On Wed, Jul 21, 2021 at 06:23:23AM -0400, Jeff King wrote: > On Mon, Jun 21, 2021 at 06:25:18PM -0400, Taylor Blau wrote: > > > When writing a new multi-pack index, write_midx_internal() attempts to > > load any existing one to fill in some pieces of information. But it uses > > load_multi_pack_index(), which ignores the configuration > > "core.multiPackIndex", which indicates whether or not Git is allowed to > > read an existing multi-pack-index. > > > > Replace this with a routine that does respect that setting, to avoid > > reading multi-pack-index files when told not to. > > > > This avoids a problem that would arise in subsequent patches due to the > > combination of 'git repack' reopening the object store in-process and > > the multi-pack index code not checking whether a pack already exists in > > the object store when calling add_pack_to_midx(). > > > > This would ultimately lead to a cycle being created along the > > 'packed_git' struct's '->next' pointer. That is obviously bad, but it > > has hard-to-debug downstream effects like saying a bitmap can't be > > loaded for a pack because one already exists (for the same pack). > > I'm not sure I completely understand the bug that this causes. Off-hand, I can't quite remember either. But it is important; I do have a distinct memory of dropping this patch and then watching a 'git repack --write-midx' (that option will be introduced in a later series) fail horribly. If I remember correctly, the bug has to do with loading a MIDX twice in the same process. When we call add_packed_git() from within prepare_midx_pack(), we load the pack without caring whether or not it's already loaded. So loading a MIDX twice in the same process will fail. So really I think that this is papering over that bug: we're just removing one of the times that we happened to load a MIDX from during the writing phase. What I do remember is that this bug was a huge pain to figure out ;). I'm happy to look further if you aren't satisfied with my vague explanation here (and I wouldn't blame you). > But another question: does this impact how > > git -c core.multipackindex=false multi-pack-index write > > behaves? I.e., do we still write, but just avoid reading the existing > midx? That itself seems like a more sensible behavior (e.g., trying to > recover from a broken midx state). Yes. Before this patch, that invocation would still load and use any existing MIDX to write a new one. Now we don't, because (unlike load_multi_pack_index()) prepare_multi_pack_index_one() does check core.multiPackIndex before returning anything. > -Peff Thanks, Taylor
