BUG: fast-import, ftruncate, and file mode

[Elijah Newren <newren@gmail.com>]

Hi,

fast-import makes use of odb_mkstemp(), which creates a secure
temporary file and opens it with mode 0444, and then uses it for its
packfile writing.  Sometimes, fast-import will call its
truncate_pack() function, which makes use of ftruncate().

According to my local manpage, "With ftruncate(), the file must be
open for writing; with truncate(), the file must be writable."

The writable requirement does not appear to be enforced by the kernel
on common filesystems like ext4 or zfs, but this is enforced on some
filesystems.  Apparently a "VxFS Veritas filesystem" got triggered by
this...and some helpful bug reporters tracked this problem down and
found a workaround (for the filter-repo usecase, they recompiled a
special copy of git using mode 0644 for odb_mkstemp, since it was just
an intermediate step anyway and won't be used elsewhere).

I'm not sure if we should (1) just stop calling truncate_pack() in
fast-import (it's merely an optimization), or (2) modify odb_mkstemp()
to allow specifying a mode and also have something like
finalize_object_file() modify the mode before renaming, or (3) if we
should do something else entirely.  Someone more familiar with the
object storage side of things probably knows.  But I'm guessing that
once someone who knows the area states what should be done, that this
might be a small micro-project suitable for the GSoC interns (or
anyone else wanting to get involved)?

Original details from here:
https://github.com/newren/git-filter-repo/issues/342#issuecomment-1047638304