midx-write: use uint32_t for preferred_pack_idx

midx-write.c has the DISABLE_SIGN_COMPARE_WARNINGS macro defined for a
few reasons, but the biggest one is the use of a signed
preferred_pack_idx member inside the write_midx_context struct. The code
currently uses -1 to indicate an unset preferred pack but pack int ids
are normally handled as uint32_t. There are also a few loops that search
for the preferred pack by name and those iterators will need updates to
uint32_t in the next change.

For now, replace the use of -1 with a 'NO_PREFERRED_PACK' macro and an
equality check. The macro stores the max value of a uint32_t, so we
cannot store a preferred pack that appears last in a list of 2^32 total
packs, but that's expected to be unreasonable already. This improves the
range from 2^31 already.

There are some careful things to worry about with initializing the
preferred pack in the struct and using that value when searching for a
preferred pack that was already incorrect but accidentally working when
the index was initialized to zero.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

midx-write.c

@@ -24,6 +24,7 @@
 #define BITMAP_POS_UNKNOWN (~((uint32_t)0))
 #define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
+#define NO_PREFERRED_PACK (~((uint32_t)0))
 
 extern int midx_checksum_valid(struct multi_pack_index *m);
 extern void clear_midx_files_ext(const char *object_dir, const char *ext,
@@ -104,7 +105,7 @@ struct write_midx_context {
 	unsigned large_offsets_needed:1;
 	uint32_t num_large_offsets;
 
-	int preferred_pack_idx;
+	uint32_t preferred_pack_idx;
 
 	int incremental;
 	uint32_t num_multi_pack_indexes_before;
@@ -260,7 +261,7 @@ static void midx_fanout_sort(struct midx_fanout *fanout)
 static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
 					struct multi_pack_index *m,
 					uint32_t cur_fanout,
-					int preferred_pack)
+					uint32_t preferred_pack)
 {
 	uint32_t start = m->num_objects_in_base, end;
 	uint32_t cur_object;
@@ -274,7 +275,7 @@ static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
 	end = m->num_objects_in_base + ntohl(m->chunk_oid_fanout[cur_fanout]);
 
 	for (cur_object = start; cur_object < end; cur_object++) {
-		if ((preferred_pack > -1) &&
+		if ((preferred_pack != NO_PREFERRED_PACK) &&
 		    (preferred_pack == nth_midxed_pack_int_id(m, cur_object))) {
 			/*
 			 * Objects from preferred packs are added
@@ -364,7 +365,8 @@ static void compute_sorted_entries(struct write_midx_context *ctx,
 						    preferred, cur_fanout);
 		}
 
-		if (-1 < ctx->preferred_pack_idx && ctx->preferred_pack_idx < start_pack)
+		if (ctx->preferred_pack_idx != NO_PREFERRED_PACK &&
+		    ctx->preferred_pack_idx < start_pack)
 			midx_fanout_add_pack_fanout(&fanout, ctx->info,
 						    ctx->preferred_pack_idx, 1,
 						    cur_fanout);
@@ -1060,7 +1062,9 @@ static int write_midx_internal(struct repository *r, const char *object_dir,
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct tempfile *incr;
-	struct write_midx_context ctx = { 0 };
+	struct write_midx_context ctx = {
+		.preferred_pack_idx = NO_PREFERRED_PACK,
+	 };
 	int bitmapped_packs_concat_len = 0;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
@@ -1168,7 +1172,7 @@ static int write_midx_internal(struct repository *r, const char *object_dir,
 		goto cleanup; /* nothing to do */
 
 	if (preferred_pack_name) {
-		ctx.preferred_pack_idx = -1;
+		ctx.preferred_pack_idx = NO_PREFERRED_PACK;
 
 		for (i = 0; i < ctx.nr; i++) {
 			if (!cmp_idx_or_pack_name(preferred_pack_name,
@@ -1178,12 +1182,12 @@ static int write_midx_internal(struct repository *r, const char *object_dir,
 			}
 		}
 
-		if (ctx.preferred_pack_idx == -1)
+		if (ctx.preferred_pack_idx == NO_PREFERRED_PACK)
 			warning(_("unknown preferred pack: '%s'"),
 				preferred_pack_name);
 	} else if (ctx.nr &&
 		   (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP))) {
-		struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
+		struct packed_git *oldest = ctx.info[0].p;
 		ctx.preferred_pack_idx = 0;
 
 		if (packs_to_drop && packs_to_drop->nr)
@@ -1211,17 +1215,17 @@ static int write_midx_internal(struct repository *r, const char *object_dir,
 			 * objects to resolve, so the preferred value doesn't
 			 * matter.
 			 */
-			ctx.preferred_pack_idx = -1;
+			ctx.preferred_pack_idx = NO_PREFERRED_PACK;
 		}
 	} else {
 		/*
 		 * otherwise don't mark any pack as preferred to avoid
 		 * interfering with expiration logic below
 		 */
-		ctx.preferred_pack_idx = -1;
+		ctx.preferred_pack_idx = NO_PREFERRED_PACK;
 	}
 
-	if (ctx.preferred_pack_idx > -1) {
+	if (ctx.preferred_pack_idx != NO_PREFERRED_PACK) {
 		struct packed_git *preferred = ctx.info[ctx.preferred_pack_idx].p;
 
 		if (open_pack_index(preferred))