The alloc and nr fields of a prio-queue tell us how much memory is
allocated and used in the array. So the natural type for them is size_t,
which prevents overflow on 64-bit systems where "int" is still 32 bits.
This is unlikely to happen in practice, as we typically use it for
storing commits, and having 2^31 of those is rather a lot. But it's good
to keep our generic data structures as flexible as possible. And as we
start to enforce -Wsign-compare, it means that callers need to use
"int", too, and the problem proliferates. Let's fix it at the source.
The changes here can be put into a few groups:
1. Changing the alloc/nr fields in the struct to size_t. This requires
swapping out int for size_t in negotiator/skipping.c, as well as
in prio_queue_get(), because those all iterate over the array.
Building with -Wsign-compare complains about these.
2. Other code that assigns or passes around indexes into the array
(e.g., the swap() and compare() functions) won't trigger
-Wsign-compare because we are simply truncating the values. These
are caught by -Wconversion, but I've adjusted them here to
future-proof us.
3. In prio_queue_reverse() we compute "queue->nr - 1" without checking
if anything is in the queue, which underflows now that nr is
unsigned. We can fix that by returning early when the queue is
empty (there is nothing to reverse).
4. The insertion_ctr variable is currently unsigned, but can likewise
grow (it is actually worse, because adding and removing an element
many times will keep increasing the counter, even though "nr" does
not). I've bumped that to size_t here, as well.
But -Wconversion notes that computing the "cmp" result by
subtracting the counters and assigning to "int" is a potential
problem. And that's true even before this patch, since we use an
unsigned counter (imagine comparing "2^32-1" and "0", which should
be a high positive value, but instead is "-1" as a signed int).
Since we only care about the sign (and not the magnitude) of the
result, we could fix this by swapping out the subtraction for a
ternary comparison. Probably the performance impact would be
negligible, since we just called into a custom compare function and
branched on its result anyway. But it's easy enough to do a
branchless version by subtracting the comparison results.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
62e745ced2