breaking-changes: deprecate support for core.commentString=auto

When "core.commentString" is set to "auto" then "git commit" will
automatically select the comment character ensuring that it is not the
first character on any of the lines in the commit message. This was
introduced by commit 84c9dc2c5a2 (commit: allow core.commentChar=auto
for character auto selection, 2014-05-17). The motivation seems to be
to avoid commenting out lines from the existing message when amending
a commit that was created with a message from a file.

Unfortunately this feature does not work with:

 * commit message templates that contain comments.

 * prepare-commit-msg hooks that introduce comments.

 * "git commit --cleanup=strip --edit -F <file>" which means that it
   is incompatible with

   - the "fixup" and "squash" commands of "git rebase -i" as the
     comments added by those commands are then treated as part of
     the commit message.

   - the conflict comments added to the commit message by "git
     cherry-pick", "git rebase" etc. as these comments are then
     treated as part of the commit message.

It is also ignored by "git notes" when amending a note.

The issues with comments coming from a template, hook or file are a
consequence of the design of this feature and are therefore hard to
fix.

As the costs of this feature outweigh the benefits, deprecate it and
remove it in Git 3.0. If someone comes up with some patches that fix
all the issues in a maintainable way then I'd be happy to see this
change reverted.

The next commits will add a warning and some advice for users on how
they can update their config settings.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/environment.c b/environment.c
index a0ac5934b3..4c87876d48 100644
--- a/environment.c
+++ b/environment.c
@@ -122,7 +122,9 @@ int protect_ntfs = PROTECT_NTFS_DEFAULT;
  */
 const char *comment_line_str = "#";
 char *comment_line_str_to_free;
+#ifndef WITH_BREAKING_CHANGES
 int auto_comment_line_char;
+#endif /* !WITH_BREAKING_CHANGES */
 
 /* This is set by setup_git_directory_gently() and/or git_default_config() */
 char *git_work_tree_cfg;
@@ -459,18 +461,22 @@ static int git_default_core_config(const char *var, const char *value,
 
 	if (!strcmp(var, "core.commentchar") ||
 	    !strcmp(var, "core.commentstring")) {
-		if (!value)
+		if (!value) {
 			return config_error_nonbool(var);
-		else if (!strcasecmp(value, "auto")) {
+#ifndef WITH_BREAKING_CHANGES
+		} else if (!strcasecmp(value, "auto")) {
 			auto_comment_line_char = 1;
 			FREE_AND_NULL(comment_line_str_to_free);
 			comment_line_str = "#";
+#endif /* !WITH_BREAKING_CHANGES */
 		} else if (value[0]) {
 			if (strchr(value, '\n'))
 				return error(_("%s cannot contain newline"), var);
 			comment_line_str = value;
 			FREE_AND_NULL(comment_line_str_to_free);
+#ifndef WITH_BREAKING_CHANGES
 			auto_comment_line_char = 0;
+#endif /* !WITH_BREAKING_CHANGES */
 		} else
 			return error(_("%s must have at least one character"), var);
 		return 0;