[PATCH v3 09/10] bulk-checkin: introduce `index_tree_bulk_checkin_incore()`

[Taylor Blau <me@ttaylorr.com>]

The remaining missing piece in order to teach the `merge-tree` builtin
how to write the contents of a merge into a pack is a function to index
tree objects into a bulk-checkin pack.

This patch implements that missing piece, which is a thin wrapper around
all of the functionality introduced in previous commits.

If and when Git gains support for a "compatibility" hash algorithm, the
changes to support that here will be minimal. The bulk-checkin machinery
will need to convert the incoming tree to compute its length under the
compatibility hash, necessary to reconstruct its header. With that
information (and the converted contents of the tree), the bulk-checkin
machinery will have enough to keep track of the converted object's hash
in order to update the compatibility mapping.

Within `deflate_tree_to_pack_incore()`, the changes should be limited
to something like:

    struct strbuf converted = STRBUF_INIT;
    if (the_repository->compat_hash_algo) {
      if (convert_object_file(&compat_obj,
                              the_repository->hash_algo,
                              the_repository->compat_hash_algo, ...) < 0)
        die(...);

      format_object_header_hash(the_repository->compat_hash_algo,
                                OBJ_TREE, size);
    }
    /* compute the converted tree's hash using the compat algorithm */
    strbuf_release(&converted);

, assuming related changes throughout the rest of the bulk-checkin
machinery necessary to update the hash of the converted object, which
are likewise minimal in size.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 bulk-checkin.c | 27 +++++++++++++++++++++++++++
 bulk-checkin.h |  4 ++++
 2 files changed, 31 insertions(+)

diff --git a/bulk-checkin.c b/bulk-checkin.c
index 9ae43648ba..d088a9c10b 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -423,6 +423,22 @@ static int deflate_blob_to_pack_incore(struct bulk_checkin_packfile *state,
 						   OBJ_BLOB, path, flags);
 }
 
+static int deflate_tree_to_pack_incore(struct bulk_checkin_packfile *state,
+				       struct object_id *result_oid,
+				       const void *buf, size_t size,
+				       const char *path, unsigned flags)
+{
+	git_hash_ctx ctx;
+	struct hashfile_checkpoint checkpoint = {0};
+
+	format_object_header_hash(the_hash_algo, &ctx, &checkpoint, OBJ_TREE,
+				  size);
+
+	return deflate_obj_contents_to_pack_incore(state, &ctx, &checkpoint,
+						   result_oid, buf, size,
+						   OBJ_TREE, path, flags);
+}
+
 static int deflate_blob_to_pack(struct bulk_checkin_packfile *state,
 				struct object_id *result_oid,
 				int fd, size_t size,
@@ -520,6 +536,17 @@ int index_blob_bulk_checkin_incore(struct object_id *oid,
 	return status;
 }
 
+int index_tree_bulk_checkin_incore(struct object_id *oid,
+				   const void *buf, size_t size,
+				   const char *path, unsigned flags)
+{
+	int status = deflate_tree_to_pack_incore(&bulk_checkin_packfile, oid,
+						 buf, size, path, flags);
+	if (!odb_transaction_nesting)
+		flush_bulk_checkin_packfile(&bulk_checkin_packfile);
+	return status;
+}
+
 void begin_odb_transaction(void)
 {
 	odb_transaction_nesting += 1;
diff --git a/bulk-checkin.h b/bulk-checkin.h
index 1b91daeaee..89786b3954 100644
--- a/bulk-checkin.h
+++ b/bulk-checkin.h
@@ -17,6 +17,10 @@ int index_blob_bulk_checkin_incore(struct object_id *oid,
 				   const void *buf, size_t size,
 				   const char *path, unsigned flags);
 
+int index_tree_bulk_checkin_incore(struct object_id *oid,
+				   const void *buf, size_t size,
+				   const char *path, unsigned flags);
+
 /*
  * Tell the object database to optimize for adding
  * multiple objects. end_odb_transaction must be called
-- 
2.42.0.408.g97fac66ae4