Re: [PATCH v3 2/5] object-file.c: handle undetermined oid in write_loose_object()

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 29, 2021, 20:44 UTC
Message-ID: <xmqqsfve669w.fsf@gitster.g>
In-Reply-To: <47b3e2ad-4fa1-040a-24c1-6da0445bd1a5@gmail.com>

Derrick Stolee <stolee@gmail.com> writes:

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> My first reaction is to not write into .git/objects/ directly, but
> instead make a .git/objects/tmp/ directory and write within that
> directory. The idea is to prevent leaving stale files in the
> .git/objects/ directory if the process terminates strangely (say,
> a power outage or segfault).

Even if we know the name of the object we are writing beforehand, I do not think it is a good idea to open-write-close the final object file. The approach we already use everywhere is to write into a tmpfile/lockfile and rename it to the final name

object-file.c::write_loose_object() uses create_tmpfile() to prepare a temporary file whose name begins with "tmp_obj_", so that "gc" can recognize stale ones and remove them.

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> If this was an interesting idea to pursue, it does leave a question:
> should we clean up the tmp/ directory when it is empty? That would
> require adding a check in finalize_object_file() that is probably
> best left unchecked (the lstat() would add a cost per loose object
> write that is probably too costly). I would rather leave an empty
> tmp/ directory than add that cost per loose object write.

I am not sure why we want a new tmp/ directory.