add -p: fix editing of intent-to-add paths

A popular way of partially staging a new file is to run `git add -N
<path>` and then use the hunk editing of `git add -p` to select the
part of the file that the user wishes to stage. Since
85953a3187 ("diff-files --raw: show correct post-image of
intent-to-add files", 2020-07-01) this has stopped working as
intent-to-add paths are now show as new files rather than changes to
an empty blob and `git apply` refused to apply a creation patch for a
path that was marked as intent-to-add. 7cfde3fa0f ("apply: allow "new
file" patches on i-t-a entries", 2020-08-06) fixed the problem with
apply but it still wasn't possible to edit the added hunk properly.

2c8bd8471a ("checkout -p: handle new files correctly", 2020-05-27)
had previously changed `add -p` to handle new files but it did not
implement patch editing correctly. The perl version simply forbade
editing and the C version opened the editor with the full diff rather
that just the hunk which meant that the user had to edit the hunk
header manually to get it to work.

The root cause of the problem is that added files store the diff header
with the hunk data rather than separating the two as we do for other
changes. Changing added files to store the diff header separately
fixes the editing problem at the expense of having to special case
empty additions as they no longer have any hunks associated with them,
only the diff header.

The changes move some existing code into a conditional changing the
indentation, they are best viewed with
--color-moved-ws=allow-indentation-change (or --ignore-space-change
works well to get an overview of the changes)

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reported-by: Thomas Sullivan <tom@msbit.com.au>
Reported-by: Yuchen Ying <ych@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 79 insertions, 61 deletions

diff --git a/add-patch.c b/add-patch.c
index 457b8c5..867720a 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -451,7 +451,7 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	pend = p + plain->len;
 	while (p != pend) {
 		char *eol = memchr(p, '\n', pend - p);
-		const char *deleted = NULL, *added = NULL, *mode_change = NULL;
+		const char *deleted = NULL, *mode_change = NULL;
 
 		if (!eol)
 			eol = pend;
@@ -468,12 +468,11 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 		} else if (p == plain->buf)
 			BUG("diff starts with unexpected line:\n"
 			    "%.*s\n", (int)(eol - p), p);
-		else if (file_diff->deleted || file_diff->added)
+		else if (file_diff->deleted)
 			; /* keep the rest of the file in a single "hunk" */
 		else if (starts_with(p, "@@ ") ||
 			 (hunk == &file_diff->head &&
-			  (skip_prefix(p, "deleted file", &deleted) ||
-			   skip_prefix(p, "new file", &added)))) {
+			  (skip_prefix(p, "deleted file", &deleted)))) {
 			if (marker == '-' || marker == '+')
 				/*
 				 * Should not happen; previous hunk did not end
@@ -491,8 +490,6 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 
 			if (deleted)
 				file_diff->deleted = 1;
-			else if (added)
-				file_diff->added = 1;
 			else if (parse_hunk_header(s, hunk) < 0)
 				return -1;
 
@@ -502,6 +499,9 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 			 */
 			marker = *p;
 		} else if (hunk == &file_diff->head &&
+			   starts_with(p, "new file")) {
+			file_diff->added = 1;
+		} else if (hunk == &file_diff->head &&
 			   skip_prefix(p, "old mode ", &mode_change) &&
 			   is_octal(mode_change, eol - mode_change)) {
 			if (file_diff->mode_change)
@@ -1362,7 +1362,8 @@ static int patch_update_file(struct add_p_state *s,
 		ALLOW_EDIT = 1 << 6
 	} permitted = 0;
 
-	if (!file_diff->hunk_nr)
+	/* Empty added files have no hunks */
+	if (!file_diff->hunk_nr && !file_diff->added)
 		return 0;
 
 	strbuf_reset(&s->buf);
@@ -1371,21 +1372,25 @@ static int patch_update_file(struct add_p_state *s,
 	for (;;) {
 		if (hunk_index >= file_diff->hunk_nr)
 			hunk_index = 0;
-		hunk = file_diff->hunk + hunk_index;
-
+		hunk = file_diff->hunk_nr
+				? file_diff->hunk + hunk_index
+				: &file_diff->head;
 		undecided_previous = -1;
-		for (i = hunk_index - 1; i >= 0; i--)
-			if (file_diff->hunk[i].use == UNDECIDED_HUNK) {
-				undecided_previous = i;
-				break;
-			}
-
 		undecided_next = -1;
-		for (i = hunk_index + 1; i < file_diff->hunk_nr; i++)
-			if (file_diff->hunk[i].use == UNDECIDED_HUNK) {
-				undecided_next = i;
-				break;
-			}
+
+		if (file_diff->hunk_nr) {
+			for (i = hunk_index - 1; i >= 0; i--)
+				if (file_diff->hunk[i].use == UNDECIDED_HUNK) {
+					undecided_previous = i;
+					break;
+				}
+
+			for (i = hunk_index + 1; i < file_diff->hunk_nr; i++)
+				if (file_diff->hunk[i].use == UNDECIDED_HUNK) {
+					undecided_next = i;
+					break;
+				}
+		}
 
 		/* Everything decided? */
 		if (undecided_previous < 0 && undecided_next < 0 &&
@@ -1393,38 +1398,40 @@ static int patch_update_file(struct add_p_state *s,
 			break;
 
 		strbuf_reset(&s->buf);
-		render_hunk(s, hunk, 0, colored, &s->buf);
-		fputs(s->buf.buf, stdout);
+		if (file_diff->hunk_nr) {
+			render_hunk(s, hunk, 0, colored, &s->buf);
+			fputs(s->buf.buf, stdout);
 
