reader: fix handling of when growing a reader

When growing a reader that is in buffered mode, the offset was not
updated correctly. This caused the reader to append overlapping data
twice and therefore corrupting the data.

This commit contains the fix as well as a test case for the metablock
reader. Generalizing the test for the generic reader is left for later.

lib/reader/reader.c

@@ -156,7 +156,8 @@ handle_buffered(struct SqshReader *reader, sqsh_index_t offset, size_t size) {
 
 	struct SqshBuffer *buffer = &reader->buffer;
 	const uint8_t *buffer_data = sqsh__buffer_data(buffer);
-	const size_t copy_size = iterator_offset - offset;
+	size_t buffer_size = sqsh__buffer_size(buffer);
+	const size_t copy_size = buffer_size - offset;
 
 	if (offset != 0) {
 		rv = sqsh__buffer_init(&new_buffer);
@@ -172,6 +173,8 @@ handle_buffered(struct SqshReader *reader, sqsh_index_t offset, size_t size) {
 			goto out;
 		}
 	}
+
+	reader->offset = buffer_size - iterator_offset;
 	rv = reader_fill_buffer(reader, size);
 
 out:

test/metablock/metablock_reader.c

@@ -197,6 +197,50 @@ advance_overlapping_2(void) {
 	sqsh__archive_cleanup(&sqsh);
 }
 
+static void
+advance_overlapping_3(void) {
+	int rv;
+	struct SqshArchive sqsh = {0};
+	struct SqshMetablockReader cursor;
+	uint8_t payload[] = {
+			/* clang-format off */
+			SQSH_HEADER,
+			[1024] = METABLOCK_HEADER(0, 8192),
+			[1024 + SQSH_SIZEOF_METABLOCK+8180] =
+			1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
+			[1024 + SQSH_SIZEOF_METABLOCK + 8192] = METABLOCK_HEADER(0, 32 - 12),
+			13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32,
+			/* clang-format on */
+	};
+	uint8_t expected[] = {1,  2,  3,  4,  5,  6,  7,  8,  9,  10, 11,
+						  12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
+						  23, 24, 25, 26, 27, 28, 29, 30, 31, 32};
+	const uint8_t *data;
+	mk_stub(&sqsh, payload, sizeof(payload));
+
+	rv = sqsh__metablock_reader_init(&cursor, &sqsh, 1024, sizeof(payload));
+	assert(rv == 0);
+
+	rv = sqsh__metablock_reader_advance(&cursor, 8180, 16);
+	assert(rv == 0);
+	assert(sqsh__metablock_reader_size(&cursor) == 16);
+	data = sqsh__metablock_reader_data(&cursor);
+	assert(data != NULL);
+	assert(memcmp(data, expected, 16) == 0);
+
+	rv = sqsh__metablock_reader_advance(&cursor, 0, 32);
+	assert(rv == 0);
+	assert(sqsh__metablock_reader_size(&cursor) == 32);
+	data = sqsh__metablock_reader_data(&cursor);
+	assert(data != NULL);
+	assert(memcmp(data, expected, 32) == 0);
+
+	rv = sqsh__metablock_reader_cleanup(&cursor);
+	assert(rv == 0);
+
+	sqsh__archive_cleanup(&sqsh);
+}
+
 static void
 advance_skip(void) {
 	int rv;
@@ -308,6 +352,7 @@ TEST(advance_once)
 TEST(advance_twice)
 TEST(advance_overlapping)
 TEST(advance_overlapping_2)
+TEST(advance_overlapping_3)
 TEST(advance_skip)
 TEST(advance_skip_2)
 TEST(advance_overflow)