Re: pread() over NFS (again) [1.5.5.4]

From: logank@sent.com <logank@sent.com>
Date: Jun 26, 2008, 23:36 UTC
Message-ID: <65688C06-BB6A-4E95-A4B9-A1A7C206BE2E@sent.com>
In-Reply-To: <7vskuzq5ix.fsf@gitster.siamese.dyndns.org>

On Jun 26, 2008, at 1:56 PM, Junio C Hamano wrote:

Show 7 quoted lines

>> "The file shouldn't be short unless someone truncated it, or there
>> is a bug in index-pack.  Neither is very likely, but I don't think
>> we would want to retry pread'ing the same block forever.
>
> I don't think we would want to retry even once.  Return value of 0  
> from
> pread is defined to be an EOF, isn't it?

No, it seems to be a simple error-out in this case. We have 2.4.20 systems with nfs-utils 0.3.3 and used to frequently get the same error while pushing. I made a similar change back in February and haven't had a problem since:

diff --git a/index-pack.c b/index-pack.c
index 5ac91ba..85c8bdb 100644
--- a/index-pack.c
+++ b/index-pack.c
@@ -313,7 +313,14 @@ static void *get_data_from_pack(struct  
object_entry *obj)
	src = xmalloc(len);
	data = src;
	do {
+		// It appears that if multiple threads read across NFS, the
+		// second read will fail. I know this is awful, but we wait for
+		// a little bit and try again.
		ssize_t n = pread(pack_fd, data + rdy, len - rdy, from + rdy);
+		if (n <= 0) {
+			sleep(1);
+			n = pread(pack_fd, data + rdy, len - rdy, from + rdy);
+		}
		if (n <= 0)
			die("cannot pread pack file: %s", strerror(errno));
		rdy += n;

I use a sleep request since it seems less likely that the other thread  
will have an outstanding request after a second of waiting.

-- 
                                                         Logan Kennelly
       ,,,
      (. .)
--ooO-(_)-Ooo--