bootconfig: Prohibit re-defining value on same key

Currently, bootconfig adds a new value on the existing key to the tail of an
array. But this looks a bit confusing because an admin can easily rewrite
the original value in the same config file.

This rejects the following value re-definition.

key = value1
...
key = value2

You should rewrite value1 to value2 in this case.

Link: http://lkml.kernel.org/r/158227282199.12842.10110929876059658601.stgit@devnote2

Suggested-by: Steven Rostedt (VMware) <[e-mail]>
Signed-off-by: Masami Hiramatsu <[e-mail]>
[ Fixed spelling of arraies to arrays ]
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

Documentation/admin-guide/bootconfig.rst

···
 In both styles, same key words are automatically merged when parsing it
 at boot time. So you can append similar trees or key-values.
 
-Note that a sub-key and a value can not co-exist under a parent key.
+Same-key Values
+---------------
+
+It is prohibited that two or more values or arrays share a same-key.
+For example,::
+
+ foo = bar, baz
+ foo = qux  # !ERROR! we can not re-define same key
+
+Also, a sub-key and a value can not co-exist under a parent key.
 For example, following config is NOT allowed.::
 
  foo = value1

lib/bootconfig.c

···
 static int __init xbc_parse_kv(char **k, char *v)
 {
 	struct xbc_node *prev_parent = last_parent;
-	struct xbc_node *node, *child;
+	struct xbc_node *child;
 	char *next;
 	int c, ret;
 
···
 		return ret;
 
 	child = xbc_node_get_child(last_parent);
-	if (child && xbc_node_is_key(child))
-		return xbc_parse_error("Value is mixed with subkey", v);
+	if (child) {
+		if (xbc_node_is_key(child))
+			return xbc_parse_error("Value is mixed with subkey", v);
+		else
+			return xbc_parse_error("Value is redefined", v);
+	}
 
 	c = __xbc_parse_value(&v, &next);
 	if (c < 0)
 		return c;
 
-	node = xbc_add_sibling(v, XBC_VALUE);
-	if (!node)
+	if (!xbc_add_sibling(v, XBC_VALUE))
 		return -ENOMEM;
 
 	if (c == ',') {	/* Array */

tools/bootconfig/samples/bad-samekey.bconf

···
+# Same key value is not allowed
+key {
+	foo = value
+	bar = value2
+}
+key.foo = value