m_property: add read_sub_validate to check if should be processed

In many cases it makes sense to early exit instead of preparing all the
data only to return type or not implemented.

options/m_property.c

@@ -432,6 +432,23 @@ int m_property_strdup_ro(int action, void* arg, const char *var)
     return M_PROPERTY_NOT_IMPLEMENTED;
 }
 
+int m_property_read_sub_validate(void *ctx, struct m_property *prop,
+                                 int action, void *arg)
+{
+    m_property_unkey(&action, &arg);
+    switch (action) {
+    case M_PROPERTY_GET_TYPE:
+        *(struct m_option *)arg = (struct m_option){.type = CONF_TYPE_NODE};
+        return M_PROPERTY_OK;
+    case M_PROPERTY_GET:
+    case M_PROPERTY_PRINT:
+    case M_PROPERTY_KEY_ACTION:
+        return M_PROPERTY_VALID;
+    default:
+        return M_PROPERTY_NOT_IMPLEMENTED;
+    };
+}
+
 // This allows you to make a list of values (like from a struct) available
 // as a number of sub-properties. The property list is set up with the current
 // property values on the stack before calling this function.

options/m_property.h

@@ -107,6 +107,9 @@ struct m_property_action_arg {
 };
 
 enum mp_property_return {
+    // Returned from validator if action should be executed.
+    M_PROPERTY_VALID = 2,
+
     // Returned on success.
     M_PROPERTY_OK = 1,
 
@@ -212,6 +215,8 @@ struct m_sub_property {
 #define SUB_PROP_PTS(f) \
     .type = {.type = &m_option_type_time}, .value = {.double_ = (f)}
 
+int m_property_read_sub_validate(void *ctx, struct m_property *prop,
+                                 int action, void *arg);
 int m_property_read_sub(const struct m_sub_property *props, int action, void *arg);