[PATCH v2] clone: error specifically with --local and symlinked objects

From: Glen Choo <chooglen@google.com>

6f054f9fb3 (builtin/clone.c: disallow --local clones with symlinks, 2022-07-28) gives a good error message when "git clone --local" fails when the repo to clone has symlinks in "$GIT_DIR/objects". In bffc762f87 (dir-iterator: prevent top-level symlinks without FOLLOW_SYMLINKS, 2023-01-24), we later extended this restriction to the case where "$GIT_DIR/objects" is itself a symlink, but we didn't update the error message then - bffc762f87's tests show that we print a generic "failed to start iterator over" message.

This is exacerbated by the fact that Documentation/git-clone.txt mentions neither restriction, so users are left wondering if this is intentional behavior or not.

Fix this by adding a check to builtin/clone.c: when doing a local clone, perform an extra check to see if "$GIT_DIR/objects" is a symlink, and if so, assume that that was the reason for the failure and report the relevant information. Ideally, dir_iterator_begin() would tell us that the real failure reason is the presence of the symlink, but (as far as I can tell) there isn't an appropriate errno value for that.

Also, update Documentation/git-clone.txt to reflect that this restriction exists.

Signed-off-by: Glen Choo <chooglen@google.com>
---
    clone: error specifically with --local and symlinked objects
    
    Thanks for the close review on v1, folks.
    
    I tried teaching dir_iterator_begin() to return an exit code to indicate
    that we found a symlink, but it ended up looking quite ugly when I
    started to consider plain files as well as generic lstat failures. I
    think the extra lstat() is fine as a one-off, but if we need another
    instance of this, we'd be better off doing the refactor.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1488%2Fchooglen%2Fpush-nymnqqnttzxz-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1488/chooglen/push-nymnqqnttzxz-v2
Pull-Request: https://github.com/git/git/pull/1488

Range-diff vs v1:

 1:  5c2d5eb10cd ! 1:  574cea062cf clone: error specifically with --local and symlinked objects
     @@ builtin/clone.c: static void copy_or_link_directory(struct strbuf *src, struct s
       
      -	if (!iter)
      +	if (!iter) {
     -+		struct stat st;
     -+		if (lstat(src->buf, &st) >= 0 && S_ISLNK(st.st_mode))
     -+			die(_("'%s' is a symlink, refusing to clone with --local"),
     -+			    src->buf);
     ++		if (errno == ENOTDIR) {
     ++			int saved_errno = errno;
     ++			struct stat st;
     ++			if (lstat(src->buf, &st) == 0 && S_ISLNK(st.st_mode))
     ++				die(_("'%s' is a symlink, refusing to clone with --local"),
     ++				    src->buf);
     ++			errno = saved_errno;
     ++		}
       		die_errno(_("failed to start iterator over '%s'"), src->buf);
      +	}
       

 Documentation/git-clone.txt |  5 +++++
 builtin/clone.c             | 11 ++++++++++-
 t/t5604-clone-reference.sh  |  2 +-
 3 files changed, 16 insertions(+), 2 deletions(-)