Re: [PATCH 2/5] parallel-checkout: make it truly parallel

[Christian Couder <christian.couder@gmail.com>]

On Wed, Mar 17, 2021 at 10:12 PM Matheus Tavares
<matheus.bernardino@usp.br> wrote:

> diff --git a/.gitignore b/.gitignore
> index 3dcdb6bb5a..26f8ddfc55 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -33,6 +33,7 @@
>  /git-check-mailmap
>  /git-check-ref-format
>  /git-checkout
> +/git-checkout--helper

I wonder if "checkout--worker" would be better than "checkout--helper".

>  /git-checkout-index
>  /git-cherry
>  /git-cherry-pick

[...]

> +#define ASSERT_PC_ITEM_RESULT_SIZE(got, exp) \
> +{ \
> +       if (got != exp) \
> +               BUG("corrupted result from checkout worker (got %dB, exp %dB)", \

Maybe precompilers are smart enough to not replace the "got" and "exp"
in the above string, but it might be a bit confusing for readers.
Anway I wonder if this macro could just be a regular (possibly inline)
function.

> +                   got, exp); \
> +} while(0)

> +static void parse_and_save_result(const char *line, int len,
> +                                 struct pc_worker *worker)
> +{
> +       struct pc_item_result *res;
> +       struct parallel_checkout_item *pc_item;
> +       struct stat *st = NULL;
> +
> +       if (len < PC_ITEM_RESULT_BASE_SIZE)
> +               BUG("too short result from checkout worker (got %dB, exp %dB)",
> +                   len, (int)PC_ITEM_RESULT_BASE_SIZE);
> +
> +       res = (struct pc_item_result *)line;
> +
> +       /*
> +        * Worker should send either the full result struct on success, or
> +        * just the base (i.e. no stat data), otherwise.
> +        */
> +       if (res->status == PC_ITEM_WRITTEN) {
> +               ASSERT_PC_ITEM_RESULT_SIZE(len, (int)sizeof(struct pc_item_result));
> +               st = &res->st;
> +       } else {
> +               ASSERT_PC_ITEM_RESULT_SIZE(len, (int)PC_ITEM_RESULT_BASE_SIZE);
> +       }
> +
> +       if (!worker->nr_items_to_complete || res->id != worker->next_item_to_complete)

Nit: maybe it could be useful to distinguish between these 2 potential
bugs and have a specific BUG() for each one.

> +               BUG("checkout worker sent unexpected item id");

> +       worker->next_item_to_complete++;
> +       worker->nr_items_to_complete--;
> +
> +       pc_item = &parallel_checkout.items[res->id];
> +       pc_item->status = res->status;
> +       if (st)
> +               pc_item->st = *st;
> +}
> +
> +

One blank line is enough.

> +static void gather_results_from_workers(struct pc_worker *workers,
> +                                       int num_workers)
> +{
> +       int i, active_workers = num_workers;
> +       struct pollfd *pfds;
> +
> +       CALLOC_ARRAY(pfds, num_workers);
> +       for (i = 0; i < num_workers; i++) {
> +               pfds[i].fd = workers[i].cp.out;
> +               pfds[i].events = POLLIN;
> +       }
> +
> +       while (active_workers) {
> +               int nr = poll(pfds, num_workers, -1);
> +
> +               if (nr < 0) {
> +                       if (errno == EINTR)
> +                               continue;
> +                       die_errno("failed to poll checkout workers");
> +               }
> +
> +               for (i = 0; i < num_workers && nr > 0; i++) {

Is it possible that nr is 0? If that happens, it looks like we would
be in an infinite `while (active_workers) { ... }` loop.

Actually in poll(2) there is: "A value of 0 indicates that the call
timed out and no file descriptors were ready." So it seems that it
could, at least theorically, happen.

> +                       struct pc_worker *worker = &workers[i];
> +                       struct pollfd *pfd = &pfds[i];
> +
> +                       if (!pfd->revents)
> +                               continue;
> +
> +                       if (pfd->revents & POLLIN) {
> +                               int len;
> +                               const char *line = packet_read_line(pfd->fd, &len);
> +
> +                               if (!line) {
> +                                       pfd->fd = -1;
> +                                       active_workers--;
> +                               } else {
> +                                       parse_and_save_result(line, len, worker);
> +                               }
> +                       } else if (pfd->revents & POLLHUP) {
> +                               pfd->fd = -1;
> +                               active_workers--;
> +                       } else if (pfd->revents & (POLLNVAL | POLLERR)) {
> +                               die(_("error polling from checkout worker"));
> +                       }
> +
> +                       nr--;
> +               }
> +       }
> +
> +       free(pfds);
> +}

[...]

> +enum pc_item_status {
> +       PC_ITEM_PENDING = 0,
> +       PC_ITEM_WRITTEN,
> +       /*
> +        * The entry could not be written because there was another file
> +        * already present in its path or leading directories. Since
> +        * checkout_entry_ca() removes such files from the working tree before
> +        * enqueueing the entry for parallel checkout, it means that there was
> +        * a path collision among the entries being written.
> +        */
> +       PC_ITEM_COLLIDED,
> +       PC_ITEM_FAILED,
> +};
> +
> +struct parallel_checkout_item {
> +       /*
> +        * In main process ce points to a istate->cache[] entry. Thus, it's not
> +        * owned by us. In workers they own the memory, which *must be* released.
> +        */
> +       struct cache_entry *ce;
> +       struct conv_attrs ca;
> +       size_t id; /* position in parallel_checkout.items[] of main process */
> +
> +       /* Output fields, sent from workers. */
> +       enum pc_item_status status;
> +       struct stat st;
> +};

Maybe the previous patch could have declared both 'enum
pc_item_status' and 'struct parallel_checkout_item' here, in
parallel-checkout.h, so that this patch wouldn't need to move them
here.