[PATCH] pathspec: warn on empty strings as pathspec

[Emily Xie <emilyxxie@gmail.com>]

Currently, passing an empty string as a pathspec results in
a match on all paths. This is because a pathspec match is a
leading substring match that honors directory boundaries.
So, just as pathspec "dir/" (or "dir") matches "dir/file",
"" matches "file".

However, this causes problems. Namely, a user's
carelessly-written script could accidentally assign an
empty string to a variable that then gets passed to a Git
command invocation, e.g.:

        git rm -r "$path"
        git add "$path"

which would unintentionally affect 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 invokes a
warning that (1) declares using empty strings to
match all paths will become invalid and (2) asks the user
to use "." if their intent was to match all.

For step two, a follow-up patch several release cycles
later will remove the warnings 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>
---
 pathspec.c     | 11 +++++++++--
 t/t3600-rm.sh  |  5 +++++
 t/t3700-add.sh |  5 +++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/pathspec.c b/pathspec.c
index c9e9b6c..02aa691 100644
--- a/pathspec.c
+++ b/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);
diff --git a/t/t3600-rm.sh b/t/t3600-rm.sh
index d046d98..14f0edc 100755
--- a/t/t3600-rm.sh
+++ b/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
diff --git a/t/t3700-add.sh b/t/t3700-add.sh
index f14a665..05379d0 100755
--- a/t/t3700-add.sh
+++ b/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
-- 
2.8.4