[PATCH] send-pack: use buffered I/O to talk to pack-objects

[Jeff King <peff@peff.net>]

On Wed, Jun 08, 2016 at 03:19:18PM -0400, Jeff King wrote:

> That made me wonder if we could repeatedly reuse a buffer attached to
> the file descriptor. And indeed, isn't that what stdio is? The whole
> reason this buffer exists is because we are using a direct descriptor
> write. If we switched this function to use fprintf(), we'd avoid the
> whole buffer question, have a fixed cap on our memory use (since we just
> flush anytime the buffer is full) _and_ we'd reduce the number of
> write syscalls we're making by almost a factor of 100.

So all of this strbuf discussion aside, I think it is worth doing
something like this for this particular case.

-- >8 --
Subject: send-pack: use buffered I/O to talk to pack-objects

We start a pack-objects process and then write all of the
positive and negative sha1s to it over a pipe. We do so by
formatting each item into a fixed-size buffer and then
writing each individually. This has two drawbacks:

  1. There's some manual computation of the buffer size,
     which is not immediately obvious is correct (though it
     is).

  2. We write() once per sha1, which means a lot more system
     calls than are necessary.

We can solve both by wrapping the pipe descriptor in a stdio
handle; this is the same technique used by upload-pack when
serving fetches.

Note that we can also simplify and improve the error
handling here. The original detected a single write error
and broke out of the loop (presumably to avoid writing the
error message over and over), but never actually acted on
seeing an error; we just fed truncated input and took
whatever pack-objects returned.

In practice, this probably didn't matter, as the likely
errors would be caused by pack-objects dying (and we'd
probably just die with SIGPIPE anyway). But we can easily
make this simpler and more robust; the stdio handle keeps an
error flag, which we can check at the end.

Signed-off-by: Jeff King <peff@peff.net>
---
 send-pack.c | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/send-pack.c b/send-pack.c
index 37ee04e..299d303 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -36,18 +36,15 @@ int option_parse_push_signed(const struct option *opt,
 	die("bad %s argument: %s", opt->long_name, arg);
 }
 
-static int feed_object(const unsigned char *sha1, int fd, int negative)
+static void feed_object(const unsigned char *sha1, FILE *fh, int negative)
 {
-	char buf[42];
-
 	if (negative && !has_sha1_file(sha1))
-		return 1;
+		return;
 
-	memcpy(buf + negative, sha1_to_hex(sha1), 40);
 	if (negative)
-		buf[0] = '^';
-	buf[40 + negative] = '\n';
-	return write_or_whine(fd, buf, 41 + negative, "send-pack: send refs");
+		putc('^', fh);
+	fputs(sha1_to_hex(sha1), fh);
+	putc('\n', fh);
 }
 
 /*
@@ -73,6 +70,7 @@ static int pack_objects(int fd, struct ref *refs, struct sha1_array *extra, stru
 		NULL,
 	};
 	struct child_process po = CHILD_PROCESS_INIT;
+	FILE *po_in;
 	int i;
 
 	i = 4;
@@ -97,21 +95,22 @@ static int pack_objects(int fd, struct ref *refs, struct sha1_array *extra, stru
 	 * We feed the pack-objects we just spawned with revision
 	 * parameters by writing to the pipe.
 	 */
+	po_in = xfdopen(po.in, "w");
 	for (i = 0; i < extra->nr; i++)
-		if (!feed_object(extra->sha1[i], po.in, 1))
-			break;
+		feed_object(extra->sha1[i], po_in, 1);
 
 	while (refs) {
-		if (!is_null_oid(&refs->old_oid) &&
-		    !feed_object(refs->old_oid.hash, po.in, 1))
-			break;
-		if (!is_null_oid(&refs->new_oid) &&
-		    !feed_object(refs->new_oid.hash, po.in, 0))
-			break;
+		if (!is_null_oid(&refs->old_oid))
+			feed_object(refs->old_oid.hash, po_in, 1);
+		if (!is_null_oid(&refs->new_oid))
+			feed_object(refs->new_oid.hash, po_in, 0);
 		refs = refs->next;
 	}
 
-	close(po.in);
+	fflush(po_in);
+	if (ferror(po_in))
+		die_errno("error writing to pack-objects");
+	fclose(po_in);
 
 	if (args->stateless_rpc) {
 		char *buf = xmalloc(LARGE_PACKET_MAX);
-- 
2.9.0.rc2.138.geb72a36