write_pack_header(): a helper function

Factor out a small logic out of the private write_pack_file() function
in builtin/pack-objects.c

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -571,7 +571,6 @@ static void write_pack_file(void)
 	uint32_t i = 0, j;
 	struct sha1file *f;
 	off_t offset;
-	struct pack_header hdr;
 	uint32_t nr_remaining = nr_result;
 	time_t last_mtime = 0;
 	struct object_entry **write_order;
@@ -596,11 +595,9 @@ static void write_pack_file(void)
 			f = sha1fd(fd, pack_tmp_name);
 		}
 
-		hdr.hdr_signature = htonl(PACK_SIGNATURE);
+		offset = write_pack_header(f, nr_remaining);
-		hdr.hdr_version = htonl(PACK_VERSION);
+		if (!offset)
-		hdr.hdr_entries = htonl(nr_remaining);
+			die_errno("unable to write pack header");
-		sha1write(f, &hdr, sizeof(hdr));
-		offset = sizeof(hdr);
 		nr_written = 0;
 		for (; i < nr_objects; i++) {
 			struct object_entry *e = write_order[i];

pack-write.c

@@ -178,6 +178,18 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 	return index_name;
 }
 
+off_t write_pack_header(struct sha1file *f, uint32_t nr_entries)
+{
+	struct pack_header hdr;
+
+	hdr.hdr_signature = htonl(PACK_SIGNATURE);
+	hdr.hdr_version = htonl(PACK_VERSION);
+	hdr.hdr_entries = htonl(nr_entries);
+	if (sha1write(f, &hdr, sizeof(hdr)))
+		return 0;
+	return sizeof(hdr);
+}
+
 /*
  * Update pack header with object_count and compute new SHA1 for pack data
  * associated to pack_fd, and write that SHA1 at the end.  That new SHA1

pack.h

@@ -2,6 +2,7 @@
 #define PACK_H
 
 #include "object.h"
+#include "csum-file.h"
 
 /*
  * Packed object header
@@ -74,6 +75,7 @@ extern const char *write_idx_file(const char *index_name, struct pack_idx_entry
 extern int check_pack_crc(struct packed_git *p, struct pack_window **w_curs, off_t offset, off_t len, unsigned int nr);
 extern int verify_pack_index(struct packed_git *);
 extern int verify_pack(struct packed_git *);
+extern off_t write_pack_header(struct sha1file *f, uint32_t);
 extern void fixup_pack_header_footer(int, unsigned char *, const char *, uint32_t, unsigned char *, off_t);
 extern char *index_pack_lockfile(int fd);
 extern int encode_in_pack_object_header(enum object_type, uintmax_t, unsigned char *);