refs: write packed_refs file using stdio

We write each line of a new packed-refs file individually
using a write() syscall (and sometimes 2, if the ref is
peeled). Since each line is only about 50-100 bytes long,
this creates a lot of system call overhead.

We can instead open a stdio handle around our descriptor and
use fprintf to write to it. The extra buffering is not a
problem for us, because nobody will read our new packed-refs
file until we call commit_lock_file (by which point we have
flushed everything).

On a pathological repository with 8.5 million refs, this
dropped the time to run `git pack-refs` from 20s to 6s.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index 4d5b76c76a..bc286ce22d 100644
--- a/cache.h
+++ b/cache.h
@@ -1395,6 +1395,8 @@ extern const char *git_mailmap_blob;
 
 /* IO helper functions */
 extern void maybe_flush_or_die(FILE *, const char *);
+__attribute__((format (printf, 2, 3)))
+extern void fprintf_or_die(FILE *, const char *fmt, ...);
 extern int copy_fd(int ifd, int ofd);
 extern int copy_file(const char *dst, const char *src, int mode);
 extern int copy_file_with_time(const char *dst, const char *src, int mode);