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hjfs: fix abort() in givebuf()

it is possible for another getbuf() on buffer b to come in
before undelayreq() calls givebuf() on a buffer again. then
givebuf() would find b already busy and abort().

instead, we now handle what getbuf() did in givebuf() and
consider the Buf* argument to givebuf() as a hint only for
the case when we have to actually flush/read a block from
disk.

--- a/sys/src/cmd/hjfs/buf.c

+++ b/sys/src/cmd/hjfs/buf.c

@@ -81,11 +81,24 @@

 {

 	Buf *c, *l;

-	markbusy(b);

+	assert(!b->busy);

 	if(req.d == b->d && req.off == b->off){

+		markbusy(b);

 		send(req.resp, &b);

 		return;

 	}

+	l = &req.d->buf[req.off & BUFHASH];

+	for(c = l->dnext; c != l; c = c->dnext)

+		if(c->off == req.off){

+			if(c->busy){

+				delayreq(req, &c->next, &c->last);

+				return;

+			}

+			markbusy(c);

+			send(req.resp, &c);

+			return;

+		}

+	markbusy(b);

 	if(b->op & BDELWRI){

 		b->op &= ~BDELWRI;

 		b->op |= BWRITE;

@@ -94,10 +107,6 @@

 		work(b->d, b);

 		return;

 	}

-	l = &req.d->buf[req.off & BUFHASH];

-	for(c = l->dnext; c != l; c = c->dnext)

-		if(c->off == req.off)

-			abort();

 	changedev(b, req.d, req.off);

 	b->op &= ~(BWRITE|BDELWRI|BWRIM);

 	if(req.nodata)

@@ -109,6 +118,19 @@

 }

 static void

+handleget(BufReq req)

+{

+	Buf *b;

+	

+	b = bfree.fnext;

+	if(b == &bfree){

+		delayreq(req, &freereq, &freereqlast);

+		return;

+	}

+	givebuf(req, b);

+}

+

+static void

 undelayreq(Buf *b, BufReq **first, BufReq **last)

 {

 	BufReq *r;

@@ -119,31 +141,6 @@

 		*last = nil;

 	givebuf(*r, b);

 	free(r);

-}

-

-static void

-handleget(BufReq req)

-{

-	Buf *b, *l;

-	Dev *d;

-	

-	d = req.d;

-	l = &d->buf[req.off & BUFHASH];

-	for(b = l->dnext; b != l; b = b->dnext)

-		if(b->off == req.off){

-			if(b->busy){

-				delayreq(req, &b->next, &b->last);

-				return;

-			}

-			givebuf(req, b);

-			return;

-		}

-	if(bfree.fnext == &bfree){

-		delayreq(req, &freereq, &freereqlast);

-		return;

-	}

-	b = bfree.fnext;

-	givebuf(req, b);

 }

 static void