X Switch SliceByRootRev to vδTSnapForTracked

Triggered one bug in ΔFtail.SliceByFileRev which was not checking
file's root to be VDEL.

wcfs/internal/xbtree/δbtail.go

@@ -1320,21 +1320,23 @@ func (δBtail *ΔBtail) GetAt(root zodb.Oid, key Key, at zodb.Tid) (value Value,
 // Only tracked keys are guaranteed to be present.
 //
 // Note: contrary to regular go slicing, low is exclusive while high is inclusive.
-func (δBtail *ΔBtail) SliceByRootRev(root zodb.Oid, lo, hi zodb.Tid) /*readonly*/[]ΔTree {
+func (δBtail *ΔBtail) SliceByRootRev(root zodb.Oid, lo, hi zodb.Tid) (/*readonly*/vδT []ΔTree) {
 	xtail.AssertSlice(δBtail, lo, hi)
-	// XXX locking
 
-	err := δBtail.rebuild1IfNeeded(root)
-	if err != nil {
-		panic(err) // XXX
+	if traceΔBtail {
+		tracefΔBtail("\nSlice root<%s> (@%s,@%s]\n", root, lo, hi)
+		defer func() {
+			tracefΔBtail("-> vδT(lo,hi]: %v\n", vδT)
+		}()
 	}
 
-	δTtail, ok := δBtail.byRoot[root]
-	if !ok {
-		return []ΔTree{}	// XXX -> panic (root must be tracked)
+	// retrieve vδT snapshot that is rebuilt to take all previous Track requests
+	vδT, err := δBtail.vδTSnapForTracked(root)
+	if err != nil {
+		panic(err) // XXX
 	}
+	debugfΔBtail("  vδT: %v\n", vδT)
 
-	vδT := δTtail.vδT
 	l := len(vδT)
 	if l == 0 {
 		return nil

wcfs/internal/zdata/δftail.go

@@ -684,7 +684,10 @@ func (δFtail *ΔFtail) SliceByFileRev(zfile *ZBigFile, lo, hi zodb.Tid) /*reado
 		//fmt.Printf("Zinblk: %v\n", Zinblk)
 
 		// vδT for current epoch
-		vδT := δFtail.δBtail.SliceByRootRev(root, epoch, head) // NOTE @head, not hi
+		var vδT []xbtree.ΔTree
+		if root != xbtree.VDEL {
+			vδT = δFtail.δBtail.SliceByRootRev(root, epoch, head) // NOTE @head, not hi
+		}
 		it := len(vδT) - 1
 		if it >= 0 {
 			ZinblkAt = vδT[it].Rev