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, 15 insertions, 0 deletions

diff --git a/write_or_die.c b/write_or_die.c
index b50f99a..e7afe7a 100644
--- a/write_or_die.c
+++ b/write_or_die.c
@@ -49,6 +49,21 @@ void maybe_flush_or_die(FILE *f, const char *desc)
 	}
 }
 
+void fprintf_or_die(FILE *f, const char *fmt, ...)
+{
+	va_list ap;
+	int ret;
+
+	va_start(ap, fmt);
+	ret = vfprintf(f, fmt, ap);
+	va_end(ap);
+
+	if (ret < 0) {
+		check_pipe(errno);
+		die_errno("write error");
+	}
+}
+
 void fsync_or_die(int fd, const char *msg)
 {
 	if (fsync(fd) < 0) {