Resurrect "diff-lib.c: adjust position of i-t-a entries in diff"

The original commit d95d728aba06a34394d15466045cbdabdada58a2 was
reverted in commit 78cc1a540ba127b13f2f3fd531777b57f3a9cd46 because we
were (and still are) not ready for a new world order. A lot more
investigation must be done to see what is impacted. See the 78cc1a5 for
details.

This patch takes a smaller and safer step. The new behavior is
controlled by SHIFT_INTENT_TO_ADD flag. We can gradually move more diff
users to the new behavior after we are sure it's safe to do so. This
flag is exposed to outside as "--shift-ita".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

6 files changed, 46 insertions, 2 deletions

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 3cb301556e..904438b8c8 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -559,5 +559,11 @@ endif::git-format-patch[]
 --no-prefix::
 	Do not show any source or destination prefix.
 
+--shift-ita::
+	By default entries added by "git add -N" appear as an existing
+	empty file in "git diff" and a new file in "git diff --cached".
+	This option makes the entry appear as a new file in "git diff"
+	and non-existent in "git diff --cached".
+
 For more detailed explanation on these common options, see also
 linkgit:gitdiffcore[7].
diff --git a/diff-lib.c b/diff-lib.c
index 27887d08d0..1248970f5a 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -212,6 +212,12 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 					       ce->sha1, !is_null_sha1(ce->sha1),
 					       ce->name, 0);
 				continue;
+			} else if (DIFF_OPT_TST(&revs->diffopt, SHIFT_INTENT_TO_ADD) &&
+				   ce_intent_to_add(ce)) {
+				diff_addremove(&revs->diffopt, '+', ce->ce_mode,
+					       EMPTY_BLOB_SHA1_BIN, 0,
+					       ce->name, 0);
+				continue;
 			}
 
 			changed = match_stat_with_submodule(&revs->diffopt, ce, &st,
@@ -376,6 +382,14 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	struct rev_info *revs = o->unpack_data;
 	int match_missing, cached;
 
+	/* i-t-a entries do not actually exist in the index */
+	if (DIFF_OPT_TST(&revs->diffopt, SHIFT_INTENT_TO_ADD) &&
+	    idx && ce_intent_to_add(idx)) {
+		idx = NULL;
+		if (!tree)
+			return;	/* nothing to diff.. */
+	}
+
 	/* if the entry is not checked out, don't examine work tree */
 	cached = o->index_only ||
 		(idx && ((idx->ce_flags & CE_VALID) || ce_skip_worktree(idx)));
diff --git a/diff.c b/diff.c
index f70425f6b3..b624e6479d 100644
--- a/diff.c
+++ b/diff.c
@@ -3910,6 +3910,8 @@ int diff_opt_parse(struct diff_options *options,
 		return parse_submodule_opt(options, arg);
 	else if (skip_prefix(arg, "--ws-error-highlight=", &arg))
 		return parse_ws_error_highlight(options, arg);
+	else if (!strcmp(arg, "--shift-ita"))
+		DIFF_OPT_SET(options, SHIFT_INTENT_TO_ADD);
 
 	/* misc options */
 	else if (!strcmp(arg, "-z"))
diff --git a/diff.h b/diff.h
index b497078c2f..9e42556dfc 100644
--- a/diff.h
+++ b/diff.h
@@ -92,6 +92,7 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
 #define DIFF_OPT_FUNCCONTEXT         (1 << 29)
 #define DIFF_OPT_PICKAXE_IGNORE_CASE (1 << 30)
 #define DIFF_OPT_DEFAULT_FOLLOW_RENAMES (1U << 31)
+#define DIFF_OPT_SHIFT_INTENT_TO_ADD (1UL << 32)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_TOUCHED(opts, flag)    ((opts)->touched_flags & DIFF_OPT_##flag)
diff --git a/t/t2203-add-intent.sh b/t/t2203-add-intent.sh
index 2a4a749b4f..4ae98f44e7 100755
--- a/t/t2203-add-intent.sh
+++ b/t/t2203-add-intent.sh
@@ -5,10 +5,24 @@ test_description='Intent to add'
 . ./test-lib.sh
 
 test_expect_success 'intent to add' '
+	test_commit 1 &&
+	git rm 1.t &&
+	echo hello >1.t &&
 	echo hello >file &&
 	echo hello >elif &&
 	git add -N file &&
-	git add elif
+	git add elif &&
+	git add -N 1.t
+'
+
+test_expect_success 'git status' '
+	git status --porcelain | grep -v actual >actual &&
+	cat >expect <<-\EOF &&
+	DA 1.t
+	A  elif
+	 A file
+	EOF
+	test_cmp expect actual
 '
 
 test_expect_success 'check result of "add -N"' '
@@ -43,7 +57,9 @@ test_expect_success 'i-t-a entry is simply ignored' '
 	git add -N nitfol &&
 	git commit -m second &&
 	test $(git ls-tree HEAD -- nitfol | wc -l) = 0 &&
-	test $(git diff --name-only HEAD -- nitfol | wc -l) = 1
+	test $(git diff --name-only HEAD -- nitfol | wc -l) = 1 &&
+	test $(git diff --name-only --shift-ita HEAD -- nitfol | wc -l) = 0 &&
+	test $(git diff --name-only --shift-ita -- nitfol | wc -l) = 1
 '
 
 test_expect_success 'can commit with an unrelated i-t-a entry in index' '
diff --git a/wt-status.c b/wt-status.c
index 4f27bd62af..cb89005696 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -497,6 +497,7 @@ static void wt_status_collect_changes_worktree(struct wt_status *s)
 	setup_revisions(0, NULL, &rev, NULL);
 	rev.diffopt.output_format |= DIFF_FORMAT_CALLBACK;
 	DIFF_OPT_SET(&rev.diffopt, DIRTY_SUBMODULES);
+	DIFF_OPT_SET(&rev.diffopt, SHIFT_INTENT_TO_ADD);
 	if (!s->show_untracked_files)
 		DIFF_OPT_SET(&rev.diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
 	if (s->ignore_submodule_arg) {
@@ -520,6 +521,7 @@ static void wt_status_collect_changes_index(struct wt_status *s)
 	setup_revisions(0, NULL, &rev, &opt);
 
 	DIFF_OPT_SET(&rev.diffopt, OVERRIDE_SUBMODULE_CONFIG);
+	DIFF_OPT_SET(&rev.diffopt, SHIFT_INTENT_TO_ADD);
 	if (s->ignore_submodule_arg) {
 		handle_ignore_submodules_arg(&rev.diffopt, s->ignore_submodule_arg);
 	} else {
@@ -555,6 +557,8 @@ static void wt_status_collect_changes_initial(struct wt_status *s)
 
 		if (!ce_path_match(ce, &s->pathspec, NULL))
 			continue;
+		if (ce_intent_to_add(ce))
+			continue;
 		it = string_list_insert(&s->change, ce->name);
 		d = it->util;
 		if (!d) {
@@ -853,6 +857,7 @@ static void wt_status_print_verbose(struct wt_status *s)
 
 	init_revisions(&rev, NULL);
 	DIFF_OPT_SET(&rev.diffopt, ALLOW_TEXTCONV);
+	DIFF_OPT_SET(&rev.diffopt, SHIFT_INTENT_TO_ADD);
 
 	memset(&opt, 0, sizeof(opt));
 	opt.def = s->is_initial ? EMPTY_TREE_SHA1_HEX : s->reference;