[PATCH v3 4/6] qapi: Apply aliases in qobject-input-visitor

[Kevin Wolf]

When looking for an object in a struct in the external representation,
check not only the currently visited struct, but also whether an alias
in the current StackObject matches and try to fetch the value from the
alias then. Providing two values for the same object through different
aliases is an error.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qapi/qobject-input-visitor.c | 227 +++++++++++++++++++++++++++++++++--
 1 file changed, 218 insertions(+), 9 deletions(-)

diff --git a/qapi/qobject-input-visitor.c b/qapi/qobject-input-visitor.c
index 16a75442ff..6193df28a5 100644
--- a/qapi/qobject-input-visitor.c
+++ b/qapi/qobject-input-visitor.c
@@ -97,6 +97,8 @@ struct QObjectInputVisitor {
     QObject *root;
     bool keyval;                /* Assume @root made with keyval_parse() */
 
+    QDict *empty_qdict;         /* Used for implicit objects */
+
     /* Stack of objects being visited (all entries will be either
      * QDict or QList). */
     QSLIST_HEAD(, StackObject) stack;
@@ -169,9 +171,190 @@ static const char *full_name(QObjectInputVisitor *qiv, 
const char *name)
     return full_name_so(qiv, name, false, tos);
 }
 
+static bool find_object_member(QObjectInputVisitor *qiv,
+                               StackObject **so, const char **name,
+                               bool *is_alias_prefix, Error **errp);
+
+/*
+ * Check whether the member @name in @so, or an alias for it, is
+ * present in the input and can be used to obtain the value.
+ */
+static bool input_present(QObjectInputVisitor *qiv, StackObject *so,
+                          const char *name)
+{
+    /*
+     * Check whether the alias member is present in the input
+     * (possibly recursively because aliases are transitive).
+     * The QAPI generator makes sure that alises cannot form loops, so
+     * the recursion guaranteed to terminate.
+     */
+    if (!find_object_member(qiv, &so, &name, NULL, NULL)) {
+        return false;
+    }
+
+    /*
+     * Every source can be used only once. If a value in the input
+     * would end up being used twice through aliases, we'll fail the
+     * second access.
+     */
+    if (!g_hash_table_contains(so->h, name)) {
+        return false;
+    }
+
+    return true;
+}
+
+/*
+ * Check whether the member @name in the object visited by @so can be
+ * specified in the input by using the alias described by @a (which
+ * must be an alias contained in so->aliases).
+ *
+ * If @name is only a prefix of the alias source, but doesn't match
+ * immediately, false is returned and *is_alias_prefix is set to true
+ * if it is non-NULL.  In all other cases, *is_alias_prefix is left
+ * unchanged.
+ */
+static bool alias_source_matches(QObjectInputVisitor *qiv,
+                                 StackObject *so, InputVisitorAlias *a,
+                                 const char *name, bool *is_alias_prefix)
+{
+    if (a->src[0] == NULL) {
+        assert(a->name == NULL);
+        return true;
+    }
+
+    if (!strcmp(a->src[0], name)) {
+        if (a->name && a->src[1] == NULL) {
+            /*
+             * We're matching an exact member, the source for this alias is
+             * immediately in @so.
+             */
+            return true;
+        } else if (is_alias_prefix) {
+            /*
+             * We're only looking at a prefix of the source path for the alias.
+             * If the input contains no object of the requested name, we will
+             * implicitly create an empty one so that the alias can still be
+             * used.
+             *
+             * We want to create the implicit object only if the alias is
+             * actually used, but we can't tell here for wildcard aliases (only
+             * a later visitor call will determine this). This means that
+             * wildcard aliases must never have optional keys in their source
+             * path. The QAPI generator checks this condition.
+             */
+            if (!a->name || input_present(qiv, a->alias_so, a->name)) {
+                *is_alias_prefix = true;
+            }
+        }
+    }
+
+    return false;
+}
+
+/*
+ * Find the place in the input where the value for the object member
+ * @name in @so is specified, considering applicable aliases.
+ *
+ * If a value could be found, true is returned and @so and @name are
+ * updated to identify the key name and StackObject where the value
+ * can be found in the input.  (This is either unchanged or the
+ * alias_so/name of an alias.)  The value of @is_alias_prefix on
+ * return is undefined in this case.
+ *
+ * If no value could be found in the input, false is returned and @so
+ * and @name are set to NULL.  This is not an error and @errp remains
+ * unchanged.  If @is_alias_prefix is non-NULL, it is set to true if
+ * the given name is a prefix of the source path of an alias for which
+ * a value may be present in the input.  It is set to false otherwise.
+ *
+ * If an error occurs (e.g. two values are specified for the member
+ * through different names), false is returned and @errp is set.  The
+ * value of @is_alias_prefix on return is undefined in this case.
+ */
+static bool find_object_member(QObjectInputVisitor *qiv,
+                               StackObject **so, const char **name,
+                               bool *is_alias_prefix, Error **errp)
+{
+    QDict *qdict = qobject_to(QDict, (*so)->obj);
+    const char *found_name = NULL;
+    StackObject *found_so = NULL;
+    bool found_is_wildcard = false;
+    InputVisitorAlias *a;
+
+    if (is_alias_prefix) {
+        *is_alias_prefix = false;
+    }
+
+    /* Directly present in the container */
+    if (qdict_haskey(qdict, *name)) {
+        found_name = *name;
+        found_so = *so;
+    }
+
+    /*
+     * Find aliases whose source path matches @name in this StackObject. We can
+     * then get the value with the key a->name from a->alias_so.
+     */
+    QSLIST_FOREACH(a, &(*so)->aliases, next) {
+        if (a->name == NULL && found_name && !found_is_wildcard) {
+            /*
+             * Skip wildcard aliases if we already have a match. This is
+             * not a conflict that should result in an error.
+             *
+             * However, multiple wildcard aliases matching is an error
+             * and will be caught below.
+             */
+            continue;
+        }
+
+        if (!alias_source_matches(qiv, *so, a, *name, is_alias_prefix)) {
+            continue;
+        }
+
+        /*
+         * For single-member aliases, an alias name is specified in the
+         * alias definition. For wildcard aliases, the alias has the same
+         * name as the member in the source object, i.e. *name.
+         */
+        if (!input_present(qiv, a->alias_so, a->name ?: *name)) {
+            continue;
+        }
+
+        /*
+         * A non-wildcard alias simply overrides a wildcard alias, but
+         * two matching non-wildcard aliases or two matching wildcard
+         * aliases conflict with each other.
+         */
+        if (found_name && (!found_is_wildcard || a->name == NULL)) {
+            error_setg(errp, "Value for parameter %s was already given "
+                       "through an alias",
+                       full_name_so(qiv, *name, false, *so));
+            return false;
+        } else {
+            found_name = a->name ?: *name;
+            found_so = a->alias_so;
+            found_is_wildcard = !a->name;
+        }
+    }
+
+    /*
+     * Chained aliases: *found_so/found_name might be the source of
+     * another alias.
+     */
+    if (found_name && (found_so != *so || found_name != *name)) {
+        find_object_member(qiv, &found_so, &found_name, NULL, errp);
+    }
+
+    *so = found_so;
+    *name = found_name;
+
+    return found_name != NULL;
+}
+
 static QObject *qobject_input_try_get_object(QObjectInputVisitor *qiv,
                                              const char *name,
-                                             bool consume)
+                                             bool consume, Error **errp)
 {
     StackObject *tos;
     QObject *qobj;
@@ -189,10 +372,31 @@ static QObject 
*qobject_input_try_get_object(QObjectInputVisitor *qiv,
     assert(qobj);
 
