On Fri, May 13, 2022 at 04:06:39PM -0700, Jonathan Tan wrote: > Taylor Blau <me@xxxxxxxxxxxx> writes: > > An alternative approach to closing this race would have been to call > > `is_pack_valid()` on _all_ packs in a multi-pack bitmap on load. This > > has a couple of problems: > > > > - it is unnecessarily expensive in the cases where we don't actually > > need to open any packs (e.g., in `git rev-list --use-bitmap-index > > --count`) > > > > - more importantly, it means any time we would have hit this race, > > we'll avoid using bitmaps altogether, leading to significant > > slowdowns by forcing a full object traversal > > This answers a question I had about why we're only opening the preferred > pack instead of all packs. (You mention in [1] that it's also answered > in that patch message, but I didn't see it.) In any case, it might be > clearer to move this part to the 1st commit. > > [1] https://lore.kernel.org/git/Yn63nDhSBIEa3%2F+%2F@nand.local/ Makes sense, will do. In [1] I was referring to why we wanted to call `is_pack_valid()` as early as we did, and not in `reuse_partial_packfile_from_bitmap()`. But the quoted part here is useful context for the first patch, too, so I moved it up. > > Work around this by calling `is_pack_valid()` from within > > `want_found_object()`, matching the behavior in > > `want_object_in_pack_one()` (which has an analogous call). Most calls to > > `is_pack_valid()` should be basically no-ops, since only the first call > > requires us to open a file (subsequent calls realize the file is already > > open, and return immediately). > > > > This does require us to make a small change in > > `want_object_in_pack_one()`, since `want_found_object()` may return `-1` > > (indicating "keep searching for other packs containing this object") > > when `*found_pack` is non-NULL. Force `want_object_in_pack_one()` to > > call `is_pack_valid()` when `p != *found_pack`, not just when > > `*found_pack` is non-NULL. > > It took me a while to realize that the relevant want_found_object() > invocation that may return -1 is not the one in > want_object_in_pack_one(), but in the latter's caller > want_object_in_pack(). But even in this case, couldn't > want_found_object() return -1 (see the very end of the function) even > before this patch? Perhaps changing the parenthetical to be: (indicating that `want_object_in_pack()` should continue searching for other packs containing this object) Yes, `want_found_object()` could have returned -1 before, but the only time when `*found_pack != NULL` and `want_found_object()` would have returned -1 is when given `--local` with at least one non-local pack. I actually think it's possible we have a bug there, since AFAICT the pre-image of this patch would have left the non-NULL `*found_pack` alone but picked a copy of the given object from a _different_ pack. So I think this change inadvertently resolves that bug. > > @@ -1424,14 +1427,15 @@ static int want_object_in_pack_one(struct packed_git *p, > > off_t *found_offset) > > { > > off_t offset; > > + int use_found = p == *found_pack; > > > > - if (p == *found_pack) > > + if (use_found) > > offset = *found_offset; > > else > > offset = find_pack_entry_one(oid->hash, p); > > > > if (offset) { > > - if (!*found_pack) { > > + if (!use_found) { > > if (!is_pack_valid(p)) > > return -1; > > *found_offset = offset; > > My understanding of the purpose of this code change is that if we reach > here with a non-NULL *found_pack, it is likely that *found_pack contains > an invalid pack, and this part overwrites *found_pack (and > *found_offset) if it finds a valid pack. This seems like a good change, > but I don't see how this is a result of something that "does require > us" (as far as I can tell, *found_pack could have already been invalid > before this patch, so the downstream code should already be able to > handle it). Maybe it's just that we couldn't tell if the pack is invalid > previously, but now we can; but if so, it would be better to say "use > this added information to overwrite *found_pack when it makes sense" or > something like that. I think my reply above indicates why this change is necessary, but if we're talking about separate issues, let me know. Thanks, Taylor
