[PATCH] patch-id: fix scan_hunk_header on diffs with -U1

[Jerry Zhang <jerry@skydio.com>]

Normally diffs will contain a hunk header of the format
"@@ -2,2 +2,15 @@ code". However when git diff is given "-U1"
indicating that diffs are surrounded by only one line of
context, it 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.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
---
 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..08d10dfc3e 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 -U1' '
+	cat diffu1 | calc_patch_id diffu1 &&
+	test_config patchid.stable true &&
+	cat diffu1 | calc_patch_id diffu1stable
+'
 test_done
-- 
2.32.0.1314.g6ed4fcc4cc