Crash on empty pickaxe argument

> I fat-fingered my enter key today and noticed this crash:
> 
> $ git --version
> git version 2.48.1
> $ mkdir foo && cd foo
> $ git init
> <snip>
> $ git commit --allow-empty -m "foo"
> [master (root-commit) ed4e080] foo
> $ git log -S ""
> BUG: diffcore-pickaxe.c:241: should have needle under -G or -S
> [1]    65421 IOT instruction (core dumped)  git log -S ""
> $ git log -G ""
> BUG: diffcore-pickaxe.c:241: should have needle under -G or -S
> [1]    65457 IOT instruction (core dumped)  git log -G ""
> 
> 
> Well, it says BUG right there. It probably shouldn't crash. Relevant
> backtrace for -S:

>
> The pickaxe options, -G and -S, need either a regex or a string to look
> through the history for.  An empty value isn't very useful since it
> would either match everything or nothing, and what's worse, we presently
> crash with a BUG like so when the user provides one:
>
>     BUG: diffcore-pickaxe.c:241: should have needle under -G or -S
>
> Since it's not very nice of us to crash and this wouldn't do anything
> useful anyway, let's simply inform the user that they must provide a
> non-empty argument and exit with an error if they provide an empty one
> instead.

> Reported-by: Jared Van Bortel <cebtenzzre@gmail.com>
> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> ---
>  diff.c                 |  4 ++++
>  t/t4209-log-pickaxe.sh | 16 ++++++++++++++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/diff.c b/diff.c
> index 019fb893a7..c89c15d98e 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -5493,6 +5493,8 @@ static int diff_opt_pickaxe_regex(const struct option *opt,
>         BUG_ON_OPT_NEG(unset);
>         options->pickaxe = arg;
>         options->pickaxe_opts |= DIFF_PICKAXE_KIND_G;
> +       if (arg && !*arg)
> +               return error(_("-G requires a non-empty argument"));
>         return 0;
>  }
>
> @@ -5504,6 +5506,8 @@ static int diff_opt_pickaxe_string(const struct option *opt,
>         BUG_ON_OPT_NEG(unset);
>         options->pickaxe = arg;
>         options->pickaxe_opts |= DIFF_PICKAXE_KIND_S;
> +       if (arg && !*arg)
> +               return error(_("-S requires a non-empty argument"));
>         return 0;
>  }

> diff --git a/t/t4209-log-pickaxe.sh b/t/t4209-log-pickaxe.sh
> index a675ace081..0e2f80a268 100755
> --- a/t/t4209-log-pickaxe.sh
> +++ b/t/t4209-log-pickaxe.sh
> @@ -93,6 +93,22 @@ test_expect_success 'usage: --no-pickaxe-regex' '
>         test_cmp expect actual
>  '
>
> +test_expect_success 'usage: -G and -S with empty argument' '
> +       cat >expect <<-\EOF &&
> +       error: -S requires a non-empty argument
> +       EOF
> +
> +       test_expect_code 129 git log -S "" 2>actual &&
> +       test_cmp expect actual &&
> +
> +       cat >expect <<-\EOF &&
> +       error: -G requires a non-empty argument
> +       EOF
> +
> +       test_expect_code 129 git log -G "" 2>actual &&
> +       test_cmp expect actual
> +'

> The pickaxe options, -G and -S, need either a regex or a string to look
> through the history for.  An empty value isn't very useful since it
> would either match everything or nothing, and what's worse, we presently
> crash with a BUG like so when the user provides one:
>
>     BUG: diffcore-pickaxe.c:241: should have needle under -G or -S

>
> Reported-by: Jared Van Bortel <cebtenzzre@gmail.com>
> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> ---
>  diff.c                 |  4 ++++
>  t/t4209-log-pickaxe.sh | 16 ++++++++++++++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/diff.c b/diff.c
> index 019fb893a7..c89c15d98e 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -5493,6 +5493,8 @@ static int diff_opt_pickaxe_regex(const struct option *opt,
>  	BUG_ON_OPT_NEG(unset);
>  	options->pickaxe = arg;
>  	options->pickaxe_opts |= DIFF_PICKAXE_KIND_G;
> +	if (arg && !*arg)
> +		return error(_("-G requires a non-empty argument"));
>  	return 0;
>  }
>  
> @@ -5504,6 +5506,8 @@ static int diff_opt_pickaxe_string(const struct option *opt,
>  	BUG_ON_OPT_NEG(unset);
>  	options->pickaxe = arg;
>  	options->pickaxe_opts |= DIFF_PICKAXE_KIND_S;
> +	if (arg && !*arg)
> +		return error(_("-S requires a non-empty argument"));
>  	return 0;
>  }
>  
> diff --git a/t/t4209-log-pickaxe.sh b/t/t4209-log-pickaxe.sh
> index a675ace081..0e2f80a268 100755
> --- a/t/t4209-log-pickaxe.sh
> +++ b/t/t4209-log-pickaxe.sh
> @@ -93,6 +93,22 @@ test_expect_success 'usage: --no-pickaxe-regex' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success 'usage: -G and -S with empty argument' '
> +	cat >expect <<-\EOF &&
> +	error: -S requires a non-empty argument
> +	EOF
> +
> +	test_expect_code 129 git log -S "" 2>actual &&
> +	test_cmp expect actual &&
> +
> +	cat >expect <<-\EOF &&
> +	error: -G requires a non-empty argument
> +	EOF
> +
> +	test_expect_code 129 git log -G "" 2>actual &&
> +	test_cmp expect actual
> +'
> +
>  test_log	expect_initial	--grep initial
>  test_log	expect_nomatch	--grep InItial
>  test_log_icase	expect_initial	--grep InItial

> I agree BUG is unwelcome.  I am not sure about the value of
> forbidding an empty string (I am sure about forbidding NULL,
> though).  
> 
> If an empty matches everything, "git log -S" would skip changes that
> would keep the number of lines, right?  For the history of a project
> that keeps track of source code, such a "feature" would not be
> useful, but I can see a complaint by somebody who may want to keep
> track of a "list of things" one-item-per-line, if we had been
> allowing an empty string.  It would be a regression for such a niche
> user.

> So I'd say that it may be a bit premature for us to declare
> "anything useful", I am perfectly fine with the patch given here.
> If somebody who wants to maintain a text file, one-item-per-line
> that keeps track of a list of things to omit commits that do not
> change the number of items, they can drop "&& !*arg" part, tweak the
> message and add their own tests, once this fix lands and the dust
> settles.