Re: [PATCH v2 2/2] diff --no-index: support reading from pipes

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 13, 2017, 23:24 UTC
Message-ID: <xmqqo9zaee6t.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170113102021.6054-3-dennis@kaarsemaker.net>

Dennis Kaarsemaker <dennis@kaarsemaker.net> writes:

Show 8 quoted lines

> +	/*
> +	 * In --no-index mode, we support reading from pipes. canon_mode, called by
> +	 * fill_filespec, gets confused by this and thinks we now have subprojects.
> +	 * Detect this and tell the rest of the diff machinery to treat pipes as
> +	 * normal files.
> +	 */
> +	if (S_ISGITLINK(s->mode))
> +		s->mode = S_IFREG | ce_permissions(mode);

Hmph. Pipes on your system may satisfy S_ISGITLINK() and confuse later code, and this hack may work it around. But a proper gitlink that was thrown at this codepath (probably by mistake) will also be caught and pretend as if it were a regular file. Do we know for certain that pipes everywhere will be munged to appear as S_ISGITLINK()? Is it possible to do the "are we looking at an end of a pipe?" check _before_ canon_mode() munges and stores the result in s->mode in diff-no-index.c somewhere, perhaps inside get_mode()?

Show 22 quoted lines

> diff --git a/diff.c b/diff.c
> index 2fc0226338..bb04eab331 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -2839,9 +2839,18 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
>  		fd = open(s->path, O_RDONLY);
>  		if (fd < 0)
>  			goto err_empty;
> -		s->data = xmmap(NULL, s->size, PROT_READ, MAP_PRIVATE, fd, 0);
> +		if (!S_ISREG(st.st_mode)) {
> +			struct strbuf sb = STRBUF_INIT;
> +			strbuf_read(&sb, fd, 0);
> +			s->size = sb.len;
> +			s->data = strbuf_detach(&sb, NULL);
> +			s->should_free = 1;
> +		}
> +		else {
> +			s->data = xmmap(NULL, s->size, PROT_READ, MAP_PRIVATE, fd, 0);
> +			s->should_munmap = 1;
> +		}
>  		close(fd);
> -		s->should_munmap = 1;

I like the fact that, by extending the !S_ISREG() check this patch introduces, we can later use the new "do not mmap but allocate to read" codepath for small files, which may be more efficient. We may want to have a small helper

	static int should_mmap_file_contents(struct stat *st)
	{
		return S_ISREG(st->st_mode);
	}

so that we can do such an enhancement later more easily.

So, I am skeptical with the "do we have pipe" check in the earlier hunk, but otherwise I think what this patch wanted to solve is a reasonable problem to tackle.

Thanks.