Change unpack_trees' 'reset' flag into an enum

Traditionally, unpack_trees_options->reset was used to signal that it
was okay to delete any untracked files in the way.  This was used by
`git read-tree --reset`, but then started appearing in other places as
well.  However, many of the other uses should not be deleting untracked
files in the way.  Change this value to an enum so that a value of 1
(i.e. "true") can be split into two:
   UNPACK_RESET_PROTECT_UNTRACKED,
   UNPACK_RESET_OVERWRITE_UNTRACKED
In order to catch accidental misuses (i.e. where folks call it the way
they traditionally used to), define the special enum value of
   UNPACK_RESET_INVALID = 1
which will trigger a BUG().

Modify existing callers so that
   read-tree --reset
   reset --hard
   checkout --force
continue using the UNPACK_RESET_OVERWRITE_UNTRACKED logic, while other
callers, including
   am
   checkout without --force
   stash  (though currently dead code; reset always had a value of 0)
   numerous callers from rebase/sequencer to reset_head()
will use the new UNPACK_RESET_PROTECT_UNTRACKED value.

Also, note that it has been reported that 'git checkout <treeish>
<pathspec>' currently also allows overwriting untracked files[1].  That
case should also be fixed, but it does not use unpack_trees() and thus
is outside the scope of the current changes.

[1] https://lore.kernel.org/git/15dad590-087e-5a48-9238-5d2826950506@gmail.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1918,9 +1918,8 @@ static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
 	opts.dst_index = &the_index;
 	opts.update = 1;
 	opts.merge = 1;
-	opts.reset = reset;
-	if (!reset)
-		opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
+	opts.reset = reset ? UNPACK_RESET_PROTECT_UNTRACKED : 0;
+	opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
 	opts.fn = twoway_merge;
 	init_tree_desc(&t[0], head->buffer, head->size);
 	init_tree_desc(&t[1], remote->buffer, remote->size);

builtin/checkout.c

@@ -646,9 +646,10 @@ static int reset_tree(struct tree *tree, const struct checkout_opts *o,
 	opts.head_idx = -1;
 	opts.update = worktree;
 	opts.skip_unmerged = !worktree;
-	opts.reset = 1;
+	opts.reset = o->force ? UNPACK_RESET_OVERWRITE_UNTRACKED :
+				UNPACK_RESET_PROTECT_UNTRACKED;
+	opts.preserve_ignored = (!o->force && !o->overwrite_ignore);
 	opts.merge = 1;
-	opts.preserve_ignored = 0;
 	opts.fn = oneway_merge;
 	opts.verbose_update = o->show_progress;
 	opts.src_index = &the_index;

builtin/read-tree.c

@@ -166,6 +166,9 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	if (1 < opts.merge + opts.reset + prefix_set)
 		die("Which one? -m, --reset, or --prefix?");
 
+	if (opts.reset)
+		opts.reset = UNPACK_RESET_OVERWRITE_UNTRACKED;
+
 	/*
 	 * NEEDSWORK
 	 *

builtin/reset.c

@@ -71,9 +71,14 @@ static int reset_index(const char *ref, const struct object_id *oid, int reset_t
 		break;
 	case HARD:
 		opts.update = 1;
-		/* fallthrough */
+		opts.reset = UNPACK_RESET_OVERWRITE_UNTRACKED;
+		break;
+	case MIXED:
+		opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+		/* but opts.update=0, so working tree not updated */
+		break;
 	default:
-		opts.reset = 1;
+		BUG("invalid reset_type passed to reset_index");
 	}
 
 	read_cache_unmerged();

builtin/stash.c

@@ -256,9 +256,9 @@ static int reset_tree(struct object_id *i_tree, int update, int reset)
 	opts.src_index = &the_index;
 	opts.dst_index = &the_index;
 	opts.merge = 1;
-	opts.reset = reset;
+	opts.reset = reset ? UNPACK_RESET_PROTECT_UNTRACKED : 0;
 	opts.update = update;
-	if (update && !reset)
+	if (update)
 		opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
 	opts.fn = oneway_merge;
 

