trailer: teach iterator about non-trailer lines

Previously the iterator did not iterate over non-trailer lines. This was
somewhat unfortunate, because trailer blocks could have non-trailer
lines in them since 146245063e (trailer: allow non-trailers in trailer
block, 2016-10-21), which was before the iterator was created in
f0939a0eb1 (trailer: add interface for iterating over commit trailers,
2020-09-27).

So if trailer API users wanted to iterate over all lines in a trailer
block (including non-trailer lines), they could not use the iterator and
were forced to use the lower-level trailer_info struct directly (which
provides a raw string array that includes all lines in the trailer
block).

Change the iterator's behavior so that we also iterate over non-trailer
lines, instead of skipping over them. The new "raw" member of the
iterator allows API users to access previously inaccessible non-trailer
lines. Reword the variable "trailer" to just "line" because this
variable can now hold both trailer lines _and_ non-trailer lines.

The new "raw" member is important because anyone currently not using the
iterator is using trailer_info's raw string array directly to access
lines to check what the combined key + value looks like. If we didn't
provide a "raw" member here, iterator users would have to re-construct
the unparsed line by concatenating the key and value back together again

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-trailer.c

@@ -1,7 +1,7 @@
 #include "test-lib.h"
 #include "trailer.h"
 
-static void t_trailer_iterator(const char *msg, size_t num_expected_trailers)
+static void t_trailer_iterator(const char *msg, size_t num_expected)
 {
 	struct trailer_iterator iter;
 	size_t i = 0;
@@ -11,7 +11,7 @@ static void t_trailer_iterator(const char *msg, size_t num_expected_trailers)
 		i++;
 	trailer_iterator_release(&iter);
 
-	check_uint(i, ==, num_expected_trailers);
+	check_uint(i, ==, num_expected);
 }
 
 static void run_t_trailer_iterator(void)
@@ -19,7 +19,7 @@ static void run_t_trailer_iterator(void)
 	static struct test_cases {
 		const char *name;
 		const char *msg;
-		size_t num_expected_trailers;
+		size_t num_expected;
 	} tc[] = {
 		{
 			"empty input",
@@ -119,7 +119,13 @@ static void run_t_trailer_iterator(void)
 			"not a trailer line\n"
 			"not a trailer line\n"
 			"Signed-off-by: x\n",
-			1
+			/*
+			 * Even though there is only really 1 real "trailer"
+			 * (Signed-off-by), we still have 4 trailer objects
+			 * because we still want to iterate through the entire
+			 * block.
+			 */
+			4
 		},
 		{
 			"with non-trailer lines (one too many) in trailer block",
@@ -162,7 +168,7 @@ static void run_t_trailer_iterator(void)
 
 	for (int i = 0; i < sizeof(tc) / sizeof(tc[0]); i++) {
 		TEST(t_trailer_iterator(tc[i].msg,
-					tc[i].num_expected_trailers),
+					tc[i].num_expected),
 		     "%s", tc[i].name);
 	}
 }

trailer.c

@@ -1146,17 +1146,15 @@ void trailer_iterator_init(struct trailer_iterator *iter, const char *msg)
 
 int trailer_iterator_advance(struct trailer_iterator *iter)
 {
-	while (iter->internal.cur < iter->internal.info.trailer_nr) {
-		char *trailer = iter->internal.info.trailers[iter->internal.cur++];
-		int separator_pos = find_separator(trailer, separators);
-
-		if (separator_pos < 1)
-			continue; /* not a real trailer */
+	if (iter->internal.cur < iter->internal.info.trailer_nr) {
+		char *line = iter->internal.info.trailers[iter->internal.cur++];
+		int separator_pos = find_separator(line, separators);
 
+		iter->raw = line;
 		strbuf_reset(&iter->key);
 		strbuf_reset(&iter->val);
 		parse_trailer(&iter->key, &iter->val, NULL,
-			      trailer, separator_pos);
+			      line, separator_pos);
 		/* Always unfold values during iteration. */
 		unfold_value(&iter->val);
 		return 1;

trailer.h

@@ -125,6 +125,13 @@ void format_trailers_from_commit(const struct process_trailer_options *,
  *   trailer_iterator_release(&iter);
  */
 struct trailer_iterator {
+	/*
+	 * Raw line (e.g., "foo: bar baz") before being parsed as a trailer
+	 * key/val pair as part of a trailer block (as the "key" and "val"
+	 * fields below). If a line fails to parse as a trailer, then the "key"
+	 * will be the entire line and "val" will be the empty string.
+	 */
+	const char *raw;
 	struct strbuf key;
 	struct strbuf val;