X ΔBtail.rebuild started to work draftly

* t2: (50 commits)
  .
  X rebuild: Serial tests now started to PASS draftly
  .
  X rebuild: Don't return nil for empty ΔPPTreeSubSet - that leads to SIGSEGV
  X treediff: Fix BUG while computing AB coverage
  X ΔBtail.Clone had bug that it was aliasing klon and orig data
  X Fix rebuild with ø @at2
  X Fix bug in PPTreeSubSet.Difference  - it was always leaving root node alive
  .
  .
  X found why TestΔBTailAllStructs was not effective to find δtkeycov bugs
  .
  .
  X wcfs: assert that keycov only grow
  .
  X another bug in δtkeyconv computation
  .
  .
  .
  .
  ...

wcfs/internal/set/set.go.in

@@ -136,6 +136,15 @@ func (a Set) Equal(b Set) bool {
 	return true
 }
 
+// Clone returns copy of the set.
+func (orig Set) Clone() Set {
+	klon := make(Set, len(orig))
+	for v := range orig {
+		klon.Add(v)
+	}
+	return klon
+}
+
 // --------
 
 func (s Set) SortedElements() []VALUE {

wcfs/internal/set/zset_i64.go

@@ -138,6 +138,15 @@ func (a SetI64) Equal(b SetI64) bool {
 	return true
 }
 
+// Clone returns copy of the set.
+func (orig SetI64) Clone() SetI64 {
+	klon := make(SetI64, len(orig))
+	for v := range orig {
+		klon.Add(v)
+	}
+	return klon
+}
+
 // --------
 
 func (s SetI64) SortedElements() []int64 {

wcfs/internal/set/zset_oid.go

@@ -138,6 +138,15 @@ func (a SetOid) Equal(b SetOid) bool {
 	return true
 }
 
+// Clone returns copy of the set.
+func (orig SetOid) Clone() SetOid {
+	klon := make(SetOid, len(orig))
+	for v := range orig {
+		klon.Add(v)
+	}
+	return klon
+}
+
 // --------
 
 func (s SetOid) SortedElements() []_Oid {

wcfs/internal/xbtree/pptreesubset.go

@@ -27,6 +27,7 @@ import (
 )
 
 const tracePPSet = false
+const debugPPSet = false
 
 // PPTreeSubSet represents PP-connected subset of tree node objects.
 //
@@ -58,9 +59,13 @@ type PPTreeSubSet map[zodb.Oid]*nodeInTree
 type nodeInTree struct {
 	parent  zodb.Oid // parent node | InvalidOid for root
 	nchild  int      // number of direct children in PPTreeSubSet referring to this node
-	// XXX + [lo,hi) range this node is coming under in its parent	XXX -> in its tree ?
 }
 
+// Has returns whether node is in the set.
+func (S PPTreeSubSet) Has(oid zodb.Oid) bool {
+	_, ok := S[oid]
+	return ok
+}
 
 // Path returns path leading to the node specified by oid.
 //
@@ -84,6 +89,8 @@ func (S PPTreeSubSet) Path(oid zodb.Oid) (path []zodb.Oid) {
 }
 
 // AddPath adds path to a node to the set.
+//
+// Note: embedded buckets (leaf node with InvalidOid) are removed from the path.
 func (S PPTreeSubSet) AddPath(path []zodb.Oid) {
 	S.verify()
 	defer S.verify()
@@ -93,18 +100,11 @@ func (S PPTreeSubSet) AddPath(path []zodb.Oid) {
 		panic("empty path")
 	}
 
-	// don't keep track of artificial empty tree
-	if l == 1 && path[0] == zodb.InvalidOid {
-		return
-	}
-
-	// don't explicitly keep track of embedded buckets - they all have
-	// InvalidOid, and thus, if kept in S, e.g. T/B1:a and another
-	// T/B2:b would lead to InvalidOid having multiple parents.
-	if l == 2 && path[1] == zodb.InvalidOid {
-		path = path[:1]
-	}
+	// normalize path: remove embedded bucket and check whether it was an
+	// artificial empty tree.
+	path = normPath(path)
 
+	// go through path and add nodes to the set
 	parent := zodb.InvalidOid
 	var pt *nodeInTree = nil
 	for _, oid := range path {
@@ -132,11 +132,33 @@ func (S PPTreeSubSet) AddPath(path []zodb.Oid) {
 	}
 }
 
-// ---- Union/Difference/Intersetctior ----
+// normPath normalizes path.
+//
+// It removes embedded buckets and artificial empty trees.
+// Returned slice is subslice of path and aliases its memory.
+func normPath(path []zodb.Oid) []zodb.Oid {
+	l := len(path)
+
+	// don't keep track of artificial empty tree
+	if l == 1 && path[0] == zodb.InvalidOid {
+		return nil
+	}
+
+	// don't explicitly keep track of embedded buckets - they all have
+	// InvalidOid, and thus, if kept in S, e.g. T/B1:a and another
+	// T/B2:b would lead to InvalidOid having multiple parents.
+	if l == 2 && path[1] == zodb.InvalidOid {
+		return path[:1]
+	}
+
+	return path
+}
+
+// ---- Union/Difference/Intersection ----
 
 // Union returns U = PP(A.leafs | B.leafs)
 //
-// In other words it returns sum of A and B.
+// In other words it adds A and B nodes.
 func (A PPTreeSubSet) Union(B PPTreeSubSet) PPTreeSubSet {
 	U := A.Clone()
 	U.UnionInplace(B)
@@ -163,7 +185,7 @@ func (A PPTreeSubSet) UnionInplace(B PPTreeSubSet) {
 
 // Difference returns D = PP(A.leafs \ B.leafs)
 //
-// In other words ... XXX
+// In other words it removes B nodes from A while still maintaining A as PP-connected.
 func (A PPTreeSubSet) Difference(B PPTreeSubSet) PPTreeSubSet {
 	D := A.Clone()
 	D.DifferenceInplace(B)
@@ -189,7 +211,7 @@ func (A PPTreeSubSet) DifferenceInplace(B PPTreeSubSet) {
 	A.xDifferenceInplace(B)
 }
 
