keyval: introduce keyval_merge

This patch introduces a function that merges two keyval-produced
(or keyval-like) QDicts.  It can be used to emulate the behavior of
.merge_lists = true QemuOpts groups, merging -readconfig sections and
command-line options in a single QDict, and also to implement -set.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/option.h

@@ -149,5 +149,6 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst, QemuOptsList *list);
 
 QDict *keyval_parse(const char *params, const char *implied_key,
                     bool *help, Error **errp);
+void keyval_merge(QDict *old, const QDict *new, Error **errp);
 
 #endif

tests/unit/test-keyval.c

@@ -747,6 +747,61 @@ static void test_keyval_visit_any(void)
     visit_free(v);
 }
 
+static void test_keyval_merge_dict(void)
+{
+    QDict *first = keyval_parse("opt1=abc,opt2.sub1=def,opt2.sub2=ghi,opt3=xyz",
+                                NULL, NULL, &error_abort);
+    QDict *second = keyval_parse("opt1=ABC,opt2.sub2=GHI,opt2.sub3=JKL",
+                                 NULL, NULL, &error_abort);
+    QDict *combined = keyval_parse("opt1=ABC,opt2.sub1=def,opt2.sub2=GHI,opt2.sub3=JKL,opt3=xyz",
+                                   NULL, NULL, &error_abort);
+    Error *err = NULL;
+
+    keyval_merge(first, second, &err);
+    g_assert(!err);
+    g_assert(qobject_is_equal(QOBJECT(combined), QOBJECT(first)));
+    qobject_unref(first);
+    qobject_unref(second);
+    qobject_unref(combined);
+}
+
+static void test_keyval_merge_list(void)
+{
+    QDict *first = keyval_parse("opt1.0=abc,opt2.0=xyz",
+                                NULL, NULL, &error_abort);
+    QDict *second = keyval_parse("opt1.0=def",
+                                 NULL, NULL, &error_abort);
+    QDict *combined = keyval_parse("opt1.0=abc,opt1.1=def,opt2.0=xyz",
+                                   NULL, NULL, &error_abort);
+    Error *err = NULL;
+
+    keyval_merge(first, second, &err);
+    g_assert(!err);
+    g_assert(qobject_is_equal(QOBJECT(combined), QOBJECT(first)));
+    qobject_unref(first);
+    qobject_unref(second);
+    qobject_unref(combined);
+}
+
+static void test_keyval_merge_conflict(void)
+{
+    QDict *first = keyval_parse("opt2=ABC",
+                                NULL, NULL, &error_abort);
+    QDict *second = keyval_parse("opt2.sub1=def,opt2.sub2=ghi",
+                                 NULL, NULL, &error_abort);
+    QDict *third = qdict_clone_shallow(first);
+    Error *err = NULL;
+
+    keyval_merge(first, second, &err);
+    error_free_or_abort(&err);
+    keyval_merge(second, third, &err);
+    error_free_or_abort(&err);
+
+    qobject_unref(first);
+    qobject_unref(second);
+    qobject_unref(third);
+}
+
 int main(int argc, char *argv[])
 {
     g_test_init(&argc, &argv, NULL);
@@ -760,6 +815,9 @@ int main(int argc, char *argv[])
     g_test_add_func("/keyval/visit/optional", test_keyval_visit_optional);
     g_test_add_func("/keyval/visit/alternate", test_keyval_visit_alternate);
     g_test_add_func("/keyval/visit/any", test_keyval_visit_any);
+    g_test_add_func("/keyval/merge/dict", test_keyval_merge_dict);
+    g_test_add_func("/keyval/merge/list", test_keyval_merge_list);
+    g_test_add_func("/keyval/merge/conflict", test_keyval_merge_conflict);
     g_test_run();
     return 0;
 }

util/keyval.c

@@ -310,6 +310,86 @@ static char *reassemble_key(GSList *key)
     return g_string_free(s, FALSE);
 }
 
+/*
+ * Recursive worker for keyval_merge.
+ *
+ * @str is the path that led to the * current dictionary (to be used for
+ * error messages).  It is modified internally but restored before the
+ * function returns.
+ */
+static void keyval_do_merge(QDict *dest, const QDict *merged, GString *str, Error **errp)
+{
+    size_t save_len = str->len;
+    const QDictEntry *ent;
+    QObject *old_value;
+
+    for (ent = qdict_first(merged); ent; ent = qdict_next(merged, ent)) {
+        old_value = qdict_get(dest, ent->key);
+        if (old_value) {
+            if (qobject_type(old_value) != qobject_type(ent->value)) {
+                error_setg(errp, "Parameter '%s%s' used inconsistently",
+                           str->str, ent->key);
+                return;
+            } else if (qobject_type(ent->value) == QTYPE_QDICT) {
+                /* Merge sub-dictionaries.  */
+                g_string_append(str, ent->key);
+                g_string_append_c(str, '.');
+                keyval_do_merge(qobject_to(QDict, old_value),
+                                qobject_to(QDict, ent->value),
+                                str, errp);
+                g_string_truncate(str, save_len);
+                continue;
+            } else if (qobject_type(ent->value) == QTYPE_QLIST) {
+                /* Append to old list.  */
+                QList *old = qobject_to(QList, old_value);
+                QList *new = qobject_to(QList, ent->value);
+                const QListEntry *item;
+                QLIST_FOREACH_ENTRY(new, item) {
+                    qobject_ref(item->value);
+                    qlist_append_obj(old, item->value);
+                }
+                continue;
+            } else {
+                assert(qobject_type(ent->value) == QTYPE_QSTRING);
+            }
+        }
+
+        qobject_ref(ent->value);
+        qdict_put_obj(dest, ent->key, ent->value);
+    }
+}
+
+/* Merge the @merged dictionary into @dest.
+ *
+ * The dictionaries are expected to be returned by the keyval parser, and
+ * therefore the only expected scalar type is the string.  In case the same
+ * path is present in both @dest and @merged, the semantics are as follows:
+ *
+ * - lists are concatenated
+ *
+ * - dictionaries are merged recursively
+ *
+ * - for scalar values, @merged wins
+ *
+ * In case an error is reported, @dest may already have been modified.
+ *
+ * This function can be used to implement semantics analogous to QemuOpts's
+ * .merge_lists = true case, or to implement -set for options backed by QDicts.
+ *
+ * Note: while QemuOpts is commonly used so that repeated keys overwrite
+ * ("last one wins"), it can also be used so that repeated keys build up
+ * a list. keyval_merge() can only be used when the options' semantics are
+ * the former, not the latter.
+ */
+void keyval_merge(QDict *dest, const QDict *merged, Error **errp)
+{
+    GString *str;
+
+    str = g_string_new("");
+    keyval_do_merge(dest, merged, str, errp);
+    g_string_free(str, TRUE);
+}
+
 /*
  * Listify @cur recursively.
  * Replace QDicts whose keys are all valid list indexes by QLists.