     if (qobject_type(qobj) == QTYPE_QDICT) {
-        assert(name);
-        ret = qdict_get(qobject_to(QDict, qobj), name);
-        if (tos->h && consume && ret) {
-            bool removed = g_hash_table_remove(tos->h, name);
+        StackObject *so = tos;
+        const char *key = name;
+        bool is_alias_prefix;
+
+        assert(key);
+        if (!find_object_member(qiv, &so, &key, &is_alias_prefix, errp)) {
+            if (is_alias_prefix) {
+                /*
+                 * The member is not present in the input, but
+                 * something inside of it might still be given through
+                 * an alias. Pretend there was an empty object in the
+                 * input.
+                 */
+                if (!qiv->empty_qdict) {
+                    qiv->empty_qdict = qdict_new();
+                }
+                return QOBJECT(qiv->empty_qdict);
+            } else {
+                return NULL;
+            }
+        }
+        ret = qdict_get(qobject_to(QDict, so->obj), key);
+        assert(ret != NULL);
+        if (so->h && consume) {
+            bool removed = g_hash_table_remove(so->h, key);
             assert(removed);
         }
     } else {
@@ -218,9 +422,10 @@ static QObject 
*qobject_input_get_object(QObjectInputVisitor *qiv,
                                          const char *name,
                                          bool consume, Error **errp)
 {
-    QObject *obj = qobject_input_try_get_object(qiv, name, consume);
+    ERRP_GUARD();
+    QObject *obj = qobject_input_try_get_object(qiv, name, consume, errp);
 
-    if (!obj) {
+    if (!obj && !*errp) {
         error_setg(errp, QERR_MISSING_PARAMETER, full_name(qiv, name));
     }
     return obj;
@@ -803,13 +1008,16 @@ static bool qobject_input_type_size_keyval(Visitor *v, 
const char *name,
 static void qobject_input_optional(Visitor *v, const char *name, bool *present)
 {
     QObjectInputVisitor *qiv = to_qiv(v);
-    QObject *qobj = qobject_input_try_get_object(qiv, name, false);
+    Error *local_err = NULL;
+    QObject *qobj = qobject_input_try_get_object(qiv, name, false, &local_err);
 
-    if (!qobj) {
+    /* If there was an error, let the caller try and run into the error */
+    if (!qobj && !local_err) {
         *present = false;
         return;
     }
 
+    error_free(local_err);
     *present = true;
 }
 
@@ -842,6 +1050,7 @@ static void qobject_input_free(Visitor *v)
         qobject_input_stack_object_free(tos);
     }
 
+    qobject_unref(qiv->empty_qdict);
     qobject_unref(qiv->root);
     if (qiv->errname) {
         g_string_free(qiv->errname, TRUE);
-- 
2.31.1