[PATCH] sequencer: support folding in rfc2822 footer

[Jonathan Tan <jonathantanmy@google.com>]

When running `git cherry-pick -x`, a blank line would be inserted if a line in
the footer was broken into multiple lines (which is inconsistent with its
behavior if no lines were broken). For example, this command would produce:

---
Sample-field: no blank line below
(cherry picked from commit ...)
---

but would also produce:

---
Sample-field: multiple-line field body
 that causes a blank line below

(cherry picked from commit ...)
---

This, among other things, trips up tools that look for the last paragraph
(including sequencer.c itself).

RFC 2822 allows field bodies to be split into multiple lines, especially (but
not only) to deal with the line-width limitations described in the
specification, referring to this as "folding".

Teach sequencer.c to treat split and unsplit field bodies in the same way (that
is, to not include the blank line).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 sequencer.c              | 19 +++++++++++++++++--
 t/t3511-cherry-pick-x.sh | 19 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 3804fa9..411dd50 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -26,10 +26,20 @@ static GIT_PATH_FUNC(git_path_opts_file, SEQ_OPTS_FILE)
 static GIT_PATH_FUNC(git_path_seq_dir, SEQ_DIR)
 static GIT_PATH_FUNC(git_path_head_file, SEQ_HEAD_FILE)
 
-static int is_rfc2822_line(const char *buf, int len)
+static int is_rfc2822_line(const char *buf, int len,
+			   int allow_folding_continuation)
 {
 	int i;
 
+	/*
+	 * Section 2.2.3 of RFC 2822 allows field bodies to continue onto
+	 * multiple lines, referred to as "folding". Such continuation lines
+	 * start with whitespace.
+	 */
+	if (allow_folding_continuation)
+		if (len && (buf[0] == ' ' || buf[0] == '\t'))
+			return 1;
+
 	for (i = 0; i < len; i++) {
 		int ch = buf[i];
 		if (ch == ':')
@@ -64,6 +74,7 @@ static int has_conforming_footer(struct strbuf *sb, struct strbuf *sob,
 	int len = sb->len - ignore_footer;
 	const char *buf = sb->buf;
 	int found_sob = 0;
+	int allow_folding_continuation = 0;
 
 	/* footer must end with newline */
 	if (!len || buf[len - 1] != '\n')
@@ -92,7 +103,11 @@ static int has_conforming_footer(struct strbuf *sb, struct strbuf *sob,
 			; /* do nothing */
 		k++;
 
-		found_rfc2822 = is_rfc2822_line(buf + i, k - i - 1);
+		found_rfc2822 = is_rfc2822_line(buf + i,
+				                k - i - 1,
+				                allow_folding_continuation);
+		allow_folding_continuation = 1;
+
 		if (found_rfc2822 && sob &&
 		    !strncmp(buf + i, sob->buf, sob->len))
 			found_sob = k;
diff --git a/t/t3511-cherry-pick-x.sh b/t/t3511-cherry-pick-x.sh
index 9cce5ae..1a50339 100755
--- a/t/t3511-cherry-pick-x.sh
+++ b/t/t3511-cherry-pick-x.sh
@@ -36,6 +36,11 @@ mesg_with_cherry_footer="$mesg_with_footer_sob
 (cherry picked from commit da39a3ee5e6b4b0d3255bfef95601890afd80709)
 Tested-by: C.U. Thor <cuthor@example.com>"
 
+mesg_with_folding_footer="$mesg_no_footer
+
+Field: This is a very long field body
+ that is continued onto another line"
+
 mesg_unclean="$mesg_one_line
 
 
@@ -68,6 +73,8 @@ test_expect_success setup '
 	git reset --hard initial &&
 	test_commit "$mesg_with_cherry_footer" foo b mesg-with-cherry-footer &&
 	git reset --hard initial &&
+	test_commit "$mesg_with_folding_footer" foo b mesg-with-folding-footer &&
+	git reset --hard initial &&
 	test_config commit.cleanup verbatim &&
 	test_commit "$mesg_unclean" foo b mesg-unclean &&
 	test_unconfig commit.cleanup &&
@@ -234,6 +241,18 @@ test_expect_success 'cherry-pick -x -s treats "(cherry picked from..." line as p
 	test_cmp expect actual
 '
 
+test_expect_success 'cherry-pick -x does not insert blank line when folding footer is found' '
+	pristine_detach initial &&
+	sha1=$(git rev-parse mesg-with-folding-footer^0) &&
+	git cherry-pick -x mesg-with-folding-footer &&
+	cat <<-EOF >expect &&
+		$mesg_with_folding_footer
+		(cherry picked from commit $sha1)
+	EOF
+	git log -1 --pretty=format:%B >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'cherry-pick preserves commit message' '
 	pristine_detach initial &&
 	printf "$mesg_unclean" >expect &&
-- 
2.8.0.rc3.226.g39d4020