-// XXX Intersection
+// TODO Intersection
 
 func (A PPTreeSubSet) xUnionInplace(B PPTreeSubSet) {
 	if tracePPSet {
@@ -269,10 +291,16 @@ func (A PPTreeSubSet) fixup(δnchild map[zodb.Oid]int) {
 	A.xfixup(+1, δnchild)
 }
 func (A PPTreeSubSet) xfixup(sign int, δnchild map[zodb.Oid]int) {
-	//fmt.Printf("\nfixup:\n")
-	//fmt.Printf("    ·: %s\n", A)
-	//fmt.Printf("    δ: %v\n", δnchild)
-	//defer fmt.Printf("  ->·: %s\n\n", A)
+	if debugPPSet {
+		ssign := "+"
+		if sign < 0 {
+			ssign = "-"
+		}
+		fmt.Printf("\n  fixup:\n")
+		fmt.Printf("      ·: %s\n", A)
+		fmt.Printf("     %sδ: %v\n", ssign, δnchild)
+		defer fmt.Printf("    ->·: %s\n\n", A)
+	}
 
 	gcq := []zodb.Oid{}
 	for oid, δnc := range δnchild {
@@ -304,7 +332,7 @@ func (S PPTreeSubSet) gc1(oid zodb.Oid) {
 	for oid != zodb.InvalidOid {
 		t := S[oid]
 		t.nchild--
-		if t.nchild > 0 || /* root node */t.parent == zodb.InvalidOid {
+		if t.nchild > 0 {
 			break
 		}
 		delete(S, oid)
@@ -395,6 +423,11 @@ func (A PPTreeSubSet) Equal(B PPTreeSubSet) bool {
 	return true
 }
 
+// Empty returns whether set is empty.
+func (S PPTreeSubSet) Empty() bool {
+	return len(S) == 0
+}
+
 func (t nodeInTree) String() string {
 	return fmt.Sprintf("{p%s c%d}", t.parent, t.nchild)
 }
@@ -440,13 +473,22 @@ func (t nodeInTree) String() string {
 //         - δ.Add, and
 //         - xfixup(+1, δnchildNonLeafs)
 //
-//     produce correctly PP-connected set.
+//     produces correctly PP-connected set.
 type ΔPPTreeSubSet struct {
 	Del PPTreeSubSet
 	Add PPTreeSubSet
 	δnchildNonLeafs map[zodb.Oid]int
 }
 
+// NewΔPPTreeSubSet creates new empty ΔPPTreeSubSet.
+func NewΔPPTreeSubSet() *ΔPPTreeSubSet {
+	return &ΔPPTreeSubSet{
+		Del: PPTreeSubSet{},
+		Add: PPTreeSubSet{},
+		δnchildNonLeafs: map[zodb.Oid]int{},
+	}
+}
+
 // Update updates δ to be combination of δ+δ2.
 func (δ *ΔPPTreeSubSet) Update(δ2 *ΔPPTreeSubSet) {
 	δ.Del.UnionInplace(δ2.Del)

wcfs/internal/xbtree/pptreesubset_test.go

@@ -20,6 +20,7 @@
 package xbtree
 
 import (
+	"strings"
 	"testing"
 
 	"lab.nexedi.com/kirr/neo/go/zodb"
@@ -79,19 +80,44 @@ func TestPPTreeSubSetOps(t *testing.T) {
 			S{a:{ø,1}, b:{a,0}},	// B
 				S{a:{ø,1}, b:{a,0}},	// U
 				S{}),			// D
-	}
 
-	for _, tt := range testv {
-		U := tt.A.Union(tt.B)
-		D := tt.A.Difference(tt.B)
+		E(
+			S{a:{ø,1}, b:{a,1}, c:{b,0}},	// A
+			S{a:{ø,1}, b:{a,1}, c:{b,0}},	// B (=A)
+				S{a:{ø,1}, b:{a,1}, c:{b,0}},	// U (=A)
+				S{}),				// D
+	}
 
-		if !U.Equal(tt.Union) {
-			t.Errorf("Union:\n  A:   %s\n  B:   %s\n  ->u: %s\n  okU: %s\n", tt.A, tt.B, U, tt.Union)
+	// assert1 asserts that result of op(A,B) == resOK.
+	assert1 := func(op string, A, B, res, resOK S) {
+		t.Helper()
+		if res.Equal(resOK) {
+			return
 		}
-		if !D.Equal(tt.Difference) {
-			t.Errorf("Difference:\n  A:   %s\n  B:   %s\n  ->d: %s\n  okD: %s\n", tt.A, tt.B, D, tt.Difference)
+		op1 := op[0:1]
+		t.Errorf("%s:\n  A:   %s\n  B:   %s\n  ->%s: %s\n  ok%s: %s\n",
+			strings.Title(op), A, B, op1, res, strings.ToUpper(op1), resOK)
 	}
 
+	for _, tt := range testv {
+		Uab := tt.A.Union(tt.B)
+		Uba := tt.B.Union(tt.A)
+		Dab := tt.A.Difference(tt.B)
+
+		assert1("union",      tt.A, tt.B, Uab, tt.Union)
+		assert1("union",      tt.B, tt.A, Uba, tt.Union)
+		assert1("difference", tt.A, tt.B, Dab, tt.Difference)
+
+		Uaa := tt.A.Union(tt.A)
+		Ubb := tt.B.Union(tt.B)
+		Daa := tt.A.Difference(tt.A)
+		Dbb := tt.B.Difference(tt.B)
+
+		assert1("union",      tt.A, tt.A, Uaa, tt.A)
+		assert1("union",      tt.B, tt.B, Ubb, tt.B)
+		assert1("difference", tt.A, tt.A, Daa, S{})
+		assert1("difference", tt.B, tt.B, Dbb, S{})
+
 		// XXX also verify U/D properties like (A+B)\B + (A+B)\A + (A^B) == (A+B) ?
 	}
 }

wcfs/internal/xbtree/testprog/treegen.py

@@ -20,7 +20,7 @@
 # See https://www.nexedi.com/licensing for rationale and options.
 """Program treegen provides infrastructure to generate ZODB BTree states.
 
-It is used as helper for ΔBTree tests.
+It is used as helper for ΔBtail tests.
 
