patchpathspec: warn on empty strings as pathspec

For any command that takes a pathspec, passing an empty string will execute the command on all files in the current directory. This results in unexpected behavior. For example, git add "" adds all files to staging, while git rm -rf "" recursively removes all files from the working tree and index. A two-step implemetation will prevent such cases.

This patch, as step one, invokes a warning whenever an empty string is detected as a pathspec, introducing users to the upcoming change. For step two, a follow-up patch several release cycles later will remove the warnings and actually implement the change by throwing an error instead.

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     | 6 +++++-
 t/t3600-rm.sh  | 6 +++++-
 t/t3700-add.sh | 4 ++++
 3 files changed, 14 insertions(+), 2 deletions(-)

Emily Xie <emilyxxie@gmail.com> writes:

OK.

Technically speaking, your expectation is what is wrong here. An empty string as a pathspec matches all paths.

That is a logical consequence that an empty string is a pathspec that matches everything. If somebody wants to add everything in their current directory, they can say 'git add ""'. If you do not want to do so, don't say 'git add ""'.

You need to argue why these are bad things to convince those who are used to the current behaviour that it is OK to break them.

Here is my attempt.

	An pathspec that is an empty string has been interpreted to
	match any path, as pathspec match is a leading substring
	match that honours directory boundary.  Just like pathspec
	"dir/" (or "dir") matches "dir/file", "" matches "file".

	However, a carelessly-written script may result in an empty
	string assigned to a variable (perhaps caused by a bug in
	it), and may end up passing an empty string to a Git command
	invocation, i.e.

        	git rm "$path"
        	git add "$path"
		
	which would affect all paths in the current directory.

	We cannot simply reject an empty string given as a pathspec
	to prevent this kind of mistake.  Because there surely are
	existing scripts that take advantage of the fact that an
	empty string matches all paths, such a change will break
	scripts that legitimately have been using an empty string
	for that purpose.

	Instead, we need two step approach.  The first step is to
	notice that the caller used an empty string as a pathspec,
	give a warning to (1) declare that the use of an empty
	string as "match all" will become invalid and (2) ask them
	to use "." instead if they mean "match all".

	After some release cycles, we can remove the warning and
	turn an empty string used as a pathspec element as an error.

	This patch is the first step.

There is some leap/gap in logic here.  

This paragraph is OK, but I think I ended up covering the whole thing in my attempt ;-).

Three issues:

 * if argv[1] and argv[2] are both emtpy, the user will see the same
   message twice.  Is it intended?  Is it acceptable?

 * "Empty strings are not valid pathspecs" is just plain wrong.  It
   has been valid, but the warning message notifies that we are
   going to make it invalid what has been valid.

 * "Will no longer be supported" is just plain useless.  We are
   notifying that we will turn what they have been using as a valid
   feature invalid.  What needs to accompany that notification is
   how to update their script that have been happily working, which
   we are going to break with the future change, in a way that will
   keep working, i.e. "please use . instead if you meant to match
   all".

As your warning is in _("..."), you would need test_i18grep here, I think.

Oops.