Re: [GSoC] How to protect cached_objects

> Hi, everyone
>
> As one of my first tasks in GSoC, I'm looking to protect the global
> states at sha1-file.c for future parallelizations. Currently, I'm
> analyzing how to deal with the cached_objects array, which is a small
> set of in-memory objects that read_object_file() is able to return
> although they don't really exist on disk. The only current user of
> this set is git-blame, which adds a fake commit containing
> non-committed changes.
>
> As it is now, if we start parallelizing blame, cached_objects won't be
> a problem since it is written to only once, at the beginning, and read
> from a couple times latter, with no possible race conditions.
>
> But should we make these operations thread safe for future uses that
> could involve potential parallel writes and reads too?
>
> If so, we have two options:
> - Make the array thread local, which would oblige us to replicate data, or
> - Protect it with locks, which could impact the sequential
> performance. We could have a macro here, to skip looking on
> single-threaded use cases. But we don't know, a priori, the number of
> threads that would want to use the pack access code.
>
> Any thought on this?