 The following subcommands are provided:
 
@@ -319,6 +319,9 @@ def AllStructs(kv1txt, kv2txt, maxdepth, maxsplit, n, seed=None):
     t1structv.insert(0, t1struct0)
     t2structv.insert(0, t2struct0)
 
+    # XXX rework allstructs to accept only 1 kv and emit n structs for that one kv only
+    #     -> iterate through the pairs/triplets in the caller (TestΔBTailAllStructs)
+
     # emit topologies for tree1->tree2 and tree1<-tree2 transitions for all
     # combinations of tree1 and tree2.
     t12travel = list(bitravel2Way(t1structv, t2structv))

wcfs/internal/xbtree/treediff.go

@@ -159,7 +159,7 @@ func δZConnectTracked(δZv []zodb.Oid, T PPTreeSubSet) (δZTC SetOid, δtopsByR
 // nodeInRange represents a Node coming under [lo, hi_] key range in its tree.
 type nodeInRange struct {
 	prefix   []zodb.Oid // path to this node goes via this objects
-	lo, hi_  Key  // [lo, hi_]	NOTE _not_ hi) not to overflow at ∞	XXX -> Range
+	lo, hi_  Key  // [lo, hi_]	NOTE _not_ hi) not to overflow at ∞	XXX -> keycov KeyRange?
 	node     Node
 	done     bool // whether this node was already taken into account while computing diff
 }
@@ -330,7 +330,7 @@ func treediff(ctx context.Context, root zodb.Oid, δtops SetOid, δZTC SetOid, t
 	defer xerr.Contextf(&err, "treediff %s..%s %s", zconnOld.At(), zconnNew.At(), root)
 
 	δT = map[Key]ΔValue{}
-	δtrack = &ΔPPTreeSubSet{Del: PPTreeSubSet{}, Add: PPTreeSubSet{}, δnchildNonLeafs: map[zodb.Oid]int{}}
+	δtrack = NewΔPPTreeSubSet()
 	δtkeycov = &RangedKeySet{}
 
 	tracefDiff("\ntreediff %s  δtops: %v  δZTC: %v\n", root, δtops, δZTC)
@@ -429,7 +429,7 @@ func diffX(ctx context.Context, a, b Node, δZTC SetOid, trackSet PPTreeSubSet)
 		var δtrack *ΔPPTreeSubSet
 		δ, err := diffB(ctx, aB, bB)
 		if δ != nil {
-			δtrack = &ΔPPTreeSubSet{}
+			δtrack = NewΔPPTreeSubSet()
 			δtkeycov = &RangedKeySet{}
 		}
 		return δ, δtrack, δtkeycov, err
@@ -445,7 +445,7 @@ func diffT(ctx context.Context, A, B *Tree, δZTC SetOid, trackSet PPTreeSubSet)
 	defer xerr.Contextf(&err, "diffT %s %s", xidOf(A), xidOf(B))
 
 	δ = map[Key]ΔValue{}
-	δtrack = &ΔPPTreeSubSet{Del: PPTreeSubSet{}, Add: PPTreeSubSet{}, δnchildNonLeafs: map[zodb.Oid]int{}}
+	δtrack = NewΔPPTreeSubSet()
 	δtkeycov = &RangedKeySet{}
 	defer func() {
 		tracefDiff("  -> δ: %v\n", δ)
@@ -487,6 +487,9 @@ func diffT(ctx context.Context, A, B *Tree, δZTC SetOid, trackSet PPTreeSubSet)
 ABcov:
 	for i := len(ABpath)-2; i >= 0; i-- {
 		xparent, err := node.PJar().Get(ctx, ABpath[i]);  /*X*/if err != nil { return nil,nil,nil, err }
+		err = xparent.PActivate(ctx);  /*X*/if err != nil { return nil,nil,nil, err}
+		defer xparent.PDeactivate()
+
 		parent := xparent.(*Tree) // must succeed
 		// find node in parent children and constrain ABlo/ABhi accordingly
 		entryv := parent.Entryv()
@@ -512,7 +515,14 @@ ABcov:
 			}
 		}
 
-		panicf("BUG: T%s points to T%s as parent in trackSet, but not found in T%s children", node.POid(), parent.POid(), parent.POid())
+		emsg := fmt.Sprintf("BUG: T%s points to T%s as parent in trackSet, but not found in T%s children\n", node.POid(), parent.POid(), parent.POid())
+		children := []string{}
+		for _, entry := range entryv {
+			children = append(children, vnode(entry.Child()))
+		}
+		emsg += fmt.Sprintf("T%s children: %v\n", parent.POid(), children)
+		emsg += fmt.Sprintf("trackSet: %s\n", trackSet)
+		panic(emsg)
 	}
 
 
@@ -565,10 +575,11 @@ ABcov:
 		}
 	}
 
-	// {} oid -> parent for all nodes in Bv: current and previously expanded - up till top B
-	BtrackSet := PPTreeSubSet{}
+	// {} oid -> nodeInRange for all nodes we've came through in Bv:
+	//                       current and previously expanded - up till top B.
+	BnodeIdx := map[zodb.Oid]*nodeInRange{}
 	if !Bempty {
-		BtrackSet.AddPath(ABpath)
+		BnodeIdx[ABoid] = btop
 	}
 
 	// δtkeycov will be = BAdd \ ADel
@@ -579,7 +590,8 @@ ABcov:
 	// by default a node contributes to δ-
 	// a node ac does not contribute to δ- and can be skipped, if:
 	// - ac is not tracked, or
-	// - ac ∉ δZTC && ∃ bc from B: ac.oid == bc.oid	(ac+ac.children were not changed, and ac stays in the tree)
+	// - ac ∉ δZTC && ∃ bc from B: ac.oid == bc.oid && ac.keycov == bc.keycov
+	//   (ac+ac.children were not changed, ac stays in the tree with the same key range coverage)
 	Aq := []*nodeInRange{atop} // queue for A nodes that contribute to δ-
 	for len(Aq) > 0 {
 		debugfDiff("\n")
@@ -606,7 +618,7 @@ ABcov:
 			ra.done = true
 
 		case *Tree:
-			// empty tree - only queue holes covered by it
+			// empty tree - queue holes covered by it
 			if len(a.Entryv()) == 0 {
 				ar := KeyRange{ra.lo, ra.hi_}
 				δtrack.Del.AddPath(ra.Path())
@@ -632,7 +644,7 @@ ABcov:
 					// ( this does not give exact answer but should be a reasonable heuristic;
 					//   the diff is the same if heuristic does not work and we
 					//   look into and load more nodes to compute δ )
-					_, found := BtrackSet[acOid]
+					bc, found := BnodeIdx[acOid]
 					if !found {
 						for {
 							blo  := Bv.Get(ac.lo)
@@ -652,11 +664,12 @@ ABcov:
 							}
 