-		strbuf_reset(&s->buf);
-		if (undecided_previous >= 0) {
-			permitted |= ALLOW_GOTO_PREVIOUS_UNDECIDED_HUNK;
-			strbuf_addstr(&s->buf, ",k");
-		}
-		if (hunk_index) {
-			permitted |= ALLOW_GOTO_PREVIOUS_HUNK;
-			strbuf_addstr(&s->buf, ",K");
-		}
-		if (undecided_next >= 0) {
-			permitted |= ALLOW_GOTO_NEXT_UNDECIDED_HUNK;
-			strbuf_addstr(&s->buf, ",j");
-		}
-		if (hunk_index + 1 < file_diff->hunk_nr) {
-			permitted |= ALLOW_GOTO_NEXT_HUNK;
-			strbuf_addstr(&s->buf, ",J");
-		}
-		if (file_diff->hunk_nr > 1) {
-			permitted |= ALLOW_SEARCH_AND_GOTO;
-			strbuf_addstr(&s->buf, ",g,/");
-		}
-		if (hunk->splittable_into > 1) {
-			permitted |= ALLOW_SPLIT;
-			strbuf_addstr(&s->buf, ",s");
-		}
-		if (hunk_index + 1 > file_diff->mode_change &&
-		    !file_diff->deleted) {
-			permitted |= ALLOW_EDIT;
-			strbuf_addstr(&s->buf, ",e");
+			strbuf_reset(&s->buf);
+			if (undecided_previous >= 0) {
+				permitted |= ALLOW_GOTO_PREVIOUS_UNDECIDED_HUNK;
+				strbuf_addstr(&s->buf, ",k");
+			}
+			if (hunk_index) {
+				permitted |= ALLOW_GOTO_PREVIOUS_HUNK;
+				strbuf_addstr(&s->buf, ",K");
+			}
+			if (undecided_next >= 0) {
+				permitted |= ALLOW_GOTO_NEXT_UNDECIDED_HUNK;
+				strbuf_addstr(&s->buf, ",j");
+			}
+			if (hunk_index + 1 < file_diff->hunk_nr) {
+				permitted |= ALLOW_GOTO_NEXT_HUNK;
+				strbuf_addstr(&s->buf, ",J");
+			}
+			if (file_diff->hunk_nr > 1) {
+				permitted |= ALLOW_SEARCH_AND_GOTO;
+				strbuf_addstr(&s->buf, ",g,/");
+			}
+			if (hunk->splittable_into > 1) {
+				permitted |= ALLOW_SPLIT;
+				strbuf_addstr(&s->buf, ",s");
+			}
+			if (hunk_index + 1 > file_diff->mode_change &&
+			    !file_diff->deleted) {
+				permitted |= ALLOW_EDIT;
+				strbuf_addstr(&s->buf, ",e");
+			}
 		}
 		if (file_diff->deleted)
 			prompt_mode_type = PROMPT_DELETION;
@@ -1438,7 +1445,9 @@ static int patch_update_file(struct add_p_state *s,
 		color_fprintf(stdout, s->s.prompt_color,
 			      "(%"PRIuMAX"/%"PRIuMAX") ",
 			      (uintmax_t)hunk_index + 1,
-			      (uintmax_t)file_diff->hunk_nr);
+			      (uintmax_t)(file_diff->hunk_nr
+						? file_diff->hunk_nr
+						: 1));
 		color_fprintf(stdout, s->s.prompt_color,
 			      _(s->mode->prompt_mode[prompt_mode_type]),
 			      s->buf.buf);
@@ -1458,16 +1467,24 @@ soft_increment:
 			hunk->use = SKIP_HUNK;
 			goto soft_increment;
 		} else if (ch == 'a') {
-			for (; hunk_index < file_diff->hunk_nr; hunk_index++) {
-				hunk = file_diff->hunk + hunk_index;
-				if (hunk->use == UNDECIDED_HUNK)
-					hunk->use = USE_HUNK;
+			if (file_diff->hunk_nr) {
+				for (; hunk_index < file_diff->hunk_nr; hunk_index++) {
+					hunk = file_diff->hunk + hunk_index;
+					if (hunk->use == UNDECIDED_HUNK)
+						hunk->use = USE_HUNK;
+				}
+			} else if (hunk->use == UNDECIDED_HUNK) {
+				hunk->use = USE_HUNK;
 			}
 		} else if (ch == 'd' || ch == 'q') {
-			for (; hunk_index < file_diff->hunk_nr; hunk_index++) {
-				hunk = file_diff->hunk + hunk_index;
-				if (hunk->use == UNDECIDED_HUNK)
-					hunk->use = SKIP_HUNK;
+			if (file_diff->hunk_nr) {
+				for (; hunk_index < file_diff->hunk_nr; hunk_index++) {
+					hunk = file_diff->hunk + hunk_index;
+					if (hunk->use == UNDECIDED_HUNK)
+						hunk->use = SKIP_HUNK;
+				}
+			} else if (hunk->use == UNDECIDED_HUNK) {
+				hunk->use = SKIP_HUNK;
 			}
 			if (ch == 'q') {
 				quit = 1;
@@ -1625,7 +1642,8 @@ soft_increment:
 		if (file_diff->hunk[i].use == USE_HUNK)
 			break;
 
-	if (i < file_diff->hunk_nr) {
+	if (i < file_diff->hunk_nr ||
+	    (!file_diff->hunk_nr && file_diff->head.use == USE_HUNK)) {
 		/* At least one hunk selected: apply */
 		strbuf_reset(&s->buf);
 		reassemble_patch(s, file_diff, 0, &s->buf);