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blk: fix allocator ordering issues (thanks mcf)

logappend inserts LogAlloc1 in case it needed to
allocate a new log block.

However, these new log blocks are allocated from
the root of the avl tree, and compresslog traverses
starting from avlmin; the current code inserts into
the current log segment, instead of the active one,
which means that logappend could insert a LogAlloc1
for the new log block before the freeing of the block
would be allocated.

Additionally, we chain the wrong block when compressing
the allocation log.

This fixes both issues.

--- a/blk.c

+++ b/blk.c

@@ -24,6 +24,7 @@

 static vlong	blkalloc(int);

 static int	blkdealloc_lk(vlong);

 static Blk*	initblk(vlong, int);

+static int	logop(Arena *, vlong, vlong, int);

 static Blk magic;

@@ -70,10 +71,10 @@

 	assert(bp != -1);

 	if((b = malloc(sizeof(Blk))) == nil)

 		return nil;

-	off = 0;

+	off = bp;

 	rem = Blksz;

 	while(rem != 0){

-		n = pread(fs->fd, b->buf+off, rem, bp+off);

+		n = pread(fs->fd, b->buf, rem, off);

 		if(n <= 0){

 			free(b);

 			return nil;

@@ -234,7 +235,7 @@

  * recursion.

  */

 static int

-logappend(Arena *_a, vlong off, vlong len, int op, Blk **tl)

+logappend(Arena *a, vlong off, vlong len, int op, Blk **tl)

 {

 	Blk *pb, *lb;

 	vlong o;

@@ -255,7 +256,7 @@

 	 */

 	if(lb == nil || lb->logsz >= Logspc - 40){

 		pb = lb;

-		if((o = blkalloc_lk(_a)) == -1)

+		if((o = blkalloc_lk(a)) == -1)

 			return -1;

 		if((lb = initblk(o, Tlog)) == nil)

 			return -1;

@@ -298,13 +299,16 @@

 	}

 	/*

 	 * The newly allocated log block needs

-	 * to be recorded.

+	 * to be recorded. If we're compressing

+	 * a log, it needs to go to the tail of

+	 * the new log, rather than after the

+	 * current allocation.

+	 *

+	 * Because we've just grown the log, we

+	 * know it won't recurse.

 	 */

-	if(o != -1){

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

-		PBIT64(p, o|LogAlloc1);

-		lb->logsz += 8;

-	}

+	if(o != -1)

+		logop(a, o, Blksz, LogAlloc);

 	/* this gets overwritten by the next append */

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

 	PBIT64(p, (uvlong)LogEnd);

@@ -478,7 +482,7 @@

 		if(logappend(a, log[i].off, log[i].len, LogFree, &tl) == -1)

 			return -1;

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

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

 	PBIT64(p, LogChain|graft);

 	free(log);

 	finalize(b);