Subject: Re: git diff -U0 header off-by-one error when deleting no lines

From: Junio C Hamano <[email-masked]>
Date: Wed, 03 May 2023 14:19:23 -0700

Lukas Tenbrink <[email-masked]> writes:

> Thank you for filling out a Git bug report!
> Please answer the following questions to help us understand your issue.
>
> What did you do before the bug happened? (Steps to reproduce your issue)
>
> Full reproduction:
>
> echo "A\nB\nC" > test1
> echo "A\nB\nC\nD\nE\nF" > test2

For many folks, the above will place one line each in these files.
For portability, you'd need to do something like

	printf "%s\n" A B C >test1
	printf "%s\n" A B C D E F >test2

> git diff -U0 test1 test2

> The actual output places the new text at line 3:
>
>> git diff -U0 test1 test2
> diff --git a/test1 b/test2
> index b1e6722..cead32e 100644
> --- a/test1
> +++ b/test2
> @@ -3,0 +4,3 @@ C
> +D
> +E
> +F

I think this is very much in line with how "diff -U0" by other
people do it, and any tool like "git am" that need to read "diff"
output need to understand how these output work correctly; "-U0"
output and ",1" that is omitted are things that have confused us
tool writers forever ;-).

With -U0, you'd get

    $ diff -U0 test1 test2
    --- test1       2023-05-03 14:02:27.718960038 -0700
    +++ test2       2023-05-03 14:02:20.094156573 -0700
    @@ -3,0 +4,3 @@
    +D
    +E
    +F

which has "-3,0" that says "there were 3 lines before this hunk that
did not have any line before the change".

This might be unintuitive, but it is specified by POSIX.  

Open

  https://pubs.opengroup.org/onlinepubs/9699919799/

and look for "@@".

    "@@-%s+%s@@", <file1 range>, <file2 range>

    Each <range> field shall be of the form:

    "%1d", <beginning line number>

    or:

    "%1d,1", <beginning line number>

    if the range contains exactly one line, and:

    "%1d,%1d", <beginning line number>, <number of lines>

    otherwise.

    If a range is empty, its beginning line number shall be the
    number of the line just before the range, or 0 if the empty
    range starts the file.

So a patch that adds lines to an empty file would look like

    $ diff -U0 /dev/null test1
    --- /dev/null   2023-04-29 22:24:54.395999895 -0700
    +++ test1       2023-05-03 14:02:27.718960038 -0700
    @@ -0,0 +1,3 @@
    +A
    +B
    +C

just as specified.  Note "-0,0" that says "there was no line before
this hunk before this change".