midx-write: extract function to test whether MIDX needs updating

In `write_midx_internal()` we know to skip writing the new multi-pack
index in case it would be the same as the existing one. This logic does
not handle the `--stdin-packs` option yet though, so we end up always
rewriting the MIDX if that option is passed to us.

Extract the logic to decide whether or not to rewrite the MIDX into a
separate function. This will allow us to extend that feature in the next
commit to address the above issue.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

midx-write.c

@@ -1015,6 +1015,41 @@ static void clear_midx_files(struct odb_source *source,
 	strbuf_release(&buf);
 }
 
+static bool midx_needs_update(struct write_midx_context *ctx)
+{
+	struct multi_pack_index *midx = ctx->m;
+	bool needed = true;
+
+	/*
+	 * Ignore incremental updates for now. The assumption is that any
+	 * incremental update would be either empty (in which case we will bail
+	 * out later) or it would actually cover at least one new pack.
+	 */
+	if (ctx->incremental)
+		goto out;
+
+	/*
+	 * If there is no MIDX then either it doesn't exist, or we're doing a
+	 * geometric repack. We cannot (yet) determine whether we need to
+	 * update the multi-pack index in the second case.
+	 */
+	if (!midx)
+		goto out;
+
+	/*
+	 * Otherwise, we need to verify that the packs covered by the existing
+	 * MIDX match the packs that we already have. This test is somewhat
+	 * lenient and will be fixed.
+	 */
+	if (ctx->nr != midx->num_packs + midx->num_packs_in_base)
+		goto out;
+
+	needed = false;
+
+out:
+	return needed;
+}
+
 static int write_midx_internal(struct odb_source *source,
 			       struct string_list *packs_to_include,
 			       struct string_list *packs_to_drop,
@@ -1112,9 +1147,7 @@ static int write_midx_internal(struct odb_source *source,
 	for_each_file_in_pack_dir(source->path, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);
 
-	if ((ctx.m && ctx.nr == ctx.m->num_packs + ctx.m->num_packs_in_base) &&
-	    !ctx.incremental &&
-	    !(packs_to_include || packs_to_drop)) {
+	if (!packs_to_include && !packs_to_drop && !midx_needs_update(&ctx)) {
 		struct bitmap_index *bitmap_git;
 		int bitmap_exists;
 		int want_bitmap = flags & MIDX_WRITE_BITMAP;