pathspec: warn on empty strings as pathspec

An empty string as a pathspec element matches all paths.  A buggy
script, however, could accidentally assign an empty string to a
variable that then gets passed to a Git command invocation, e.g.:

	path=... compute a path to be removed in $path ...
        git rm -r "$paht"

which would unintentionally remove all paths in the current
directory.

The fix for this issue requires a two-step approach. As there may be
existing scripts that knowingly use empty strings in this manner,
the first step simply gives a warning that (1) tells that an empty
string will become an invalid pathspec element and (2) asks the user
to use "." if they mean to match all.

For step two, a follow-up patch several release cycles later will
remove the warning and throw an error instead.

This patch is the first step.

Signed-off-by: Emily Xie <emilyxxie@gmail.com>
Reported-by: David Turner <novalis@novalis.org>
Mentored-by: Michail Denchev <mdenchev@gmail.com>
Thanks-to: Sarah Sharp <sarah@thesharps.us> and James Sharp <jamey@minilop.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.c

@@ -364,7 +364,7 @@ void parse_pathspec(struct pathspec *pathspec,
 {
 	struct pathspec_item *item;
 	const char *entry = argv ? *argv : NULL;
-	int i, n, prefixlen, nr_exclude = 0;
+	int i, n, prefixlen, warn_empty_string, nr_exclude = 0;
 
 	memset(pathspec, 0, sizeof(*pathspec));
 
@@ -402,8 +402,15 @@ void parse_pathspec(struct pathspec *pathspec,
 	}
 
 	n = 0;
-	while (argv[n])
+	warn_empty_string = 1;
+	while (argv[n]) {
+		if (*argv[n] == '\0' && warn_empty_string) {
+			warning(_("empty strings as pathspecs will be made invalid in upcoming releases. "
+				  "please use . instead if you meant to match all paths"));
+			warn_empty_string = 0;
+		}
 		n++;
+	}
 
 	pathspec->nr = n;
 	ALLOC_ARRAY(pathspec->items, n);

t/t3600-rm.sh

@@ -881,4 +881,9 @@ test_expect_success 'rm files with two different errors' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'rm empty string should invoke warning' '
+	git rm -rf "" 2>output &&
+	test_i18ngrep "warning: empty strings" output
+'
+
 test_done

t/t3700-add.sh

@@ -332,4 +332,9 @@ test_expect_success 'git add --dry-run --ignore-missing of non-existing file out
 	test_i18ncmp expect.err actual.err
 '
 
+test_expect_success 'git add empty string should invoke warning' '
+	git add "" 2>output &&
+	test_i18ngrep "warning: empty strings" output
+'
+
 test_done