built-in add -p: adjust hunk headers as needed

When skipping a hunk that adds a different number of lines than it
removes, we need to adjust the subsequent hunk headers of non-skipped
hunks: in pathological cases, the context is not enough to determine
precisely where the patch should be applied.

This problem was identified in 23fea4c240 (t3701: add failing test for
pathological context lines, 2018-03-01) and fixed in the Perl version in
fecc6f3a68 (add -p: adjust offsets of subsequent hunks when one is
skipped, 2018-03-01).

And this patch fixes it in the C version of `git add -p`.

In contrast to the Perl version, we try to keep the extra text on the
hunk header (which typically contains the signature of the function
whose code is changed in the hunk) intact.

Note: while the C version does not support staging mode changes at this
stage, we already prepare for this by simply skipping the hunk header if
both old and new offset is 0 (this cannot happen for regular hunks, and
we will use this as an indicator that we are looking at a special hunk).

Likewise, we already prepare for hunk splitting by handling the absence
of extra text in the hunk header gracefully: only the first split hunk
will have that text, the others will not (indicated by an empty extra
text start/end range). Preparing for hunk splitting already at this
stage avoids an indentation change of the entire hunk header-printing
block later, and is almost as easy to review as without that handling.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 133 insertions, 12 deletions

diff --git a/add-patch.c b/add-patch.c
index 79eefa9..e266a96 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -5,14 +5,26 @@
 #include "argv-array.h"
 #include "pathspec.h"
 #include "color.h"
+#include "diff.h"
+
+struct hunk_header {
+	unsigned long old_offset, old_count, new_offset, new_count;
+	/*
+	 * Start/end offsets to the extra text after the second `@@` in the
+	 * hunk header, e.g. the function signature. This is expected to
+	 * include the newline.
+	 */
+	size_t extra_start, extra_end, colored_extra_start, colored_extra_end;
+};
 
 struct hunk {
 	size_t start, end, colored_start, colored_end;
 	enum { UNDECIDED_HUNK = 0, SKIP_HUNK, USE_HUNK } use;
+	struct hunk_header header;
 };
 
 struct add_p_state {
-	struct repository *r;
+	struct add_i_state s;
 	struct strbuf answer, buf;
 
 	/* parsed diff */
@@ -35,7 +47,70 @@ static void setup_child_process(struct add_p_state *s,
 
 	cp->git_cmd = 1;
 	argv_array_pushf(&cp->env_array,
-			 INDEX_ENVIRONMENT "=%s", s->r->index_file);
+			 INDEX_ENVIRONMENT "=%s", s->s.r->index_file);
+}
+
+static int parse_range(const char **p,
+		       unsigned long *offset, unsigned long *count)
+{
+	char *pend;
+
+	*offset = strtoul(*p, &pend, 10);
+	if (pend == *p)
+		return -1;
+	if (*pend != ',') {
+		*count = 1;
+		*p = pend;
+		return 0;
+	}
+	*count = strtoul(pend + 1, (char **)p, 10);
+	return *p == pend + 1 ? -1 : 0;
+}
+
+static int parse_hunk_header(struct add_p_state *s, struct hunk *hunk)
+{
+	struct hunk_header *header = &hunk->header;
+	const char *line = s->plain.buf + hunk->start, *p = line;
+	char *eol = memchr(p, '\n', s->plain.len - hunk->start);
+
+	if (!eol)
+		eol = s->plain.buf + s->plain.len;
+
+	if (!skip_prefix(p, "@@ -", &p) ||
+	    parse_range(&p, &header->old_offset, &header->old_count) < 0 ||
+	    !skip_prefix(p, " +", &p) ||
+	    parse_range(&p, &header->new_offset, &header->new_count) < 0 ||
+	    !skip_prefix(p, " @@", &p))
+		return error(_("could not parse hunk header '%.*s'"),
+			     (int)(eol - line), line);
+
+	hunk->start = eol - s->plain.buf + (*eol == '\n');
+	header->extra_start = p - s->plain.buf;
+	header->extra_end = hunk->start;
+
+	if (!s->colored.len) {
+		header->colored_extra_start = header->colored_extra_end = 0;
+		return 0;
+	}
+
+	/* Now find the extra text in the colored diff */
+	line = s->colored.buf + hunk->colored_start;
+	eol = memchr(line, '\n', s->colored.len - hunk->colored_start);
+	if (!eol)
+		eol = s->colored.buf + s->colored.len;
+	p = memmem(line, eol - line, "@@ -", 4);
+	if (!p)
+		return error(_("could not parse colored hunk header '%.*s'"),
+			     (int)(eol - line), line);
+	p = memmem(p + 4, eol - p - 4, " @@", 3);
+	if (!p)
+		return error(_("could not parse colored hunk header '%.*s'"),
+			     (int)(eol - line), line);
+	hunk->colored_start = eol - s->colored.buf + (*eol == '\n');
+	header->colored_extra_start = p + 3 - s->colored.buf;
+	header->colored_extra_end = hunk->colored_start;
+
+	return 0;
 }
 
