midx-write.c: pass start_pack to compute_sorted_entries()

The function `compute_sorted_entries()` is broadly responsible for
building an array of the objects to be written into a MIDX based on the
provided list of packs.

If we have loaded an existing MIDX, however, we may not use all of its
packs, despite loading them into the ctx->info array.

The existing implementation simply skips past the first
ctx->m->num_packs (if ctx->m is non-NULL, indicating that we loaded an
existing MIDX). This is because we read objects in packs from an
existing MIDX via the MIDX itself, rather than from the pack-level
fanout to guarantee a de-duplicated result (see: a40498a (midx: use
existing midx when writing new one, 2018-07-12)).

Future changes (outside the scope of this patch series) to the MIDX code
will require us to skip *at most* that number[^1].

We could tag each pack with a bit that indicates the pack's contents
should be included in the MIDX. But we can just as easily determine the
number of packs to skip by passing in the number of packs we learned
about after processing an existing MIDX.

[^1]: Kind of. The real number will be bounded by the number of packs in
  a MIDX layer, and the number of packs in its base layer(s), but that
  concept hasn't been fully defined yet.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

midx-write.c

@@ -299,12 +299,12 @@ static void midx_fanout_add_pack_fanout(struct midx_fanout *fanout,
  * Copy only the de-duplicated entries (selected by most-recent modified time
  * of a packfile containing the object).
  */
-static void compute_sorted_entries(struct write_midx_context *ctx)
+static void compute_sorted_entries(struct write_midx_context *ctx,
+				   uint32_t start_pack)
 {
 	uint32_t cur_fanout, cur_pack, cur_object;
 	size_t alloc_objects, total_objects = 0;
 	struct midx_fanout fanout = { 0 };
-	uint32_t start_pack = ctx->m ? ctx->m->num_packs : 0;
 
 	for (cur_pack = start_pack; cur_pack < ctx->nr; cur_pack++)
 		total_objects = st_add(total_objects,
@@ -883,7 +883,7 @@ static int write_midx_internal(const char *object_dir,
 {
 	struct strbuf midx_name = STRBUF_INIT;
 	unsigned char midx_hash[GIT_MAX_RAWSZ];
-	uint32_t i;
+	uint32_t i, start_pack;
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct write_midx_context ctx = { 0 };
@@ -951,6 +951,8 @@ static int write_midx_internal(const char *object_dir,
 		}
 	}
 
+	start_pack = ctx.nr;
+
 	ctx.pack_paths_checked = 0;
 	if (flags & MIDX_PROGRESS)
 		ctx.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
@@ -1048,7 +1050,7 @@ static int write_midx_internal(const char *object_dir,
 		}
 	}
 
-	compute_sorted_entries(&ctx);
+	compute_sorted_entries(&ctx, start_pack);
 
 	ctx.large_offsets_needed = 0;
 	for (i = 0; i < ctx.entries_nr; i++) {