Re: fatal error when diffing changed symlinks

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 24, 2017, 19:51 UTC
Message-ID: <xmqqtw7j1i6d.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <xmqqshn34gsh.fsf@gitster.mtv.corp.google.com>

Junio C Hamano <gitster@pobox.com> writes:

Show 15 quoted lines

>> cd /tmp
>> mkdir a
>> cd a
>> git init
>> touch b
>> ln -s b c
>> git add .
>> git commit -m 'first'
>> touch d
>> rm c
>> ln -s d c
>> git difftool --dir-diff
>
> A slightly worse is that the upcoming Git will ship with a rewritten
> "difftool" that makes the above sequence segfault.

The culprit seems to be these lines in run_dir_diff():

		if (S_ISLNK(lmode)) {
			char *content = read_sha1_file(loid.hash, &type, &size);
			add_left_or_right(&symlinks2, src_path, content, 0);
			free(content);
		}

		if (S_ISLNK(rmode)) {
			char *content = read_sha1_file(roid.hash, &type, &size);
			add_left_or_right(&symlinks2, dst_path, content, 1);
			free(content);
		}

When viewing a working tree file, oid.hash could be 0{40} and read_sha1_file() is not the right function to use to obtain the contents.

Both of these two need to pay attention to 0{40}, I think, as the user may be running "difftool -R --dir-diff" in which case the working tree would appear in the left hand side instead.

I didn't follow the codepath for regular files closely, but the code that follows the above excerpt does quite different things to lstate and rstate, which makes me suspect that the code is not prepared to see "-R"(everse) diff (and I further suspect that these issues were inherited from the original scripted Porcelain).