[asterisk-dev] watch out usages of AST_LIST_INSERT_TAIL

[Luigi Rizzo]

i just realised "the hard way" that AST_LIST_INSERT_TAIL can only
append a single element to a list, and it is not good for appending
two lists. The macro also assumes that the element has the 'tail'
pointer properly initialized.

I am not 100% sure that all places in asterisk where AST_LIST_INSERT_TAIL
is used honor the assumption.
Other than a scrutiny of all places where AST_LIST_INSERT_TAIL is used,
i would suggest a simple fix - changing the macro to handle the case
of list merging, as follows (more or less):

   #define AST_LIST_INSERT_TAIL(head, elm, field) do {                     \
      if (!(head)->first) {                                             \
                (head)->first = (elm);                                  \
      } else {                                                          \
                (head)->last->field.next = (elm);                       \
      }                                                                 \
	(head)->last = (elm);                                   	\
	while ((head)->last->field.next)		\
		(head)->last = (head)->last->field.next;		\
   } while (0)

The extra overhead is negligible in the normal case
((head)->last->field.next = NULL)

Of course, with all these pointer dereferences, one wonders if it
isn't better/safer to use the macro just as a frontend to a
function, using offsetof(field) to locate link field within the
structure.

cheers
luigi