterated_cache_free);
+
+#define _cleanup_iterated_cache_free_ _cleanup_(iterated_cache_freep)