On 2020-09-18 at 11:32:56, Thomas Guyot-Sionnest wrote:
> diff --git a/diff-no-index.c b/diff-no-index.c
> index 7814eabfe0..779c686d23 100644
> --- a/diff-no-index.c
> +++ b/diff-no-index.c
> @@ -41,6 +41,33 @@ static int read_directory_contents(const char *path, struct string_list *list)
>   */
>  static const char file_from_standard_input[] = "-";
>  
> +/* Check that file is - (STDIN) or unnamed pipe - explicitly
> + * avoid on-disk named pipes which could block
> + */
> +static int ispipe(const char *name)
> +{
> +	struct stat st;
> +
> +	if (name == file_from_standard_input)
> +		return 1;  /* STDIN */
> +
> +	if (!lstat(name, &st)) {
> +		if (S_ISLNK(st.st_mode)) {
> +			/* symlink - read it and check it doesn't exists
> +			 * as a file yet link to a pipe */
> +			struct strbuf sb = STRBUF_INIT;
> +			strbuf_realpath(&sb, name, 0);
> +			/* We're abusing strbuf_realpath here, it may append
> +			 * pipe:[NNNNNNNNN] to an abs path */
> +			if (!stat(sb.buf, &st))
> +				return 0; /* link target exists , not pipe */
> +			if (!stat(name, &st))
> +				return S_ISFIFO(st.st_mode);

This makes a lot of assumptions about the implementation which are
specific to Linux, namely that an anonymous pipe will be a symlink to a
FIFO.  That's not the case on macOS, where the /dev/fd entries are
actually named pipes themselves.

Granted, in that case, Git just chokes and fails instead of diffing the
symlink values, but I suspect you'll want this to work on macOS as well.
I don't use macOS that often, but I would appreciate it if it worked
when I did, and I'm sure others will as well.

I think we can probably get away with just doing a regular stat and
seeing if S_ISFIFO is true, which is both simpler and cheaper.

> diff --git a/t/t4053-diff-no-index.sh b/t/t4053-diff-no-index.sh
> index 0168946b63..e49f773515 100755
> --- a/t/t4053-diff-no-index.sh
> +++ b/t/t4053-diff-no-index.sh
> @@ -144,4 +144,193 @@ test_expect_success 'diff --no-index allows external diff' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success 'diff --no-index can diff piped subshells' '
> +	echo 1 >non/git/c &&
> +	test_expect_code 0 git diff --no-index non/git/b <(cat non/git/c) &&
> +	test_expect_code 0 git diff --no-index <(cat non/git/b) non/git/c &&
> +	test_expect_code 0 git diff --no-index <(cat non/git/b) <(cat non/git/c) &&
> +	test_expect_code 0 cat non/git/b | git diff --no-index - non/git/c &&
> +	test_expect_code 0 cat non/git/c | git diff --no-index non/git/b - &&
> +	test_expect_code 0 cat non/git/b | git diff --no-index - <(cat non/git/c) &&
> +	test_expect_code 0 cat non/git/c | git diff --no-index <(cat non/git/b) -
> +'

As mentioned by others, this requires non-POSIX syntax.  /bin/sh on my
Debian system is dash, which doesn't support this.  You can either use a
prerequisite, or just test by piping from standard input and assume that
Git can handle the rest.  I would recommend at least adding some POSIX
testcases that use only a pipe from standard input to avoid regressing
this behavior on Debian and Ubuntu.
-- 
brian m. carlson: Houston, Texas, US