[RFC PATCH] object-file API: add a format_loose_header() function

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add a convenience function to wrap the xsnprintf() command that
generates loose object headers. This code was copy/pasted in various
parts of the codebase, let's define it in one place and re-use it from
there.

All except one caller of it had a valid "enum object_type" for us,
it's only write_object_file_prepare() which might need to deal with
"git hash-object --literally" and a potential garbage type. Let's have
the primary API use an "enum object_type", and define an *_extended()
function that can take an arbitrary "const char *" for the type.

See [1] for the discussion that prompted this patch, i.e. new code in
object-file.c that wanted to copy/paste the xsnprintf() invocation.

1. https://lore.kernel.org/git/211213.86bl1l9bfz.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---

On Fri, Dec 17 2021, Han Xin wrote:

> From: Han Xin <hanxin.hx@alibaba-inc.com>
>
> There are 3 places where "xsnprintf" is used to generate the object
> header, and I originally planned to add a fourth in the latter patch.
>
> According to Ævar Arnfjörð Bjarmason’s suggestion, although it's just
> one line, it's also code that's very central to git, so reafactor them
> into a function which will help later readability.
>
> Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> Signed-off-by: Han Xin <hanxin.hx@alibaba-inc.com>

I came up with this after my comment on the earlier round suggesting
to factor out that header formatting. I don't know if this more
thorough approach is worth it or if you'd like to replace your change
with this one, but just posting it here as an RFC.

 builtin/index-pack.c |  3 +--
 bulk-checkin.c       |  4 ++--
 cache.h              | 21 +++++++++++++++++++++
 http-push.c          |  2 +-
 object-file.c        | 14 +++++++++++---
 5 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index c23d01de7dc..900c6539f68 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -449,8 +449,7 @@ static void *unpack_entry_data(off_t offset, unsigned long size,
 	int hdrlen;
 
 	if (!is_delta_type(type)) {
-		hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX,
-				   type_name(type),(uintmax_t)size) + 1;
+		hdrlen = format_loose_header(hdr, sizeof(hdr), type, (uintmax_t)size);
 		the_hash_algo->init_fn(&c);
 		the_hash_algo->update_fn(&c, hdr, hdrlen);
 	} else
diff --git a/bulk-checkin.c b/bulk-checkin.c
index 8785b2ac806..446dea7c516 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -220,8 +220,8 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
 	if (seekback == (off_t) -1)
 		return error("cannot find the current offset");
 
-	header_len = xsnprintf((char *)obuf, sizeof(obuf), "%s %" PRIuMAX,
-			       type_name(type), (uintmax_t)size) + 1;
+	header_len = format_loose_header((char *)obuf, sizeof(obuf),
+					 type, (uintmax_t)size);
 	the_hash_algo->init_fn(&ctx);
 	the_hash_algo->update_fn(&ctx, obuf, header_len);
 
diff --git a/cache.h b/cache.h
index d5cafba17d4..ccece21a4a2 100644
--- a/cache.h
+++ b/cache.h
@@ -1309,6 +1309,27 @@ enum unpack_loose_header_result unpack_loose_header(git_zstream *stream,
 						    unsigned long bufsiz,
 						    struct strbuf *hdrbuf);
 
+/**
+ * format_loose_header() is a thin wrapper around s xsnprintf() that
+ * writes the initial "<type> <obj-len>" part of the loose object
+ * header. It returns the size that snprintf() returns + 1.
+ *
+ * The format_loose_header_extended() function allows for writing a
+ * type_name that's not one of the "enum object_type" types. This is
+ * used for "git hash-object --literally". Pass in a OBJ_NONE as the
+ * type, and a non-NULL "type_str" to do that.
+ *
+ * format_loose_header() is a convenience wrapper for
+ * format_loose_header_extended().
+ */
+int format_loose_header_extended(char *str, size_t size, enum object_type type,
+				 const char *type_str, size_t objsize);
+static inline int format_loose_header(char *str, size_t size,
+				      enum object_type type, size_t objsize)
+{
+	return format_loose_header_extended(str, size, type, NULL, objsize);
+}
+
 /**
  * parse_loose_header() parses the starting "<type> <len>\0" of an
  * object. If it doesn't follow that format -1 is returned. To check
diff --git a/http-push.c b/http-push.c
index 3309aaf004a..d1a8619e0af 100644
--- a/http-push.c
+++ b/http-push.c
@@ -363,7 +363,7 @@ static void start_put(struct transfer_request *request)
 	git_zstream stream;
 
 	unpacked = read_object_file(&request->obj->oid, &type, &len);
-	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX , type_name(type), (uintmax_t)len) + 1;
+	hdrlen = format_loose_header(hdr, sizeof(hdr), type, (uintmax_t)len);
 
 	/* Set it up */
 	git_deflate_init(&stream, zlib_compression_level);
diff --git a/object-file.c b/object-file.c
index eac67f6f5f9..d94609ee48d 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1009,6 +1009,14 @@ void *xmmap(void *start, size_t length,
 	return ret;
 }
 
+int format_loose_header_extended(char *str, size_t size, enum object_type type,
+				 const char *typestr, size_t objsize)
+{
+	const char *s = type == OBJ_NONE ? typestr : type_name(type);
+
+	return xsnprintf(str, size, "%s %"PRIuMAX, s, (uintmax_t)objsize) + 1;
+}
+
 /*
  * With an in-core object data in "map", rehash it to make sure the
  * object name actually matches "oid" to detect object corruption.
@@ -1037,7 +1045,7 @@ int check_object_signature(struct repository *r, const struct object_id *oid,
 		return -1;
 
 	/* Generate the header */
-	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX , type_name(obj_type), (uintmax_t)size) + 1;
+	hdrlen = format_loose_header(hdr, sizeof(hdr), obj_type, size);
 
 	/* Sha1.. */
 	r->hash_algo->init_fn(&c);
@@ -1737,7 +1745,7 @@ static void write_object_file_prepare(const struct git_hash_algo *algo,
 	git_hash_ctx c;
 
 	/* Generate the header */
-	*hdrlen = xsnprintf(hdr, *hdrlen, "%s %"PRIuMAX , type, (uintmax_t)len)+1;
+	*hdrlen = format_loose_header_extended(hdr, *hdrlen, OBJ_NONE, type, len);
 
 	/* Sha1.. */
 	algo->init_fn(&c);
@@ -2009,7 +2017,7 @@ int force_object_loose(const struct object_id *oid, time_t mtime)
 	buf = read_object(the_repository, oid, &type, &len);
 	if (!buf)
 		return error(_("cannot read object for %s"), oid_to_hex(oid));
-	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX , type_name(type), (uintmax_t)len) + 1;
+	hdrlen = format_loose_header(hdr, sizeof(hdr), type, len);
 	ret = write_loose_object(oid, hdr, hdrlen, buf, len, mtime, 0);
 	free(buf);
 
-- 
2.34.1.1119.g606023410ba