pack-bitmap: do not use gcc packed attribute

The "__attribute__" flag may be a noop on some compilers.
That's OK as long as the code is correct without the
attribute, but in this case it is not. We would typically
end up with a struct that is 2 bytes too long due to struct
padding, breaking both reading and writing of bitmaps.

Instead of marshalling the data in a struct, let's just
provide helpers for reading and writing the appropriate
types. Besides being correct on all platforms, the result is
more efficient and simpler to read.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

csum-file.h

@@ -39,4 +39,15 @@ extern void sha1flush(struct sha1file *f);
 extern void crc32_begin(struct sha1file *);
 extern uint32_t crc32_end(struct sha1file *);
 
+static inline void sha1write_u8(struct sha1file *f, uint8_t data)
+{
+	sha1write(f, &data, sizeof(data));
+}
+
+static inline void sha1write_be32(struct sha1file *f, uint32_t data)
+{
+	data = htonl(data);
+	sha1write(f, &data, sizeof(data));
+}
+
 #endif

pack-bitmap-write.c

@@ -473,7 +473,6 @@ static void write_selected_commits_v1(struct sha1file *f,
 
 	for (i = 0; i < writer.selected_nr; ++i) {
 		struct bitmapped_commit *stored = &writer.selected[i];
-		struct bitmap_disk_entry on_disk;
 
 		int commit_pos =
 			sha1_pos(stored->commit->object.sha1, index, index_nr, sha1_access);
@@ -481,11 +480,10 @@ static void write_selected_commits_v1(struct sha1file *f,
 		if (commit_pos < 0)
 			die("BUG: trying to write commit not in index");
 
-		on_disk.object_pos = htonl(commit_pos);
-		on_disk.xor_offset = stored->xor_offset;
-		on_disk.flags = stored->flags;
+		sha1write_be32(f, commit_pos);
+		sha1write_u8(f, stored->xor_offset);
+		sha1write_u8(f, stored->flags);
 
-		sha1write(f, &on_disk, sizeof(on_disk));
 		dump_bitmap(f, stored->write_as);
 	}
 }

pack-bitmap.c

@@ -197,13 +197,24 @@ static struct stored_bitmap *store_bitmap(struct bitmap_index *index,
 	return stored;
 }
 
+static inline uint32_t read_be32(const unsigned char *buffer, size_t *pos)
+{
+	uint32_t result = get_be32(buffer + *pos);
+	(*pos) += sizeof(result);
+	return result;
+}
+
+static inline uint8_t read_u8(const unsigned char *buffer, size_t *pos)
+{
+	return buffer[(*pos)++];
+}
+
 static int load_bitmap_entries_v1(struct bitmap_index *index)
 {
 	static const size_t MAX_XOR_OFFSET = 160;
 
 	uint32_t i;
 	struct stored_bitmap **recent_bitmaps;
-	struct bitmap_disk_entry *entry;
 
 	recent_bitmaps = xcalloc(MAX_XOR_OFFSET, sizeof(struct stored_bitmap));
 
@@ -214,15 +225,12 @@ static int load_bitmap_entries_v1(struct bitmap_index *index)
 		uint32_t commit_idx_pos;
 		const unsigned char *sha1;
 
-		entry = (struct bitmap_disk_entry *)(index->map + index->map_pos);
-		index->map_pos += sizeof(struct bitmap_disk_entry);
+		commit_idx_pos = read_be32(index->map, &index->map_pos);
+		xor_offset = read_u8(index->map, &index->map_pos);
+		flags = read_u8(index->map, &index->map_pos);
 
-		commit_idx_pos = ntohl(entry->object_pos);
 		sha1 = nth_packed_object_sha1(index->pack, commit_idx_pos);
 
-		xor_offset = (int)entry->xor_offset;
-		flags = (int)entry->flags;
-
 		bitmap = read_bitmap_1(index);
 		if (!bitmap)
 			return -1;

pack-bitmap.h

@@ -5,12 +5,6 @@
 #include "khash.h"
 #include "pack-objects.h"
 
-struct bitmap_disk_entry {
-	uint32_t object_pos;
-	uint8_t xor_offset;
-	uint8_t flags;
-} __attribute__((packed));
-
 struct bitmap_disk_header {
 	char magic[4];
 	uint16_t version;