reset.c

@@ -59,7 +59,7 @@ int reset_head(struct repository *r, struct object_id *oid, const char *action,
 	unpack_tree_opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
 	init_checkout_metadata(&unpack_tree_opts.meta, switch_to_branch, oid, NULL);
 	if (!detach_head)
-		unpack_tree_opts.reset = 1;
+		unpack_tree_opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
 
 	if (repo_read_index_unmerged(r) < 0) {
 		ret = error(_("could not read index"));

t/t2500-untracked-overwriting.sh

@@ -92,7 +92,7 @@ test_setup_checkout_m () {
 	)
 }
 
-test_expect_failure 'checkout -m does not nuke untracked file' '
+test_expect_success 'checkout -m does not nuke untracked file' '
 	test_setup_checkout_m &&
 	(
 		cd checkout &&
@@ -138,7 +138,7 @@ test_setup_sequencing () {
 	)
 }
 
-test_expect_failure 'git rebase --abort and untracked files' '
+test_expect_success 'git rebase --abort and untracked files' '
 	test_setup_sequencing rebase_abort_and_untracked &&
 	(
 		cd sequencing_rebase_abort_and_untracked &&
@@ -155,7 +155,7 @@ test_expect_failure 'git rebase --abort and untracked files' '
 	)
 '
 
-test_expect_failure 'git rebase fast forwarding and untracked files' '
+test_expect_success 'git rebase fast forwarding and untracked files' '
 	test_setup_sequencing rebase_fast_forward_and_untracked &&
 	(
 		cd sequencing_rebase_fast_forward_and_untracked &&

unpack-trees.c

@@ -1694,6 +1694,9 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	int free_pattern_list = 0;
 	struct dir_struct dir = DIR_INIT;
 
+	if (o->reset == UNPACK_RESET_INVALID)
+		BUG("o->reset had a value of 1; should be UNPACK_TREES_*_UNTRACKED");
+
 	if (len > MAX_UNPACK_TREES)
 		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
 	if (o->dir)
@@ -1708,6 +1711,10 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		ensure_full_index(o->dst_index);
 	}
 
+	if (o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED &&
+	    o->preserve_ignored)
+		BUG("UNPACK_RESET_OVERWRITE_UNTRACKED incompatible with preserved ignored files");
+
 	if (!o->preserve_ignored) {
 		o->dir = &dir;
 		o->dir->flags |= DIR_SHOW_IGNORED;
@@ -2231,7 +2238,8 @@ static int verify_absent_1(const struct cache_entry *ce,
 	int len;
 	struct stat st;
 
-	if (o->index_only || o->reset || !o->update)
+	if (o->index_only || !o->update ||
+	    o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED)
 		return 0;
 
 	len = check_leading_path(ce->name, ce_namelen(ce), 0);

unpack-trees.h

@@ -45,9 +45,15 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
  */
 void clear_unpack_trees_porcelain(struct unpack_trees_options *opts);
 
+enum unpack_trees_reset_type {
+	UNPACK_RESET_NONE = 0,    /* traditional "false" value; still valid */
+	UNPACK_RESET_INVALID = 1, /* "true" no longer valid; use below values */
+	UNPACK_RESET_PROTECT_UNTRACKED,
+	UNPACK_RESET_OVERWRITE_UNTRACKED
+};
+
 struct unpack_trees_options {
-	unsigned int reset,
-		     merge,
+	unsigned int merge,
 		     update,
 		     preserve_ignored,
 		     clone,
@@ -65,6 +71,7 @@ struct unpack_trees_options {
 		     exiting_early,
 		     show_all_errors,
 		     dry_run;
+	enum unpack_trees_reset_type reset;
 	const char *prefix;
 	int cache_bottom;
 	struct pathspec *pathspec;