When _cacheLocal loads the label of a block from the disk and not its
contents (if called with mode = OOverWrite), mark the cache entry
iostate as BioLabel.  It used to be marked BioClean, which causes
problems when doRemoveLink calls cacheLocalData with OOverWrite and
then leaves the data invalid, where it can be found by a concurrent
archThread and written to venti.

Reference: /n/sources/patch/applied/fossil-archive-corruption
Date: Thu Mar 22 17:26:04 CET 2012
Signed-off-by: miller@hamnavoe.com

--- /sys/src/cmd/fossil/cache.c	Thu Mar 22 17:25:54 2012
+++ /sys/src/cmd/fossil/cache.c	Thu Mar 22 17:25:50 2012
@@ -561,12 +561,13 @@
 		switch(b->iostate){
 		default:
 			abort();
-		case BioEmpty:
 		case BioLabel:
 			if(mode == OOverWrite){
-				blockSetIOState(b, BioClean);
+				/* leave iostate as BioLabel because data hasn't been read */
 				return b;
 			}
+			/* fall through */
+		case BioEmpty:
 			diskRead(c->disk, b);
 			vtSleep(b->ioready);
 			break;
@@ -1098,7 +1099,7 @@
 
 	if(b->iostate == BioDirty)
 		return 1;
-	assert(b->iostate == BioClean);
+	assert(b->iostate == BioClean || b->iostate == BioLabel);
 
 	vtLock(c->dirtylk);
 	vtLock(c->lk);