Get rid of warning messages (1) "could not write super block; waiting
10 seconds" and (2) "blistAlloc: called on clean block".

(1) Add a waitlock parameter to blockWrite, so superWrite can tell it
to wait when it finds the dependent root block locked when trying to
write out the new epoch's super block.

(2) Don't try to allocate an unneeded BList to remove the old epoch's
root block from the active file system, in the rare case when the
super block has already been written out.

Reference: /n/sources/patch/applied/fossil-superblock-write
Date: Mon Mar 26 12:15:15 CES 2012
Signed-off-by: miller@hamnavoe.com

--- /sys/src/cmd/fossil/9fsys.c	Mon Mar 26 12:14:40 2012
+++ /sys/src/cmd/fossil/9fsys.c	Mon Mar 26 12:14:37 2012
@@ -662,7 +662,7 @@
 			goto Out1;
 		n = 0;
 		for(;;){
-			if(blockWrite(bb)){
+			if(blockWrite(bb, Waitlock)){
 				while(bb->iostate != BioClean){
 					assert(bb->iostate == BioWriting);
 					vtSleep(bb->ioready);
--- /sys/src/cmd/fossil/cache.c	Mon Mar 26 12:14:47 2012
+++ /sys/src/cmd/fossil/cache.c	Mon Mar 26 12:14:43 2012
@@ -9,9 +9,6 @@
 typedef struct BAddr BAddr;
 
 enum {
-	Nowaitlock,
-	Waitlock,
-
 	BadHeap = ~0,
 };
 
@@ -1171,7 +1168,7 @@
  *	Otherwise, bail.
  */
 int
-blockWrite(Block *b)
+blockWrite(Block *b, int waitlock)
 {
 	uchar *dmap;
 	Cache *c;
@@ -1195,7 +1192,7 @@
 		}
 
 		lockfail = 0;
-		bb = _cacheLocalLookup(c, p->part, p->addr, p->vers, Nowaitlock,
+		bb = _cacheLocalLookup(c, p->part, p->addr, p->vers, waitlock,
 			&lockfail);
 		if(bb == nil){
 			if(lockfail)
@@ -1476,10 +1473,13 @@
 	bl.next = nil;
 	bl.recurse = recurse;
 
-	p = blistAlloc(b);
+	if(b->part == PartSuper && b->iostate == BioClean)
+		p = nil;
+	else
+		p = blistAlloc(b);
 	if(p == nil){
 		/*
-		 * We were out of blists so blistAlloc wrote b to disk.
+		 * b has already been written to disk.
 		 */
 		doRemoveLink(b->c, &bl);
 		return;
@@ -2008,7 +2008,7 @@
 		b = _cacheLocalLookup(c, p->part, p->addr, p->vers, Nowaitlock,
 			&lockfail);
 
-		if(b && blockWrite(b)){
+		if(b && blockWrite(b, Nowaitlock)){
 			c->nflush++;
 			blockPut(b);
 			return 1;
--- /sys/src/cmd/fossil/dat.h	Mon Mar 26 12:14:50 2012
+++ /sys/src/cmd/fossil/dat.h	Mon Mar 26 12:14:48 2012
@@ -34,6 +34,9 @@
 };
 
 enum {
+	Nowaitlock,
+	Waitlock,
+
 	NilBlock	= (~0UL),
 	MaxBlock	= (1UL<<31),
 };
--- /sys/src/cmd/fossil/fns.h	Mon Mar 26 12:14:54 2012
+++ /sys/src/cmd/fossil/fns.h	Mon Mar 26 12:14:52 2012
@@ -39,7 +39,7 @@
 void	blockSetIOState(Block*, int);
 Block*	_blockSetLabel(Block*, Label*);
 int	blockSetLabel(Block*, Label*, int);
-int	blockWrite(Block*);
+int	blockWrite(Block*, int);
 
 Disk*	diskAlloc(int);
 int	diskBlockSize(Disk*);
--- /sys/src/cmd/fossil/fs.c	Mon Mar 26 12:14:58 2012
+++ /sys/src/cmd/fossil/fs.c	Mon Mar 26 12:14:56 2012
@@ -204,8 +204,8 @@
 	superPack(super, b->data);
 	blockDirty(b);
 	if(forceWrite){
-		while(!blockWrite(b)){
-			/* BUG: what should really happen here? */
+		while(!blockWrite(b, Waitlock)){
+			/* this should no longer happen */
 			fprint(2, "%s: could not write super block; "
 				"waiting 10 seconds\n", argv0);
 			sleep(10*1000);