Subject: Re: [PATCH] Enable core.fsyncObjectFiles by default

From: Junio C Hamano <[email-masked]>
Date: Tue, 23 Jun 2015 22:32:08 -0700

Theodore Ts'o <[email-masked]> writes:

> The main issue is that non-expert users might not realize that they
> really need to run "git fsck" after a crash; otherwise, what might
> happen is that although git is only appending, that you might have
> some zero-length (or partially written) git object or pack files, and
> while you might not notice at that moment, it might come and bite you
> later.

Regarding loose object files, given that we write to a temporary,
optionally fsync, close and then move to the final name, would we
still see partially written file if we omit the fsync, or would the
corruption be limited to either empty or missing?

The reason I am wondering is because the codepath to create an
object (i.e. "update-index --add", "hash-object -w", or "add") first
checks if a packed or a loose object file _exists_ and if so
bypasses writing the same thing anew, but the existence check for a
loose object is to merely making sure that access(F_OK) (and
optionally utime()) succeeds.  If the potential breakage is limited
to truncation to empty, then we could replace it with stat(2) and
st.st_size check, as no loose object file can be empty.