sys: dlist: Add sys_dnode_is_linked

The original implementation allows a list to be corrupted by list
operations on the removed node.  Existing code attempts to avoid this by
using external state to determine whether a node is in a list, but this
is fragile and fails when the state that holds the flag value is changed
after the node is removed, e.g. in preparation for re-using the node.

Follow Linux in invalidating the link pointers in a removed node.  Add
API so that detection of particpation in a list is available at the node
abstraction.

This solution relies on the following steady-state invariants:
* A node (as opposed to a list) will never be adjacent to itself in a
  list;
* The next and prev pointers of a node are always either both null or
  both non-null.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

include/misc/dlist.h

@@ -181,7 +181,7 @@ typedef struct _dnode sys_dnode_t;
 	     __cns = SYS_DLIST_PEEK_NEXT_CONTAINER(__dl, __cn, __n))
 
 /**
- * @brief initialize list
+ * @brief initialize list to its empty state
  *
  * @param list the doubly-linked list
  *
@@ -196,6 +196,33 @@ static inline void sys_dlist_init(sys_dlist_t *list)
 
 #define SYS_DLIST_STATIC_INIT(ptr_to_list) { {(ptr_to_list)}, {(ptr_to_list)} }
 
+/**
+ * @brief initialize node to its state when not in a list
+ *
+ * @param node the node
+ *
+ * @return N/A
+ */
+
+static inline void sys_dnode_init(sys_dnode_t *node)
+{
+	node->next = NULL;
+	node->prev = NULL;
+}
+
+/**
+ * @brief check if a node is a member of any list
+ *
+ * @param node the node
+ *
+ * @return true if node is linked into a list, false if it is not
+ */
+
+static inline bool sys_dnode_is_linked(const sys_dnode_t *node)
+{
+	return node->next != NULL;
+}
+
 /**
  * @brief check if a node is the list's head
  *
@@ -500,6 +527,7 @@ static inline void sys_dlist_remove(sys_dnode_t *node)
 {
 	node->prev->next = node->next;
 	node->next->prev = node->prev;
+	sys_dnode_init(node);
 }
 
 /**

tests/kernel/common/src/dlist.c

@@ -189,9 +189,13 @@ void test_dlist(void)
 		     "test_list head/tail are wrong");
 
 	/* Finding and removing node 1 */
+	zassert_true(sys_dnode_is_linked(&test_node_1.node),
+		     "node1 is not linked");
 	sys_dlist_remove(&test_node_1.node);
 	zassert_true((verify_emptyness(&test_list)),
 		     "test_list should be empty");
+	zassert_false(sys_dnode_is_linked(&test_node_1.node),
+		      "node1 is still linked");
 
 	/* Prepending node 1 */
 	sys_dlist_prepend(&test_list, &test_node_1.node);