[PATCH v2 1/7] bulk-checkin: factor out `format_object_header_hash()`

[Taylor Blau <me@ttaylorr.com>]

Before deflating a blob into a pack, the bulk-checkin mechanism prepares
the pack object header by calling `format_object_header()`, and writing
into a scratch buffer, the contents of which eventually makes its way
into the pack.

Future commits will add support for deflating multiple kinds of objects
into a pack, and will likewise need to perform a similar operation as
below.

This is a mostly straightforward extraction, with one notable exception.
Instead of hard-coding `the_hash_algo`, pass it in to the new function
as an argument. This isn't strictly necessary for our immediate purposes
here, but will prove useful in the future if/when the bulk-checkin
mechanism grows support for the hash transition plan.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 bulk-checkin.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/bulk-checkin.c b/bulk-checkin.c
index 6ce62999e5..fd3c110d1c 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -247,6 +247,22 @@ static void prepare_to_stream(struct bulk_checkin_packfile *state,
 		die_errno("unable to write pack header");
 }
 
+static void format_object_header_hash(const struct git_hash_algo *algop,
+				      git_hash_ctx *ctx,
+				      struct hashfile_checkpoint *checkpoint,
+				      enum object_type type,
+				      size_t size)
+{
+	unsigned char header[16384];
+	unsigned header_len = format_object_header((char *)header,
+						   sizeof(header),
+						   type, size);
+
+	algop->init_fn(ctx);
+	algop->update_fn(ctx, header, header_len);
+	algop->init_fn(&checkpoint->ctx);
+}
+
 static int deflate_blob_to_pack(struct bulk_checkin_packfile *state,
 				struct object_id *result_oid,
 				int fd, size_t size,
@@ -254,8 +270,6 @@ static int deflate_blob_to_pack(struct bulk_checkin_packfile *state,
 {
 	off_t seekback, already_hashed_to;
 	git_hash_ctx ctx;
-	unsigned char obuf[16384];
-	unsigned header_len;
 	struct hashfile_checkpoint checkpoint = {0};
 	struct pack_idx_entry *idx = NULL;
 
@@ -263,11 +277,8 @@ static int deflate_blob_to_pack(struct bulk_checkin_packfile *state,
 	if (seekback == (off_t) -1)
 		return error("cannot find the current offset");
 
-	header_len = format_object_header((char *)obuf, sizeof(obuf),
-					  OBJ_BLOB, size);
-	the_hash_algo->init_fn(&ctx);
-	the_hash_algo->update_fn(&ctx, obuf, header_len);
-	the_hash_algo->init_fn(&checkpoint.ctx);
+	format_object_header_hash(the_hash_algo, &ctx, &checkpoint, OBJ_BLOB,
+				  size);
 
 	/* Note: idx is non-NULL when we are writing */
 	if ((flags & HASH_WRITE_OBJECT) != 0)
-- 
2.42.0.405.gdb2a2f287e