 							bchildren := Bv.Expand(blo)
-							for _, bc := range bchildren {
-									bcOid := bc.node.POid()
-									BtrackSet.AddPath(bc.Path())
-									if acOid == bcOid {
+							for _, bc_ := range bchildren {
+									bc_Oid := bc_.node.POid()
+									BnodeIdx[bc_Oid] = bc_
+									if acOid == bc_Oid {
 										found = true
+										bc = bc_
 									}
 							}
 							if found {
@@ -665,31 +678,25 @@ ABcov:
 						}
 					}
 					if found {
-						// ac can be skipped
+						// ac can be skipped if key coverage stays the same
+						ar := KeyRange{ac.lo, ac.hi_}
+						br := KeyRange{bc.lo, bc.hi_}
+						if ar == br {
 							// adjust trackSet since path to the node could have changed
-						apath := trackSet.Path(acOid)
-						bpath := BtrackSet.Path(acOid)
+							apath := ac.Path()
+							bpath := bc.Path()
 							if !pathEqual(apath, bpath) {
 								δtrack.Del.AddPath(apath)
 								δtrack.Add.AddPath(bpath)
 								if nc := at.nchild; nc != 0 {
 									δtrack.δnchildNonLeafs[acOid] = nc
-								// XXX debugDiff δtrack [) ...
-							} else {
-								// adjust δtkeycov only if it was leaf bucket
-								ar := KeyRange{ac.lo, ac.hi_}
-								bc := Bv.Get(ac.lo)
-								br := KeyRange{bc.lo, bc.hi_}
-								δtkeycovADel.AddRange(ar)
-								δtkeycovBAdd.AddRange(br)
-								debugfDiff("  δtrack - %s %v KKK\n", ar, apath)
-								debugfDiff("  δtrack + %s %v KKK\n", br, bpath)
 								}
 							}
 
 							continue
 						}
 					}
+				}
 
 				// ac cannot be skipped
 				push(&Aq, ac)
@@ -751,11 +758,10 @@ ABcov:
 					b.done = true
 				}
 
-				// stop if r coverage is complete
+				// continue with next right bucket until r coverage is complete
 				if r.hi_ <= b.hi_ {
 					break
 				}
-				// continue with next right bucket
 				lo = b.hi_ + 1
 			}
 		}

wcfs/internal/xbtree/δbtail.go

@@ -33,10 +33,6 @@ import (
 	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xtail"
 )
 
-// XXX malfancitioning rebuild currently breaks wcfs tests
-// TODO kill this after rebuild is finished
-const XXX_killWhenRebuildWorks = true
-
 const traceΔBtail = false
 const debugΔBtail = false
 
@@ -89,30 +85,25 @@ const debugΔBtail = false
 // See also zodb.ΔTail
 // XXX naming -> ΔBTail ?
 type ΔBtail struct {
-	// raw ZODB changes; Kept to rebuild .byRoot after new Track.
+	// raw ZODB changes; Kept to rebuild .vδTbyRoot after new Track.
 	// includes all changed objects, not only tracked ones.
 	δZtail *zodb.ΔTail
 
-	// XXX vvv keys ∈ tracked -> keys ∈ kadj[tracked] ?
-//	vδB []ΔB // data with δB changes; Noted only by keys ∈ tracked subset
-	byRoot map[zodb.Oid]*ΔTtail // {} root -> [] k/v change history; only for keys ∈ tracked subset
-
-	// handle to make connections to access database.
-	// TODO allow client to optionally provide zconnOld/zconnNew on e.g. Update()
-	db     *zodb.DB    // to open connections to load new/old tree|buckets
+	vδBroots  []ΔBroots            // [] (rev, roots changed in this rev)
+	vδTbyRoot map[zodb.Oid]*ΔTtail // {} root -> [] k/v change history; only for keys ∈ tracked subset	XXX -> byRoot?
 
 	// set of tracked nodes as of @head state.
+	// For this set all vδT are fully computed.
+	// The set of nodes that were requested to be tracked, but were not yet
+	// taken into account, is kept in ΔTtail.trackNew & co.
 	trackSet PPTreeSubSet
 
-	// set of nodes that were requested to be tracked, but for which vδB was not yet rebuilt
-	trackNew PPTreeSubSet
-}
-
+	// set of trees for which .trackNew is non-empty
+	trackNewRoots SetOid
 
-// ΔB represents a change in BTrees space.
-type ΔB struct {
-	Rev    zodb.Tid
-	ByRoot map[zodb.Oid]map[Key]ΔValue // {} root -> {}(key, δvalue)
+	// handle to make connections to access database.
+	// TODO allow client to optionally provide zconnOld/zconnNew on e.g. Update()
+	db *zodb.DB    // to open connections to load new/old tree|buckets
 }
 
 // ΔTtail represent tail of revisional changes to one BTree.
@@ -121,6 +112,11 @@ type ΔB struct {
 type ΔTtail struct {
 	vδT []ΔTree // changes to tree keys; rev↑. covers keys ∈ tracked subset
 
+	// set of nodes that were requested to be tracked in this tree, but for
+	// which vδT was not yet rebuilt
+	trackNew PPTreeSubSet
+	// XXX + trackNewKeys RangedKeySet
+
 	// {}k/v @tail for keys that are changed in (tail, head].
 	KVAtTail map[Key]Value		// XXX not needed since vδT has ΔValue ?
 
@@ -128,6 +124,20 @@ type ΔTtail struct {
 	lastRevOf map[Key]zodb.Tid // {} key -> last
 }
 
+// ΔBroots represents roots-only part of ΔB.
+//
+// It describes which trees were changed, but does not provide δkv details for changed trees.
+type ΔBroots struct {
+	Rev    zodb.Tid
+	ΔRoots SetOid   // which roots changed in this revision
+}
+
+// ΔB represents a change in BTrees space.
+type ΔB struct {
+	Rev     zodb.Tid
+	ΔByRoot map[zodb.Oid]map[Key]ΔValue // {} root -> {}(key, δvalue)	XXX -> ByRoot?
+}
+
 // ΔTree describes changes to one BTree in one revision.
 // XXX -> ΔT ?
 type ΔTree struct {
@@ -146,15 +156,25 @@ type ΔTree struct {
 func NewΔBtail(at0 zodb.Tid, db *zodb.DB) *ΔBtail {
 	return &ΔBtail{
 		δZtail:        zodb.NewΔTail(at0),
-		byRoot:        map[zodb.Oid]*ΔTtail{},
+		vδBroots:      nil,
+		vδTbyRoot:     map[zodb.Oid]*ΔTtail{},
 		trackSet:      PPTreeSubSet{},
-		trackNew:      PPTreeSubSet{},
+		trackNewRoots: SetOid{},
 		db:            db,
 	}
 }
 
