One concrete issue with this technique: it doesn't "loop". Meaning, for example, if initially method #3 doesn't work but method #4 does, it'll settle on method #4. If later, method #3 starts working, it won't switch it. Worse, if method #4 stops working, it'll just always fail.
I imagine this isn't an issue for rsync's use but could be a concern if applying it more generally.
But it's an "optimization", and avoiding previously attempted methods is the whole point of the trick. Otherwise it would be an easy and uninteresting function.
That‘ll probably try all methods on every single file that is processed, so if method #16 is the one that works, you get 15 unnecessary syscalls for every file.
I think (I haven't actually tested it) you can do what the parent commenter wants by wrapping the whole thing in a do-loop but the original implementation is questionable enough, sticking an extra loop in there isn't doing anything to help make it less horrible.
int set_the_mtime(...) {
static int switch_step = 0;
int loop_again = 1;
do {
switch (switch_step) {
case 0:
if (method_0_works(...))
return 0;
switch_step++;
/* FALLTHROUGH */
case 1:
if (method_1_works(...))
return 0;
switch_step++;
/* FALLTHROUGH */
case 2:
if (method_2_works(...))
return 0;
}
switch_step = 0;
} while (loop_again--);
return -1; /* ultimate failure */
}