Subject: Re: [PATCH v3 2/3] diff: Let "git diff -O" read orderfile from any file, failing when appropriate

From: Junio C Hamano <[email-masked]>
Date: Mon, 16 Dec 2013 10:43:15 -0800

Samuel Bronson <[email-masked]> writes:

> The -O flag really shouldn't silently fail to do anything when given a
> path that it can't read from.
>
> However, it should be able to read from un-mappable files, such as
> pipes/fifos, /dev/null (as we document in the next patch), or in fact
> *any* empty file (since Linux 2.6.12).

Could you enlighten the commit log readers a bit better here?  Those
who know the change in 2.6.12 (i.e. "'mmapping with length 0 must
fail', says SUSv3, so we fail") you have in mind would know what you
mean by "in fact any empty file" even if you did not have "(since
Linux 2.6.12)", but those who do not know it would not be helped
with just "(since Linux 2.6.12)".

> (Especially since we will be
> documenting "-O/dev/null" to override "diff.orderfile" when we add that.)
>
> (Note: "-O/dev/null" did have the right effect, since the existing error
> handling essentially worked out to "silently ignore the orderfile".)
>
> So lets toss all of that logic to get the file mmapped and just use
> strbuf_read_file() instead, which gives us decent error handling
> practically for free.

Sounds good.  In the longer term, we may want to move this
file-scope static to per-infocation "struct diff_options" and clean
up the storage used to hold the list of path patterns after we are
done with the diff, but that is outside the scope of this series.

Thanks.

> Signed-off-by: Samuel Bronson <[email-masked]>
> ---
>  diffcore-order.c      | 23 ++++++++---------------
>  t/t4056-diff-order.sh | 23 +++++++++++++++++++++++
>  2 files changed, 31 insertions(+), 15 deletions(-)
>
> diff --git a/diffcore-order.c b/diffcore-order.c
> index 23e9385..a63f332 100644
> --- a/diffcore-order.c
> +++ b/diffcore-order.c
> @@ -10,28 +10,21 @@ static int order_cnt;
>  
>  static void prepare_order(const char *orderfile)
>  {
> -	int fd, cnt, pass;
> +	int cnt, pass;
> +	struct strbuf sb = STRBUF_INIT;
>  	void *map;
>  	char *cp, *endp;
> -	struct stat st;
> -	size_t sz;
> +	ssize_t sz;
>  
>  	if (order)
>  		return;
>  
> -	fd = open(orderfile, O_RDONLY);
> -	if (fd < 0)
> -		return;
> -	if (fstat(fd, &st)) {
> -		close(fd);
> -		return;
> -	}
> -	sz = xsize_t(st.st_size);
> -	map = mmap(NULL, sz, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
> -	close(fd);
> -	if (map == MAP_FAILED)
> -		return;
> +	sz = strbuf_read_file(&sb, orderfile, 0);
> +	if (sz < 0)
> +		die_errno(_("failed to read orderfile '%s'"), orderfile);
> +	map = strbuf_detach(&sb, NULL);
>  	endp = (char *) map + sz;
> +
>  	for (pass = 0; pass < 2; pass++) {
>  		cnt = 0;
>  		cp = map;
> diff --git a/t/t4056-diff-order.sh b/t/t4056-diff-order.sh
> index 398b3f6..eb471e7 100755
> --- a/t/t4056-diff-order.sh
> +++ b/t/t4056-diff-order.sh
> @@ -61,12 +61,35 @@ test_expect_success "no order (=tree object order)" '
>  	test_cmp expect_none actual
>  '
>  
> +test_expect_success 'missing orderfile' '
> +	rm -f bogus_file &&
> +	test_must_fail git diff -Obogus_file --name-only HEAD^..HEAD
> +'
> +
> +test_expect_success 'unreadable orderfile' '
> +	touch unreadable_file &&
> +	chmod -r unreadable_file &&
> +	test_must_fail git diff -Ounreadable_file --name-only HEAD^..HEAD
> +'
> +
> +test_expect_success 'orderfile is a directory' '
> +	test_must_fail git diff -O/ --name-only HEAD^..HEAD
> +'
> +
>  for i in 1 2
>  do
>  	test_expect_success "orderfile using option ($i)" '
>  	git diff -Oorder_file_$i --name-only HEAD^..HEAD >actual &&
>  	test_cmp expect_$i actual
>  '
> +
> +	test_expect_success PIPE "orderfile is fifo ($i)" '
> +	rm -f order_fifo &&
> +	mkfifo order_fifo &&
> +	cat order_file_$i >order_fifo &
> +	git diff -O order_fifo --name-only HEAD^..HEAD >actual &&
> +	test_cmp expect_$i actual
> +'
>  done
>  
>  test_done