commit 6610dac99c6e7370aa0a1d6d0521e301533dc61e
from: Russ Cox <rsc@swtch.com>
date: Thu Aug 23 15:10:58 2007 UTC

venti: avoid race in mirrorarenas (Michael Kaminsky)

commit - 92baf59b693e2f2a624e6f4fc7262c438103664d
commit + 6610dac99c6e7370aa0a1d6d0521e301533dc61e
blob - b16472bb587cf7c37a80a122fe2ee210a873a009
blob + 8b72f1a514077e8f07a80485175ab237d1d17e8b
--- src/cmd/venti/srv/mirrorarenas.c
+++ src/cmd/venti/srv/mirrorarenas.c
@@ -93,6 +93,7 @@ ewritepart(Part *p, u64int offset, u8int *buf, u32int 
  * src with writing dst during copy.  This is an easy factor of two
  * (almost) in performance.
  */
+static Write wsync;
 static void
 writeproc(void *v)
 {
@@ -100,7 +101,7 @@ writeproc(void *v)
 	
 	USED(v);
 	while((w = recvp(writechan)) != nil){
-		if(w->n == 0)
+		if(w == &wsync)
 			continue;
 		if(ewritepart(dst, w->o, w->p, w->n) < 0)
 			w->error = 1;
@@ -147,11 +148,7 @@ copy(uvlong start, uvlong end, char *what, DigestState
 	/*
 	 * wait for queued write to finish
 	 */
-	w[i].p = nil;
-	w[i].o = 0;
-	w[i].n = 0;
-	w[i].error = 0;
-	sendp(writechan, &w[i]);
+	sendp(writechan, &wsync);
 	i = 1-i;
 	if(w[i].error)
 		return -1;