-// xverifyΔBTail_rebuild needs ΔBTree.clone because otherwise it is too slow to run that test.
-func (orig *ΔBtail) clone() *ΔBtail {
+// newΔTtail creates new empty ΔTtail object.
+func newΔTtail() *ΔTtail {
+	return &ΔTtail{
+		trackNew:  PPTreeSubSet{},
+		KVAtTail:  make(map[Key]Value),
+		lastRevOf: make(map[Key]zodb.Tid),
+	}
+}
+
+// Clone returns copy of ΔBtail.
+func (orig *ΔBtail) Clone() *ΔBtail {
 	klon := &ΔBtail{db: orig.db}
 
 	// δZtail
@@ -163,35 +183,51 @@ func (orig *ΔBtail) clone() *ΔBtail {
 		klon.δZtail.Append(δZ.Rev, δZ.Changev)
 	}
 
-	// trackSet, trackNew
-	klon.trackSet = orig.trackSet.Clone()
-	klon.trackNew = orig.trackNew.Clone()
-
-	// byRoot
-	klon.byRoot = make(map[zodb.Oid]*ΔTtail, len(orig.byRoot))
-	for root, origΔTtail := range orig.byRoot {
-		klonΔTtail := &ΔTtail{}
-		klonΔTtail.vδT = append(klonΔTtail.vδT, origΔTtail.vδT...)
-		klonΔTtail.KVAtTail = make(map[Key]Value, len(origΔTtail.KVAtTail))
-		for k, v := range origΔTtail.KVAtTail {
-			klonΔTtail.KVAtTail[k] = v
+	// vδBroots
+	for _, origδBroots := range orig.vδBroots {
+		klonδBroots := ΔBroots{
+			Rev:    origδBroots.Rev,
+			ΔRoots: origδBroots.ΔRoots.Clone(),
 		}
-		klonΔTtail.lastRevOf = make(map[Key]zodb.Tid, len(origΔTtail.lastRevOf))
-		for k, rev := range origΔTtail.lastRevOf {
-			klonΔTtail.lastRevOf[k] = rev
+		klon.vδBroots = append(klon.vδBroots, klonδBroots)
 	}
-		klon.byRoot[root] = klonΔTtail
+
+	// vδTbyRoot
+	klon.vδTbyRoot = make(map[zodb.Oid]*ΔTtail, len(orig.vδTbyRoot))
+	for root, origΔTtail := range orig.vδTbyRoot {
+		klon.vδTbyRoot[root] = origΔTtail.Clone()
 	}
 
+	// trackSet, trackNewRoots
+	klon.trackSet = orig.trackSet.Clone()
+	klon.trackNewRoots = orig.trackNewRoots.Clone()
+
 	return klon
 }
 
-// newΔTtail creates new empty ΔTtail object.
-func newΔTtail() *ΔTtail {
-	return &ΔTtail{
-		KVAtTail:  make(map[Key]Value),
-		lastRevOf: make(map[Key]zodb.Tid),
+// Clone returns copy of ΔTtail.
+func (orig *ΔTtail) Clone() *ΔTtail {
+	klon := &ΔTtail{}
+	for _, origδT := range orig.vδT {
+		klonδT := ΔTree{
+			Rev: origδT.Rev,
+			ΔKV: make(map[Key]ΔValue, len(origδT.ΔKV)),
+		}
+		for k, δv := range origδT.ΔKV {
+			klonδT.ΔKV[k] = δv
+		}
+		klon.vδT = append(klon.vδT, klonδT)
 	}
+	klon.trackNew = orig.trackNew.Clone()
+	klon.KVAtTail = make(map[Key]Value, len(orig.KVAtTail))
+	for k, v := range orig.KVAtTail {
+		klon.KVAtTail[k] = v
+	}
+	klon.lastRevOf = make(map[Key]zodb.Tid, len(orig.lastRevOf))
+	for k, rev := range orig.lastRevOf {
+		klon.lastRevOf[k] = rev
+	}
+	return klon
 }
 
 // (tail, head] coverage
@@ -199,7 +235,6 @@ func (δBtail *ΔBtail) Head() zodb.Tid { return δBtail.δZtail.Head() }
 func (δBtail *ΔBtail) Tail() zodb.Tid { return δBtail.δZtail.Tail() }
 
 // XXX SliceByRev?
-// XXX ForgetPast
 
 // Track adds tree path to tracked set.
 //
@@ -215,59 +250,103 @@ func (δBtail *ΔBtail) Tail() zodb.Tid { return δBtail.δZtail.Tail() }
 // XXX path -> []oid ?
 //
 // XXX catch cycles on add?
-// XXX no need to pass keyPresent since holeIdx was removed
-func (δBtail *ΔBtail) Track(key Key, keyPresent bool, nodePath []Node) error { // XXX Tree|Bucket; path[0] = root
+// XXX no need to pass in key?  (-> all keys, covered by leaf keyrange, will be added to tracking set of keys)
+func (δBtail *ΔBtail) Track(key Key, nodePath []Node) error { // XXX Tree|Bucket; path[0] = root
 
 	path := nodePathToPath(nodePath)
 
 	pathv := []string{}
 	for _, node := range nodePath { pathv = append(pathv, vnode(node)) }
 	tracefΔBtail("\nTrack [%v] %s\n", key, strings.Join(pathv, " -> "))
+	tracefΔBtail("trackSet: %s\n", δBtail.trackSet) // XXX locking
 
-	return δBtail.track(key, keyPresent, path)
+	return δBtail.track(key, path)
 }
 
-func (δBtail *ΔBtail) track(key Key, keyPresent bool, path []zodb.Oid) error {
+func (δBtail *ΔBtail) track(key Key, path []zodb.Oid) error {
 	// XXX locking
 
+	// first normalize path: remove embedded bucket and check if it was an
+	// empty artificial tree. We need to do the normalization because we
+	// later check whether leaf path[-1] ∈ trackSet and without
+	// normalization path[-1] can be InvalidOid.
+	path = normPath(path)
+	if len(path) == 0 {
+		return nil // empty tree
+	}
+
 	root := path[0]
-	δBtail.trackNew.AddPath(path)
 
-	// track is track of path[-1]  (i.e. leaf)
+	// nothing to do if key is already tracked
+	leaf := path[len(path)-1]
+	if δBtail.trackSet.Has(leaf) {
+		tracefΔBtail("->T: nop\n")
+		path_ := δBtail.trackSet.Path(leaf)
+		if !pathEqual(path, path_) {
+			panicf("BUG: key %s is already tracked via path=%v\ntrack requests path=%v",  kstr(key), path_, path)
+		}
+		return nil
+	}
 
