shared/json: optimize appending objects to arrays

When repeatedly appending an object to a growing array, we would create a new
array larger by one slot, insert all the old entries and the new element with
ref count bumps into the new array, and then unref the old array.

This would cause problems when building an array with more than a few thousand
elements. If userdbctl is modified to construct an array,
'userdbctl --json=pretty group >/dev/null' with 31k groups:
0.74s   (existing code)
102.17s (returning an array)
0.79s   (with this patch)

We append arrays in various places, so it seems nice to make this generally
fast.

src/shared/json.c

@@ -553,9 +553,34 @@ static void json_variant_copy_source(JsonVariant *v, JsonVariant *from) {
         v->source = json_source_ref(from->source);
 }
 
+static int _json_variant_array_put_element(JsonVariant *array, JsonVariant *element) {
+        assert(array);
+        JsonVariant *w = array + 1 + array->n_elements;
+
+        uint16_t d = json_variant_depth(element);
+        if (d >= DEPTH_MAX) /* Refuse too deep nesting */
+                return -ELNRNG;
+        if (d >= array->depth)
+                array->depth = d + 1;
+        array->n_elements ++;
+
+        *w = (JsonVariant) {
+                .is_embedded = true,
+                .parent = array,
+        };
+
+        json_variant_set(w, element);
+        json_variant_copy_source(w, element);
+
+        if (!json_variant_is_normalized(element))
+                array->normalized = false;
+
+        return 0;
+}
+
 int json_variant_new_array(JsonVariant **ret, JsonVariant **array, size_t n) {
         _cleanup_(json_variant_unrefp) JsonVariant *v = NULL;
-        bool normalized = true;
+        int r;
 
         assert_return(ret, -EINVAL);
         if (n == 0) {
@@ -571,33 +596,15 @@ int json_variant_new_array(JsonVariant **ret, JsonVariant **array, size_t n) {
         *v = (JsonVariant) {
                 .n_ref = 1,
                 .type = JSON_VARIANT_ARRAY,
+                .normalized = true,
         };
 
-        for (v->n_elements = 0; v->n_elements < n; v->n_elements++) {
-                JsonVariant *w = v + 1 + v->n_elements,
-                            *c = array[v->n_elements];
-                uint16_t d;
-
-                d = json_variant_depth(c);
-                if (d >= DEPTH_MAX) /* Refuse too deep nesting */
-                        return -ELNRNG;
-                if (d >= v->depth)
-                        v->depth = d + 1;
-
-                *w = (JsonVariant) {
-                        .is_embedded = true,
-                        .parent = v,
-                };
-
-                json_variant_set(w, c);
-                json_variant_copy_source(w, c);
-
-                if (!json_variant_is_normalized(c))
-                        normalized = false;
+        while (v->n_elements < n) {
+                r = _json_variant_array_put_element(v, array[v->n_elements]);
+                if (r < 0)
+                        return r;
         }
 
-        v->normalized = normalized;
-
         *ret = TAKE_PTR(v);
         return 0;
 }
@@ -823,6 +830,19 @@ static void json_variant_free_inner(JsonVariant *v, bool force_sensitive) {
                 explicit_bzero_safe(v, json_variant_size(v));
 }
 
+static unsigned json_variant_n_ref(const JsonVariant *v) {
+        /* Return the number of references to v.
+         * 0  => NULL or not a regular object or embedded.
+         * >0 => number of references
+         */
+
+        if (!v || !json_variant_is_regular(v) || v->is_embedded)
+                return 0;
+
+        assert(v->n_ref > 0);
+        return v->n_ref;
+}
+
 JsonVariant *json_variant_ref(JsonVariant *v) {
         if (!v)
                 return NULL;
@@ -2072,28 +2092,54 @@ int json_variant_append_array(JsonVariant **v, JsonVariant *element) {
 
         if (!*v || json_variant_is_null(*v))
                 blank = true;
-        else if (!json_variant_is_array(*v))
-                return -EINVAL;
-        else
+        else if (json_variant_is_array(*v))
                 blank = json_variant_elements(*v) == 0;
+        else
+                return -EINVAL;
 
-        if (blank)
+        if (blank) {
                 r = json_variant_new_array(&nv, (JsonVariant*[]) { element }, 1);
-        else {
-                _cleanup_free_ JsonVariant **array = new(JsonVariant*, json_variant_elements(*v) + 1);
+                if (r < 0)
+                        return r;
+        } else if (json_variant_n_ref(*v) == 1) {
+                /* Let's bump the reference count on element. We can't do the realloc if we're appending *v
+                 * to itself, or one of the objects embedded in *v to *v. If the reference count grows, we
+                 * need to fall back to the other method below. */
+
+                _unused_ _cleanup_(json_variant_unrefp) JsonVariant *dummy = json_variant_ref(element);
+                if (json_variant_n_ref(*v) == 1) {
+                        /* We hold the only reference. Let's mutate the object. */
+                        size_t size = json_variant_elements(*v);
+                        void *old = *v;
+
+                        if (!GREEDY_REALLOC(*v, size + 1 + 1))
+                                return -ENOMEM;
+
+                        if (old != *v)
+                                /* Readjust the parent pointers to the new address */
+                                for (size_t i = 1; i < size; i++)
+                                        (*v)[1 + i].parent = *v;
+
+                        return _json_variant_array_put_element(*v, element);
+                }
+        }
+
+        if (!blank) {
+                size_t size = json_variant_elements(*v);
+
+                _cleanup_free_ JsonVariant **array = new(JsonVariant*, size + 1);
                 if (!array)
                         return -ENOMEM;
 
-                size_t size = json_variant_elements(*v);
                 for (size_t i = 0; i < size; i++)
                         array[i] = json_variant_by_index(*v, i);
 
                 array[size] = element;
 
                 r = json_variant_new_array(&nv, array, size + 1);
+                if (r < 0)
+                        return r;
         }
-        if (r < 0)
-                return r;
 
         json_variant_propagate_sensitive(*v, nv);
         JSON_VARIANT_REPLACE(*v, TAKE_PTR(nv));