worktree: send "chatty" messages to stderr

The order in which the stdout and stderr streams are flushed is not
guaranteed to be the same across platforms or `libc` implementations.
This lack of determinism can lead to anomalous and potentially confusing
output if normal (stdout) output is flushed after error (stderr) output.
For instance, the following output which clearly indicates a failure due
to a fatal error:

    % git worktree add ../foo bar
    Preparing worktree (checking out 'bar')
    fatal: 'bar' is already checked out at '.../wherever'

has been reported[1] on Microsoft Windows to appear as:

    % git worktree add ../foo bar
    fatal: 'bar' is already checked out at '.../wherever'
    Preparing worktree (checking out 'bar')

which may confuse the reader into thinking that the command somehow
recovered and ran to completion despite the error.

This problem crops up because the "chatty" status message "Preparing
worktree" is sent to stdout, whereas the "fatal" error message is sent
to stderr. One way to fix this would be to flush stdout manually before
git-worktree reports any errors to stderr.

However, common practice in Git is for "chatty" messages to be sent to
stderr. Therefore, a more appropriate fix is to adjust git-worktree to
conform to that practice by sending its "chatty" messages to stderr
rather than stdout as is currently the case.

There may be concern that relocating messages from stdout to stderr
could break existing tooling, however, these messages are already
internationalized, thus are unstable. And, indeed, the "Preparing
worktree" message has already been the subject of somewhat significant
changes in 2c27002a0a (worktree: improve message when creating a new
worktree, 2018-04-24). Moreover, there is existing precedent, such as
68b939b2f0 (clone: send diagnostic messages to stderr, 2013-09-18) which
likewise relocated "chatty" messages from stdout to stderr for
git-clone.

[1]: https://lore.kernel.org/git/CA+34VNLj6VB1kCkA=MfM7TZR+6HgqNi5-UaziAoCXacSVkch4A@mail.gmail.com/T/

Reported-by: Baruch Burstein <bmburstein@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/builtin/worktree.c b/builtin/worktree.c
index d22ece93e1..a57fcd0f3c 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -72,7 +72,7 @@ static void delete_worktrees_dir_if_empty(void)
 static void prune_worktree(const char *id, const char *reason)
 {
 	if (show_only || verbose)
-		printf_ln(_("Removing %s/%s: %s"), "worktrees", id, reason);
+		fprintf_ln(stderr, _("Removing %s/%s: %s"), "worktrees", id, reason);
 	if (!show_only)
 		delete_git_dir(id);
 }
@@ -418,24 +418,24 @@ static void print_preparing_worktree_line(int detach,
 	if (force_new_branch) {
 		struct commit *commit = lookup_commit_reference_by_name(new_branch);
 		if (!commit)
-			printf_ln(_("Preparing worktree (new branch '%s')"), new_branch);
+			fprintf_ln(stderr, _("Preparing worktree (new branch '%s')"), new_branch);
 		else
-			printf_ln(_("Preparing worktree (resetting branch '%s'; was at %s)"),
+			fprintf_ln(stderr, _("Preparing worktree (resetting branch '%s'; was at %s)"),
 				  new_branch,
 				  find_unique_abbrev(&commit->object.oid, DEFAULT_ABBREV));
 	} else if (new_branch) {
-		printf_ln(_("Preparing worktree (new branch '%s')"), new_branch);
+		fprintf_ln(stderr, _("Preparing worktree (new branch '%s')"), new_branch);
 	} else {
 		struct strbuf s = STRBUF_INIT;
 		if (!detach && !strbuf_check_branch_ref(&s, branch) &&
 		    ref_exists(s.buf))
-			printf_ln(_("Preparing worktree (checking out '%s')"),
+			fprintf_ln(stderr, _("Preparing worktree (checking out '%s')"),
 				  branch);
 		else {
 			struct commit *commit = lookup_commit_reference_by_name(branch);
 			if (!commit)
 				die(_("invalid reference: %s"), branch);
-			printf_ln(_("Preparing worktree (detached HEAD %s)"),
+			fprintf_ln(stderr, _("Preparing worktree (detached HEAD %s)"),
 				  find_unique_abbrev(&commit->object.oid, DEFAULT_ABBREV));
 		}
 		strbuf_release(&s);
@@ -1006,7 +1006,7 @@ static int remove_worktree(int ac, const char **av, const char *prefix)
 static void report_repair(int iserr, const char *path, const char *msg, void *cb_data)
 {
 	if (!iserr) {
-		printf_ln(_("repair: %s: %s"), msg, path);
+		fprintf_ln(stderr, _("repair: %s: %s"), msg, path);
 	} else {
 		int *exit_status = (int *)cb_data;
 		fprintf_ln(stderr, _("error: %s: %s"), msg, path);