-// XXX hack - until rebuild is implemented
-if XXX_killWhenRebuildWorks {
-	_, ok := δBtail.byRoot[root]
+	// queue path into trackNew
+	δTtail, ok := δBtail.vδTbyRoot[root]
 	if !ok {
-		δBtail.byRoot[root] = newΔTtail()
+		δTtail = newΔTtail()
+		δBtail.vδTbyRoot[root] = δTtail
 	}
+
+	δBtail.trackNewRoots.Add(root)
+	δTtail.trackNew.AddPath(path)
+	tracefΔBtail("->T: [%s].trackNew -> %s\n", root, δTtail.trackNew)
+	return nil
 }
 
-	// XXX update diff	XXX here? or as separate step?
-	// XXX update lastRevOf
+// rebuildAll rebuilds ΔBtail taking all trackNew requests into account.
+func (δBtail *ΔBtail) rebuildAll() (err error) {
+	defer xerr.Context(&err, "ΔBtail rebuildAll")
+	// XXX locking
+
+	trackNewRoots := δBtail.trackNewRoots
+
+	tracefΔBtail("\nRebuildAll @%s..@%s    trackNewRoots: %s\n", δBtail.Tail(), δBtail.Head(), trackNewRoots)
+
+	for root := range trackNewRoots {
+		δTtail := δBtail.vδTbyRoot[root] // must be there
+		δtrackSet, err := δTtail.rebuild(root, δBtail.δZtail, δBtail.db)
+		if err != nil {
+			return err
+		}
+		δBtail.trackSet.UnionInplace(δtrackSet)
+	}
+
+	δBtail.trackNewRoots = SetOid{}
 	return nil
 }
 
-// rebuild rebuilds ΔBtail taking trackNew requests into account.
+// rebuild rebuilds ΔTtail taking trackNew requests into account.
+//
+// It returns set of nodes that must be added to ΔBtail.trackSet to account for
+// keys that becomes tracked. Note: this set is potentially wider compared to what was in .trackNew.
 // XXX place
-func (δBtail *ΔBtail) rebuild() (err error) {
-	defer xerr.Context(&err, "ΔBtail rebuild")
+func (δTtail *ΔTtail) rebuild(root zodb.Oid, δZtail *zodb.ΔTail, db *zodb.DB) (δtrackSet PPTreeSubSet, err error) {
+	defer xerr.Context(&err, "ΔTtail rebuild")
 	// XXX locking
 
-	tracefΔBtail("\nRebuild @%s .. @%s\n", δBtail.Tail(), δBtail.Head())
-	tracefΔBtail("trackSet:       %v\n", δBtail.trackSet)
-	tracefΔBtail("trackNew:       %v\n", δBtail.trackNew)
+	tracefΔBtail("\nRebuild %s @%s .. @%s\n", root, δZtail.Tail(), δZtail.Head())
+	tracefΔBtail("trackNew:       %v\n", δTtail.trackNew)
 
-	trackNew := δBtail.trackNew
-	δBtail.trackNew = PPTreeSubSet{}
+	trackNew := δTtail.trackNew
+	δTtail.trackNew = PPTreeSubSet{}
 
 	if len(trackNew) == 0 {
-		return
+		return nil, nil
 	}
 
 	// go backwards and merge vδT <- treediff(lo..hi/trackNew)
-	vδZ := δBtail.δZtail.Data()
-	vδtrack := []*ΔPPTreeSubSet{}
+	vδZ := δZtail.Data()
+	for {
+		δtkeycov := &RangedKeySet{}     // all keys coming into tracking set during this lo<-hi scan
+		trackNewCur := trackNew.Clone() // trackNew adjusted as of when going to i<- entry
 		for i := len(vδZ)-1; i>=0; i-- {
 			δZ := vδZ[i]
 
@@ -277,57 +356,141 @@ func (δBtail *ΔBtail) rebuild() (err error) {
 			if i > 0 {
 				atPrev = vδZ[i-1].Rev
 			} else {
-			atPrev = δBtail.δZtail.Tail()
+				atPrev = δZtail.Tail()
 			}
 
-		δZTC, δtopsByRoot := δZConnectTracked(δZ.Changev, trackNew)
+			δtrackNew, δtkeycov_, err := δTtail.rebuild1(atPrev, δZ, trackNewCur, db)
+			if err != nil {
+				return nil, err
+			}
+
+			trackNewCur.ApplyΔ(δtrackNew)
+			δtkeycov.UnionInplace(δtkeycov_)
+
+			// XXX update .KVAtTail, .lastRevOf
+		}
+
+		// an iteration closer to tail may turn out to add a key to the tracking set.
+		// We have to recheck all entries newer that revision for changes to that key,
+		// for example:
+		//
+		//              8        5*
+		//             / \  <-  / \
+		//            2   8    2*  7
+		//
+		//     here initial tracked set is 5*-2*. Going to earlier revision
+		//     2'th keycov range is widen from [-∞,5) to [-∞,7), so 5*-7 in
+		//     later revision have to be rechecked because 7 was added into
+		//     tracking set.
+		//
+		// Implement this via restarting from head and cycling until
+		// set of tracked keys does not grow anymore.
+		if δtkeycov.Empty() {
+			break
+		}
+
+		err := widenTrackNew(trackNew, δtkeycov, root, δZtail.Head(), db)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return trackNew, nil
+}
+
+// widenTrackNew widens trackNew to cover δtkeycov.
+// XXX -> widenTrackSet?
+func widenTrackNew(trackNew PPTreeSubSet, δtkeycov *RangedKeySet, root zodb.Oid, at zodb.Tid, db *zodb.DB) (err error) {
+	// XXX errctx, debug
+	defer xerr.Contextf(&err, "widenTrackNew tree<%s> @%s +%s", root, at, δtkeycov)
+
+	txn, ctx := transaction.New(context.TODO()) // XXX
+	defer txn.Abort()
+
+	zhead, err := db.Open(ctx, &zodb.ConnOptions{At: at});  /*X*/ if err != nil { return err }
+	xtree, err := zgetNodeOrNil(ctx, zhead, root);          /*X*/ if err != nil { return err }
+	if xtree == nil {
+		// root deleted -> root node covers [-∞,∞)
+		trackNew.AddPath([]zodb.Oid{root})
+		return nil
+	}
+	tree := xtree.(*Tree) // must succeed	XXX better explicit panic?
+
+	top := &nodeInRange{prefix: nil, lo: KeyMin, hi_: KeyMax, node: tree}
+	V   := rangeSplit{top}
+	for _, r := range δtkeycov.AllRanges() {
+		lo := r.lo
+		for {
+			b, err := V.GetToLeaf(ctx, lo);  /*X*/ if err != nil { return err }
+			trackNew.AddPath(b.Path())
+
+			// continue with next right bucket until r coverage is complete
+			if r.hi_ <= b.hi_ {
+				break
+			}
+			lo = b.hi_ + 1
+		}
+	}
+	return nil
+}
+
 
