On Tue, Mar 07, 2023 at 09:52:19AM -0500, Derrick Stolee wrote: > On 3/6/2023 7:28 PM, Taylor Blau wrote: > > On Mon, Mar 06, 2023 at 02:54:00PM -0800, Junio C Hamano wrote: > >> "Derrick Stolee via GitGitGadget" <gitgitgadget@xxxxxxxxx> writes: > >> > >>> @@ -1008,6 +1008,7 @@ void reprepare_packed_git(struct repository *r) > >>> struct object_directory *odb; > >>> > >>> obj_read_lock(); > >>> + reprepare_alt_odb(r); > >>> for (odb = r->objects->odb; odb; odb = odb->next) > >>> odb_clear_loose_cache(odb); > >> > >> Hmph, if there was an old alternate ODB from which we took some > >> loose object from and cached, and if that ODB no longer is on the > >> updated alternate list, would we now fail to clear the loose objects > >> cache for the ODB? Or are we only prepared for seeing "more" > >> alternates and assume no existing alternates go away? > > > > Based on my understanding of the patch, we are only prepared to see > > "more" alternates, rather than some existing alternate going away. > > > > That being said, I am not certain that is how it works. Perhaps an > > alternate "goes away", but does not actually get removed from the list > > of alternate ODBs. If that's the case, any object lookup in that > > now-missing ODB would fail, but any subsequent ODBs which were added > > after calling reprepare_alt_odb() would succeed on that object lookup. > > > > So, I don't know. I don't have the implementation details of the > > alternates ODB mechanism paged in enough to say for sure. Hopefully > > Stolee can point us in the right direction. > > The prepare_alt_odb() call only _adds_ to the linked odb list. It > will not remove any existing ODBs. Adding this reprepare_*() method > makes it such that we can use the union of the alternates available > across the lifetime of the process. Right, that matches my understanding. What I am asking is: since we only add ODBs to the list, what happens if we can no longer access an *existing* alternate at the time we call reprepare_alt_odb()? It's clear that that now-inaccessible alternate remains in our list of alternate ODBs, but do all object lookups hitting that ODB fail-over to the new ODB? I believe so, but it isn't totally clear to me. Thanks, Taylor
