[PATCH V2] patch-id: fix scan_hunk_header on diffs with 1 line of before/after

[Jerry Zhang <jerry@skydio.com>]

Normally diffs will contain a hunk header of the format
"@@ -2,2 +2,15 @@ code". However in certain situations git can
omit the second comma separated value in either before or after
line counts in the case where there is only one line.

This can produce hunk headers that look like
"@@ -2 +2,18 @@ code" or "@@ -2,2 +2 @@ code".
As a result, scan_hunk_header mistakenly returns the line
number as line count, which then results in unpredictable
parsing errors with the rest of the patch, including giving
multiple lines of output for a single commit.

Fix by explicitly setting line count to 1 when there is
no comma, and add a test.

apply.c contains this same logic except it is correct. A
worthwhile future project might be to unify these two diff
parsers so they both benefit from fixes.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
---
V1->V2:
- Removed references to -U1 since this isn't limited to that
- Added notes for future tasks

 builtin/patch-id.c  |  9 +++++++--
 t/t4204-patch-id.sh | 32 +++++++++++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/builtin/patch-id.c b/builtin/patch-id.c
index 822ffff51f..881fcf3273 100644
--- a/builtin/patch-id.c
+++ b/builtin/patch-id.c
@@ -30,26 +30,31 @@ static int scan_hunk_header(const char *p, int *p_before, int *p_after)
 
 	q = p + 4;
 	n = strspn(q, digits);
 	if (q[n] == ',') {
 		q += n + 1;
+		*p_before = atoi(q);
 		n = strspn(q, digits);
+	} else {
+		*p_before = 1;
 	}
+
 	if (n == 0 || q[n] != ' ' || q[n+1] != '+')
 		return 0;
 
 	r = q + n + 2;
 	n = strspn(r, digits);
 	if (r[n] == ',') {
 		r += n + 1;
+		*p_after = atoi(r);
 		n = strspn(r, digits);
+	} else {
+		*p_after = 1;
 	}
 	if (n == 0)
 		return 0;
 
-	*p_before = atoi(q);
-	*p_after = atoi(r);
 	return 1;
 }
 
 static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 			   struct strbuf *line_buf, int stable)
diff --git a/t/t4204-patch-id.sh b/t/t4204-patch-id.sh
index 80f4a65b28..34a386aee2 100755
--- a/t/t4204-patch-id.sh
+++ b/t/t4204-patch-id.sh
@@ -36,11 +36,11 @@ test_expect_success 'patch-id output is well-formed' '
 calc_patch_id () {
 	patch_name="$1"
 	shift
 	git patch-id "$@" >patch-id.output &&
 	sed "s/ .*//" patch-id.output >patch-id_"$patch_name" &&
-	test_line_count -gt 0 patch-id_"$patch_name"
+	test_line_count -eq 1 patch-id_"$patch_name"
 }
 
 get_top_diff () {
 	git log -p -1 "$@" -O bar-then-foo --
 }
@@ -200,6 +200,36 @@ EOF
 test_expect_success 'patch-id handles no-nl-at-eof markers' '
 	cat nonl | calc_patch_id nonl &&
 	cat withnl | calc_patch_id withnl &&
 	test_cmp patch-id_nonl patch-id_withnl
 '
+
+cat >diffu1 <<\EOF
+diff --git a/bar b/bar
+index bdaf90f..31051f6 100644
+--- a/bar
++++ b/bar
+@@ -2 +2,2 @@
+ b
++c
+diff --git a/car b/car
+index 00750ed..2ae5e34 100644
+--- a/car
++++ b/car
+@@ -1 +1,2 @@
+ 3
++d
+diff --git a/foo b/foo
+index e439850..7146eb8 100644
+--- a/foo
++++ b/foo
+@@ -2 +2,2 @@
+ a
++e
+EOF
+
+test_expect_success 'patch-id handles diffs with one line of before/after'
+	cat diffu1 | calc_patch_id diffu1 &&
+	test_config patchid.stable true &&
+	cat diffu1 | calc_patch_id diffu1stable
+'
 test_done
-- 
2.32.0.1314.g6ed4fcc4cc