-		debugfΔBtail("\n  rebuild @%s <- @%s\n", atPrev, δZ.Rev)
+// rebuild1 rebuilds δT for single δZ.
+//
+// δtrackNew/δtkeycov represents how trackNew changes when going through `atPrev <- δZ.Rev` .
+func (δTtail *ΔTtail) rebuild1(atPrev zodb.Tid, δZ zodb.ΔRevEntry, trackNew PPTreeSubSet, db *zodb.DB) (δtrackNew *ΔPPTreeSubSet, δtkeycov *RangedKeySet, err error) {
+	defer xerr.Contextf(&err, "rebuild1 %s<-%s", atPrev, δZ.Rev)
+
+	debugfΔBtail("\n  rebuild1 @%s <- @%s\n", atPrev, δZ.Rev)
 	debugfΔBtail("  δZ:\t%v\n", δZ.Changev)
 	debugfΔBtail("  trackNew:  %v\n", trackNew)
-		debugfΔBtail("  trackSet:  %v\n", δBtail.trackSet)	// XXX needed?
-		defer debugfΔBtail("\n\n")
+	defer func() {
+		debugfΔBtail("-> δtrackNew: %v\n", δtrackNew)
+		debugfΔBtail("-> δtkeycov:  %v\n", δtkeycov)
+		debugfΔBtail("\n\n")
+	}()
 
-		// XXX len(δtopsByRoot) == 0  -> skip
+	δZTC, δtopsByRoot := δZConnectTracked(δZ.Changev, trackNew)
+
+	// skip opening DB connections if there is no change to this tree
+	if len(δtopsByRoot) == 0 {
+		return NewΔPPTreeSubSet(), &RangedKeySet{}, nil
+	}
+
+	if len(δtopsByRoot) != 1 {
+		panicf("BUG: δtopsByRoot has > 1 entries: %v\ntrackNew: %v\nδZ: %v", δtopsByRoot, trackNew, δZ)
+	}
+	var root  zodb.Oid
+	var δtops SetOid
+	for root_, δtops_ := range δtopsByRoot {
+		root  = root_
+		δtops = δtops_
+	}
 
 
 	// open ZODB connection corresponding to "current" and "prev" states
 	txn, ctx := transaction.New(context.TODO()) // XXX
-		defer txn.Abort() // XXX -> into func() or don't use defer
+	defer txn.Abort()
 
-		zconnPrev, err := δBtail.db.Open(ctx, &zodb.ConnOptions{At: atPrev})
+	zconnPrev, err := db.Open(ctx, &zodb.ConnOptions{At: atPrev})
 	if err != nil {
-			return err
+		return nil, nil, err
 	}
-		zconnCurr, err := δBtail.db.Open(ctx, &zodb.ConnOptions{At: δZ.Rev})
+	zconnCurr, err := db.Open(ctx, &zodb.ConnOptions{At: δZ.Rev})
 	if err != nil {
-			return err
+		return nil, nil, err
 	}
 
-		for root, δtops := range δtopsByRoot {
 	// diff backwards curr -> prev
 	δT, δtrack, δtkeycov, err := treediff(ctx, root, δtops, δZTC, trackNew, zconnCurr, zconnPrev)
 	if err != nil {
-				return err
+		return nil, nil, err
 	}
 
-			// FIXME use δtkeycov to recompute track coverage
-			_ = δtkeycov
-
-			debugfΔBtail("  -> root<%s>  δkv*: %v  δtrack*: %v\n", root, δT, δtrack)
-
-			trackNew.ApplyΔ(δtrack)
-			vδtrack = append([]*ΔPPTreeSubSet{δtrack}, vδtrack...)
+	debugfΔBtail("  -> root<%s>  δkv*: %v  δtrack*: %v  δtkeycov*: %v\n", root, δT, δtrack, δtkeycov)
 
 	if len(δT) == 0 { // an object might be resaved without change
-				continue
-			}
-
-			δTtail, ok := δBtail.byRoot[root]
-			if !ok {
-				// this root was not tracked before -> create δTtail for it with empty changes
-				δTtail = newΔTtail()
-				δBtail.byRoot[root] = δTtail
+		return δtrack, δtkeycov, nil
 	}
 
 	// δTtail.vδT <- merge δT*
@@ -344,7 +507,6 @@ func (δBtail *ΔBtail) rebuild() (err error) {
 	}
 	δTcurr := δTtail.vδT[j]
 
-
 	for k, δv := range δT {
 		// the diff was backward; δTtail entries are with diff forward
 		δv.New, δv.Old = δv.Old, δv.New
@@ -359,22 +521,11 @@ func (δBtail *ΔBtail) rebuild() (err error) {
 		}
 	}
 
-			// XXX update .KVAtTail, .lastRevOf
-		}
-	}
-
-	// trackNew was adjusted to correspond to @tail potentially growing its key coverage.
-	// Remap it back to @head and merge to .trackSet
-	for _, δtrack := range vδtrack {
-		δtrack.Reverse() // we saved it as lo<-hi; now we go lo->hi
-		trackNew.ApplyΔ(δtrack)
-	}
-	δBtail.trackSet.UnionInplace(trackNew)
-
-	return nil
+	// XXX update .KVAtTail, .lastRevOf	(here?)
+	return δtrack, δtkeycov, nil
 }
 
