midx-write.c: support reading an existing MIDX with packs_to_include

Avoid unconditionally copying all packs from an existing MIDX into a new
MIDX by checking that packs added via `fill_packs_from_midx()` don't
appear in the `to_include` set, if one was provided.

Do so by calling `should_include_pack()` from both `add_pack_to_midx()`
and `fill_packs_from_midx()`.

In order to make this work, teach `should_include_pack()` a new
"exclude_from_midx" parameter, which allows skipping the first check.
This is done so that the caller in `fill_packs_from_midx()` doesn't
reject all of the packs it provided since they appear in an existing
MIDX by definition.

The sum total of this change is that we are now able to read and
reference objects in an existing MIDX even when given a non-NULL
`packs_to_include`. This is a prerequisite step for incremental MIDXs,
which need to load any existing MIDX (if one is present) in order to
determine whether or not an object already appears in an earlier portion
of the MIDX to avoid duplicating it across multiple portions.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx-write.c

@@ -101,27 +101,13 @@ struct write_midx_context {
 };
 
 static int should_include_pack(const struct write_midx_context *ctx,
-			       const char *file_name)
+			       const char *file_name,
+			       int exclude_from_midx)
 {
-	/*
-	 * Note that at most one of ctx->m and ctx->to_include are set,
-	 * so we are testing midx_contains_pack() and
-	 * string_list_has_string() independently (guarded by the
-	 * appropriate NULL checks).
-	 *
-	 * We could support passing to_include while reusing an existing
-	 * MIDX, but don't currently since the reuse process drags
-	 * forward all packs from an existing MIDX (without checking
-	 * whether or not they appear in the to_include list).
-	 *
-	 * If we added support for that, these next two conditional
-	 * should be performed independently (likely checking
-	 * to_include before the existing MIDX).
-	 */
-	if (ctx->m && midx_contains_pack(ctx->m, file_name))
+	if (exclude_from_midx && ctx->m && midx_contains_pack(ctx->m, file_name))
 		return 0;
-	else if (ctx->to_include &&
-		 !string_list_has_string(ctx->to_include, file_name))
+	if (ctx->to_include && !string_list_has_string(ctx->to_include,
+						       file_name))
 		return 0;
 	return 1;
 }
@@ -135,7 +121,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 	if (ends_with(file_name, ".idx")) {
 		display_progress(ctx->progress, ++ctx->pack_paths_checked);
 
-		if (!should_include_pack(ctx, file_name))
+		if (!should_include_pack(ctx, file_name, 1))
 			return;
 
 		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
@@ -888,6 +874,9 @@ static int fill_packs_from_midx(struct write_midx_context *ctx,
 	uint32_t i;
 
 	for (i = 0; i < ctx->m->num_packs; i++) {
+		if (!should_include_pack(ctx, ctx->m->pack_names[i], 0))
+			continue;
+
 		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
 
 		if (flags & MIDX_WRITE_REV_INDEX || preferred_pack_name) {
@@ -942,15 +931,7 @@ static int write_midx_internal(const char *object_dir,
 		die_errno(_("unable to create leading directories of %s"),
 			  midx_name.buf);
 
-	if (!packs_to_include) {
-		/*
-		 * Only reference an existing MIDX when not filtering which
-		 * packs to include, since all packs and objects are copied
-		 * blindly from an existing MIDX if one is present.
-		 */
-		ctx.m = lookup_multi_pack_index(the_repository, object_dir);
-	}
-
+	ctx.m = lookup_multi_pack_index(the_repository, object_dir);
 	if (ctx.m && !midx_checksum_valid(ctx.m)) {
 		warning(_("ignoring existing multi-pack-index; checksum mismatch"));
 		ctx.m = NULL;
@@ -959,6 +940,7 @@ static int write_midx_internal(const char *object_dir,
 	ctx.nr = 0;
 	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
 	ctx.info = NULL;
+	ctx.to_include = packs_to_include;
 	ALLOC_ARRAY(ctx.info, ctx.alloc);
 
 	if (ctx.m && fill_packs_from_midx(&ctx, preferred_pack_name,
@@ -975,8 +957,6 @@ static int write_midx_internal(const char *object_dir,
 	else
 		ctx.progress = NULL;
 
-	ctx.to_include = packs_to_include;
-
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);