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blk: use blocks from the arena block cache for log compression

the arena blocks don't get freed, so if we pluck new ones
from the cache they'll get leaked; don't do that.

--- a/blk.c

+++ b/blk.c

@@ -403,7 +403,7 @@

 	char *p;

 	if(a->logtl != nil && checkflag(a->logtl, Bdirty))

-		enqueue(a->logtl);

+		syncblk(a->logtl);

 	/*

 	 * Prepare what we're writing back.

 	 * Arenas must be sized so that we can

@@ -444,16 +444,13 @@

 	initblk(b, blks[i++], -1, Tlog);

 	for(r = (Arange*)avlmin(a->free); r != nil; r = (Arange*)avlnext(r)){

 		if(b->logsz >= Logspc - Logslop){

-			if((nb = cachepluck()) == nil)

-				return -1;

 			a->nlog++;

+			nb = a->logbuf[a->lbidx++ % nelem(a->logbuf)];

 			initblk(nb, blks[i++], -1, Tlog);

-

 			b->logp = nb->bp;

 			setflag(b, Bdirty);

 			finalize(b);

-			enqueue(b);

-			dropblk(b);

+			syncblk(b);

 			b = nb;

 		}

 		p = b->data + b->logsz;

@@ -464,7 +461,7 @@

 		setflag(b, Bdirty);

 	}

 	finalize(b);

-	enqueue(b);

+	syncblk(b);

 	/*

 	 * now we have a valid freelist, and we can start

@@ -471,7 +468,6 @@

 	 * appending stuff to it. Clean up the old logs

 	 * and the eagerly allocated extra blocks.

 	 */

-	dropblk(a->logtl);

 	a->loghd = hd;

 	a->logtl = b;

 	for(; i < nblks; i++){

--- a/fs.c

+++ b/fs.c

@@ -259,6 +259,14 @@

 	return btupsert(mnt->root, m, nm);

 }

+/*

+ * When truncating a file, mutations need

+ * to wait for the sweeper to finish; this

+ * means the mutator needs to release the

+ * mutation lock, exit the epoch, and

+ * allow the sweeper to finish its job

+ * before resuming.

+ */

 static void

 truncwait(Dent *de, int id)

 {

--- a/load.c

+++ b/load.c

@@ -104,6 +104,8 @@

 		a = &fs->arenas[i];

 		a->logbuf[0] = cachepluck();

 		a->logbuf[1] = cachepluck();

+		a->logbuf[0]->bp = (Bptr){-1, -1, -1};

+		a->logbuf[1]->bp = (Bptr){-1, -1, -1};

 		if(loadlog(a, a->loghd) == -1)

 			sysfatal("load log %B: %r", a->loghd);

 	}