 static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
@@ -109,6 +184,9 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 			hunk->start = p - plain->buf;
 			if (colored)
 				hunk->colored_start = colored_p - colored->buf;
+
+			if (parse_hunk_header(s, hunk) < 0)
+				return -1;
 		}
 
 		p = eol == pend ? pend : eol + 1;
@@ -130,8 +208,43 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 }
 
 static void render_hunk(struct add_p_state *s, struct hunk *hunk,
-			int colored, struct strbuf *out)
+			ssize_t delta, int colored, struct strbuf *out)
 {
+	struct hunk_header *header = &hunk->header;
+
+	if (hunk->header.old_offset != 0 || hunk->header.new_offset != 0) {
+		/*
+		 * Generate the hunk header dynamically, except for special
+		 * hunks (such as the diff header).
+		 */
+		const char *p;
+		size_t len;
+		unsigned long old_offset = header->old_offset;
+		unsigned long new_offset = header->new_offset;
+
+		if (!colored) {
+			p = s->plain.buf + header->extra_start;
+			len = header->extra_end - header->extra_start;
+		} else {
+			strbuf_addstr(out, s->s.fraginfo_color);
+			p = s->colored.buf + header->colored_extra_start;
+			len = header->colored_extra_end
+				- header->colored_extra_start;
+		}
+
+		new_offset += delta;
+
+		strbuf_addf(out, "@@ -%lu,%lu +%lu,%lu @@",
+			    old_offset, header->old_count,
+			    new_offset, header->new_count);
+		if (len)
+			strbuf_add(out, p, len);
+		else if (colored)
+			strbuf_addf(out, "%s\n", GIT_COLOR_RESET);
+		else
+			strbuf_addch(out, '\n');
+	}
+
 	if (colored)
 		strbuf_add(out, s->colored.buf + hunk->colored_start,
 			   hunk->colored_end - hunk->colored_start);
@@ -144,13 +257,17 @@ static void reassemble_patch(struct add_p_state *s, struct strbuf *out)
 {
 	struct hunk *hunk;
 	size_t i;
+	ssize_t delta = 0;
 
-	render_hunk(s, &s->head, 0, out);
+	render_hunk(s, &s->head, 0, 0, out);
 
 	for (i = 0; i < s->hunk_nr; i++) {
 		hunk = s->hunk + i;
-		if (hunk->use == USE_HUNK)
-			render_hunk(s, hunk, 0, out);
+		if (hunk->use != USE_HUNK)
+			delta += hunk->header.old_count
+				- hunk->header.new_count;
+		else
+			render_hunk(s, hunk, delta, 0, out);
 	}
 }
 
@@ -178,7 +295,7 @@ static int patch_update_file(struct add_p_state *s)
 		return 0;
 
 	strbuf_reset(&s->buf);
-	render_hunk(s, &s->head, colored, &s->buf);
+	render_hunk(s, &s->head, 0, colored, &s->buf);
 	fputs(s->buf.buf, stdout);
 	for (;;) {
 		if (hunk_index >= s->hunk_nr)
@@ -205,7 +322,7 @@ static int patch_update_file(struct add_p_state *s)
 			break;
 
 		strbuf_reset(&s->buf);
-		render_hunk(s, hunk, colored, &s->buf);
+		render_hunk(s, hunk, 0, colored, &s->buf);
 		fputs(s->buf.buf, stdout);
 
 		strbuf_reset(&s->buf);
@@ -272,13 +389,13 @@ soft_increment:
 		strbuf_reset(&s->buf);
 		reassemble_patch(s, &s->buf);
 
-		discard_index(s->r->index);
+		discard_index(s->s.r->index);
 		setup_child_process(s, &cp, "apply", "--cached", NULL);
 		if (pipe_command(&cp, s->buf.buf, s->buf.len,
 				 NULL, 0, NULL, 0))
 			error(_("'git apply --cached' failed"));
-		if (!repo_read_index(s->r))
-			repo_refresh_and_write_index(s->r, REFRESH_QUIET, 0,
+		if (!repo_read_index(s->s.r))
+			repo_refresh_and_write_index(s->s.r, REFRESH_QUIET, 0,
 						     1, NULL, NULL, NULL);
 	}
 
@@ -288,7 +405,11 @@ soft_increment:
 
 int run_add_p(struct repository *r, const struct pathspec *ps)
 {
-	struct add_p_state s = { r, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT };
+	struct add_p_state s = {
+		{ r }, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
+	};
+
+	init_add_i_state(&s.s, r);
 
 	if (discard_index(r->index) < 0 || repo_read_index(r) < 0 ||
 	    repo_refresh_and_write_index(r, REFRESH_QUIET, 0, 1,