Re: [PATCH 2/2] packfile: replace lseek+read with pread

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 26, 2019, 18:23 UTC
Message-ID: <xmqqzhffkku2.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20191226104220.27325-3-e@80x24.org>

Eric Wong <e@80x24.org> writes:

Show 6 quoted lines

> We already have pread emulation for portability, so there's
> there's no reason to make two syscalls where one suffices.
>
> Furthermore, readers of the packfile will be using mmap
> (or pread to emulate mmap), anyways, so the file description
> offset does not matter in this case.

s/description/descriptor/ probably.

After seeking to the packfile trailer and reading the pack id hash using lseek+read, this helper function does not read from the file descriptor, and the sole caller of it closes the file descriptor immediately after it returns, which means the read file offset after reading the packfile trailer does not matter.

So this conversion is correct.  Thanks for a careful analysis.

Will queue both patches.

Show 21 quoted lines

> Signed-off-by: Eric Wong <e@80x24.org>
> ---
>  packfile.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/packfile.c b/packfile.c
> index 1821cb7a3d..7e7c04e4d8 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -576,9 +576,8 @@ static int open_packed_git_1(struct packed_git *p)
>  			     " while index indicates %"PRIu32" objects",
>  			     p->pack_name, ntohl(hdr.hdr_entries),
>  			     p->num_objects);
> -	if (lseek(p->pack_fd, p->pack_size - hashsz, SEEK_SET) == -1)
> -		return error("end of packfile %s is unavailable", p->pack_name);
> -	read_result = read_in_full(p->pack_fd, hash, hashsz);
> +	read_result = pread_in_full(p->pack_fd, hash, hashsz,
> +					p->pack_size - hashsz);
>  	if (read_result < 0)
>  		return error_errno("error reading from %s", p->pack_name);
>  	if (read_result != hashsz)