core: support receiving multipart objects

In order to support an upcoming patch implementing support for
requesting bridge vlan information for a single interface, a
modification to the pickup code is needed to allow handling of multipart
messages.

Based on the implementation of the bridge vlan dumping code in the
kernel, information relating to a single interface may not all be
contained within a single message. If the maximum message size is
exceeded before all entries can be filled in, a new message will be
started with the same ifindex but containing the remaining entries.

The code is currently only designed for handling netlink responses where
the provided parser function will be called a maximum of one time. This
is based on the assumption that all data relating to an object will be
contained within a single response message. If multiple messages are
received and the parser function is invoked multiple times, previously
stored objects will be leaked and the resulting object will only contain
information from the last parsed message.

In order to handle the case where multiple messages need to be parsed to
construct a single object, use the already existing oo_update function
that may be defined for an object. The function will be called to merge
partial objects (that are the result of each invocation of the parser
function) together into a single object that is returned at the end.

Author: ronand-atl

include/netlink/errno.h

@@ -45,8 +45,9 @@ extern "C" {
 #define NLE_IMMUTABLE		32
 #define NLE_DUMP_INTR		33
 #define NLE_ATTRSIZE		34
+#define NLE_MERGE_FAILURE	35
 
-#define NLE_MAX		NLE_ATTRSIZE
+#define NLE_MAX		NLE_MERGE_FAILURE
 
 extern const char *	nl_geterror(int);
 extern void		nl_perror(int, const char *);

lib/error.c

@@ -43,6 +43,7 @@ static const char *errmsg[NLE_MAX+1] = {
 [NLE_IMMUTABLE]		= "Immutable attribute",
 [NLE_DUMP_INTR]		= "Dump inconsistency detected, interrupted",
 [NLE_ATTRSIZE]		= "Attribute max length exceeded",
+[NLE_MERGE_FAILURE]	= "Unable to merge objects",
 };
 
 /**

lib/nl.c

@@ -1137,12 +1137,21 @@ struct pickup_param
 static int __store_answer(struct nl_object *obj, struct nl_parser_param *p)
 {
 	struct pickup_param *pp = p->pp_arg;
-	/*
-	 * the parser will put() the object at the end, expecting the cache
-	 * to take the reference.
-	 */
-	nl_object_get(obj);
-	pp->result =  obj;
+
+	if (pp->result == NULL) {
+		/* The parser will put() the object at the end, expecting the
+		 * cache to take the reference. This line ensures the object
+		 * won't get deleted when that happens.
+		 */
+		nl_object_get(obj);
+
+		pp->result = obj;
+
+		return 0;
+	}
+
+	if (nl_object_update(pp->result, obj) < 0)
+		return -NLE_MERGE_FAILURE;
 
 	return 0;
 }
@@ -1168,11 +1177,15 @@ static int __pickup_answer_syserr(struct sockaddr_nl *nla, struct nlmsgerr *nler
 /** @endcond */
 
 /**
- * Pickup netlink answer, parse is and return object
+ * Pickup netlink answer, parse it and return an object
  * @arg sk              Netlink socket
  * @arg parser          Parser function to parse answer
  * @arg result          Result pointer to return parsed object
  *
+ * @note If this function returns success, result may be NULL, so the caller
+ *       should check that before accessing the result. This can happen if the
+ *       kernel doesn't return any object data and reports success.
+ *
  * @return 0 on success or a negative error code.
  */
 int nl_pickup(struct nl_sock *sk,
@@ -1184,11 +1197,17 @@ int nl_pickup(struct nl_sock *sk,
 }
 
 /**
- * Pickup netlink answer, parse is and return object with preserving system error
+ * Pickup netlink answer, parse it and return the object while preserving the
+ * system error
  * @arg sk              Netlink socket
  * @arg parser          Parser function to parse answer
  * @arg result          Result pointer to return parsed object
- * @arg syserr          Result pointer for the system error in case of failure
+ * @arg syserror        Result pointer for the system error in case of failure
+ *                      (optional)
+ *
+ * @note If this function returns success, result may be NULL, so the caller
+ *       should check that before accessing the result. This can happen if the
+ *       kernel doesn't return any object data and reports success.
  *
  * @return 0 on success or a negative error code.
  */
@@ -1216,8 +1235,10 @@ int nl_pickup_keep_syserr(struct nl_sock *sk,
 	}
 
 	err = nl_recvmsgs(sk, cb);
-	if (err < 0)
+	if (err < 0) {
+		nl_object_put(pp.result);
 		goto errout;
+	}
 
 	*result = pp.result;
 errout: