Subject: Re: [PATCH] diff: add support for reading files literally with --no-index

From: Jonathan Nieder <[email-masked]>
Date: Thu, 3 Jan 2019 18:18:55 -0800

Hi,

brian m. carlson wrote:

> In some shells, such as bash and zsh, it's possible to use a command
> substitution to provide the output of a command as a file argument to
> another process, like so:
>
>   diff -u <(printf "a\nb\n") <(printf "a\nc\n")
>
> However, this syntax does not produce useful results with git diff
> --no-index.

Thanks much for fixing this.  It's something I've run into, too.

[...]
> --- a/Documentation/git-diff.txt
> +++ b/Documentation/git-diff.txt
> @@ -111,6 +111,11 @@ include::diff-options.txt[]
>  	"Unmerged".  Can be used only when comparing the working tree
>  	with the index.
>  
> +--literally::
> +  Read the specified files literally, as `diff` would,
> +  dereferencing any symlinks and reading data from pipes.
> +  This option only works with `--no-index`.

I may be a minority in this opinion, but I had trouble understanding
what --literally would do from its name.  I suspect we can come up
with a better name.

Unfortunately, I'm terrible at coming up with names. :-P

--dereference would be a good name when it comes to symlinks, but
it's not a good name for reading what is on the other side of a pipe.
On the plus side, it matches "diff" and "cp"'s name for the "follow
symbolic links" option.

--plain captures the desire a little better --- we want a plain
read(2) from the file instead of trying to be smart and look at
whether it is e.g. a block device.  But in the context of "diff", that
would seem more like an option that affects the output.

What would you think of

 - always reading from fifos instead of describing them, since I've
   never encountered a use case where people want the latter

 - --dereference to control whether to follow symlinks

?

[...]
> --- a/diff-no-index.c
> +++ b/diff-no-index.c
> @@ -75,7 +75,25 @@ static int populate_from_stdin(struct diff_filespec *s)
>  	return 0;
>  }
>  
> -static struct diff_filespec *noindex_filespec(const char *name, int mode)
> +static int populate_literally(struct diff_filespec *s)
> +{
> +	struct strbuf buf = STRBUF_INIT;
> +	size_t size = 0;
> +	int fd = xopen(s->path, O_RDONLY);
> +
> +	if (strbuf_read(&buf, fd, 0) < 0)
> +		return error_errno("error while reading from '%s'", s->path);
> +
> +	s->should_munmap = 0;
> +	s->data = strbuf_detach(&buf, &size);
> +	s->size = size;
> +	s->should_free = 1;
> +	s->read_literally = 1;

Oh!  --read-literally works perfectly for me as a name. :)

Jonathan