[PATCH v2] builtin/stash: report failure to write to index

The git-stash(1) command needs to write to the index for many of its operations. When the index is locked by a concurrent writer it will thus fail to operate, which is expected. What is not expected though is that we do not print any error message at all in this case. The user can thus easily miss the fact that the command didn't do what they expected it to do and would be left wondering why that is.

Fix this bug and report failures to write to the index. Add tests for the subcommands which hit the respective code paths.

While at it, unify error messages when writing to the index fails. The chosen error message is already used in "builtin/stash.c".

Reported-by: moti sd <motisd8@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
Range-diff against v1:
1:  f114688eac < -:  ---------- drop dot
2:  b13a5a10ac ! 1:  cb098cf88c builtin/stash: report failure to write to index
    @@ Commit message
         Fix this bug and report failures to write to the index. Add tests for
         the subcommands which hit the respective code paths.
     
    -    Note that the chosen error message ("Cannot write to the index") does
    -    not match our guidelines as it starts with a capitalized letter. This is
    -    intentional though and matches the style of all the other messages used
    -    in git-stash(1).
    +    While at it, unify error messages when writing to the index fails. The
    +    chosen error message is already used in "builtin/stash.c".
     
         Reported-by: moti sd <motisd8@gmail.com>
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
      ## builtin/stash.c ##
    +@@ builtin/stash.c: static void unstage_changes_unless_new(struct object_id *orig_tree)
    + 	repo_hold_locked_index(the_repository, &lock, LOCK_DIE_ON_ERROR);
    + 	if (write_locked_index(&the_index, &lock,
    + 			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
    +-		die(_("Unable to write index."));
    ++		die(_("could not write index"));
    + }
    + 
    + static int do_apply_stash(const char *prefix, struct stash_info *info,
     @@ builtin/stash.c: static int do_apply_stash(const char *prefix, struct stash_info *info,
      	repo_read_index_preload(the_repository, NULL, 0);
      	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET, 0, 0,
      					 NULL, NULL, NULL))
     -		return -1;
    -+		return error(_("Cannot write to the index"));
    ++		return error(_("could not write index"));
      
      	if (write_index_as_tree(&c_tree, &the_index, get_index_file(), 0,
      				NULL))
    @@ builtin/stash.c: static int do_create_stash(const struct pathspec *ps, struct st
      	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET, 0, 0,
      					 NULL, NULL, NULL) < 0) {
     -		ret = -1;
    -+		ret = error(_("Cannot write to the index"));
    ++		ret = error(_("could not write index"));
      		goto done;
      	}
      
    @@ builtin/stash.c: static int do_push_stash(const struct pathspec *ps, const char
      	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET, 0, 0,
      					 NULL, NULL, NULL)) {
     -		ret = -1;
    -+		ret = error(_("Cannot write to the index"));
    ++		ret = error(_("could not write index"));
      		goto done;
      	}
      
    @@ t/t3903-stash.sh: test_expect_success 'restore untracked files even when we hit
     +		touch .git/index.lock &&
     +
     +		cat >expect <<-EOF &&
    -+		error: Cannot write to the index
    ++		error: could not write index
     +		EOF
     +		test_must_fail git stash create 2>err &&
     +		test_cmp expect err
    @@ t/t3903-stash.sh: test_expect_success 'restore untracked files even when we hit
     +		touch .git/index.lock &&
     +
     +		cat >expect <<-EOF &&
    -+		error: Cannot write to the index
    ++		error: could not write index
     +		EOF
     +		test_must_fail git stash push 2>err &&
     +		test_cmp expect err
    @@ t/t3903-stash.sh: test_expect_success 'restore untracked files even when we hit
     +		touch .git/index.lock &&
     +
     +		cat >expect <<-EOF &&
    -+		error: Cannot write to the index
    ++		error: could not write index
     +		EOF
     +		test_must_fail git stash apply 2>err &&
     +		test_cmp expect err

 builtin/stash.c  |  8 ++++----
 t/t3903-stash.sh | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 4 deletions(-)