-// Update updates δB with per-object level ZODB changes.
+// Update updates δB with per-object-level ZODB changes.
 //
 // Only those objects from δZ that belong to tracked set are guaranteed to be
 // taken into account. In other words a tree history will assuredly include
@@ -384,36 +535,81 @@ func (δBtail *ΔBtail) rebuild() (err error) {
 //
 // TODO optionally accept zconnOld/zconnNew from client
 func (δBtail *ΔBtail) Update(δZ *zodb.EventCommit) (_ ΔB, err error) {
-	δB, _, err := δBtail._Update(δZ)
+	headOld := δBtail.Head()
+	defer xerr.Contextf(&err, "ΔBtail.Update %s -> %s", headOld, δZ.Tid)
+
+	δB1, err := δBtail._Update1(δZ)
+
+	δB := ΔB{Rev: δZ.Tid, ΔByRoot: make(map[zodb.Oid]map[Key]ΔValue)}
+	for root, δT1 := range δB1.ByRoot {
+		δTtail := δBtail.vδTbyRoot[root] // must succeed
+
+		// δtkeycov1 != ø   ->  rebuild δTtail with trackNew ~= δtkeycov1
+		if !δT1.δtkeycov1.Empty() && δBtail.δZtail.Len() > 1 {
+			trackNew := PPTreeSubSet{}
+			err := widenTrackNew(trackNew, δT1.δtkeycov1, root, δBtail.Head(), δBtail.db)
+			if err != nil {
+				return ΔB{}, err
+			}
+			// XXX assert δTtail.trackNew.Empty()
+			δTtail.trackNew = trackNew
+
+			// XXX vvv we can skip computing diff for HEAD~..HEAD (we just
+			//     computed it in _Update1)? (or not - trackNew is as of @head ?)
+			δtrackSet, err := δTtail.rebuild(root, δBtail.δZtail, δBtail.db)
+			if err != nil {
+				return ΔB{}, err
+			}
+
+			δBtail.trackSet.UnionInplace(δtrackSet)
+		}
+
+		// build δB. Even if δT=ø after _Update1, but δtkeycov1 != ø, above
+		// rebuild could result in head δT becoming != ø. Detect that δTtail head
+		// is anew by comparing to δZ.Rev.
+		l := len(δTtail.vδT)
+		if l > 0 {
+			δT := δTtail.vδT[l-1] // δT head
+			if δT.Rev == δZ.Tid {
+				δB.ΔByRoot[root] = δT.ΔKV
+			}
+		}
+
+		// XXX rebuild KVAtTail
+		// XXX rebuild lastRevOf
+	}
 	return δB, err
 }
 
-// _Update serves Update and also returns δtkeycov from treediff for testing.
-type _ΔTrackKeyCov struct {
-	ByRoot map[zodb.Oid]*RangedKeySet // {} root -> δtrackedKeys  (always grow)
+// _Update1 serves Update and performs direct update of δTtail head elements from δZ.
+// On key coverage growth rebuilding tail of the history is done by Update itself.
+//
+// _Update1 is also used in tests to verify δtkeycov return from treediff.
+type _ΔBUpdate1 struct {
+	ByRoot map[zodb.Oid]*_ΔTUpdate1
+}
+type _ΔTUpdate1 struct {
+	δtkeycov1 *RangedKeySet   // {} root -> δtrackedKeys after first treediff (always grow)
+	δtrack    *ΔPPTreeSubSet
 }
-func (δBtail *ΔBtail) _Update(δZ *zodb.EventCommit) (_ ΔB, δTKeyCov _ΔTrackKeyCov, err error) {
+func (δBtail *ΔBtail) _Update1(δZ *zodb.EventCommit) (δB1 _ΔBUpdate1, err error) {
 	headOld := δBtail.Head()
-	defer xerr.Contextf(&err, "ΔBtail update %s -> %s", headOld, δZ.Tid)
+	defer xerr.Contextf(&err, "ΔBtail.update1 %s -> %s", headOld, δZ.Tid)
 
 	tracefΔBtail("\nUpdate  @%s -> @%s    δZ: %v\n", δBtail.Head(), δZ.Tid, δZ.Changev)
 	tracefΔBtail("trackSet:       %v\n", δBtail.trackSet)
-	tracefΔBtail("trackNew:       %v\n", δBtail.trackNew)
+	for _, root := range δBtail.trackNewRoots.SortedElements() {
+		δTtail := δBtail.vδTbyRoot[root]
+		tracefΔBtail("[%s].trackNew:  %v\n", root, δTtail.trackNew)
+	}
 
-	δTKeyCov = _ΔTrackKeyCov{ByRoot: make(map[zodb.Oid]*RangedKeySet)}
+	δB1 = _ΔBUpdate1{ByRoot: make(map[zodb.Oid]*_ΔTUpdate1)}
 
-if XXX_killWhenRebuildWorks {
-// XXX hack - until vvv is reenabled
-	δBtail.trackSet.UnionInplace(δBtail.trackNew)
-	δBtail.trackNew = PPTreeSubSet{}
-} else {
-// XXX reenable (currently breaks wcfs tests)
 	// update .trackSet and vδB from .trackNew
-	err = δBtail.rebuild()
+	err = δBtail.rebuildAll()
 	if err != nil {
-		return ΔB{}, δTKeyCov, err
+		return δB1, err
 	}
-}
 
 	// XXX dup wrt rebuild?
 
@@ -421,11 +617,9 @@ if XXX_killWhenRebuildWorks {
 
 	δZTC, δtopsByRoot := δZConnectTracked(δZ.Changev, δBtail.trackSet)
 
-	δB := ΔB{Rev: δZ.Tid, ByRoot: make(map[zodb.Oid]map[Key]ΔValue)}
-
 	// skip opening DB connections if there is no change to any tree node
 	if len(δtopsByRoot) == 0 {
-		return δB, δTKeyCov, nil
+		return δB1, nil
 	}
 
 	// open ZODB connections corresponding to "old" and "new" states
@@ -434,55 +628,41 @@ if XXX_killWhenRebuildWorks {
 	defer txn.Abort()
 	zconnOld, err := δBtail.db.Open(ctx, &zodb.ConnOptions{At: headOld})
 	if err != nil {
-		return ΔB{}, δTKeyCov, err
+		return δB1, err
 	}
 	zconnNew, err := δBtail.db.Open(ctx, &zodb.ConnOptions{At: δZ.Tid})
 	if err != nil {
-		return ΔB{}, δTKeyCov, err
+		return δB1, err
 	}
 
 	for root, δtops := range δtopsByRoot {
 		δT, δtrack, δtkeycov, err := treediff(ctx, root, δtops, δZTC, δBtail.trackSet, zconnOld, zconnNew)
 		if err != nil {
-			return ΔB{}, δTKeyCov, err
+			return δB1, err
 		}
 
 		tracefΔBtail("\n-> root<%s>  δkv: %v  δtrack: %v  δtkeycov: %v\n", root, δT, δtrack, δtkeycov)
 
+		δTtail := δBtail.vδTbyRoot[root] // must be there
 		if len(δT) > 0 { // an object might be resaved without change
-			δB.ByRoot[root] = δT
-			δTtail, ok := δBtail.byRoot[root]
-			if !ok {
-				// this root was not tracked before -> create δTtail for it with empty changes
-				δTtail = newΔTtail()
-				δBtail.byRoot[root] = δTtail
-			}
-
 			δTtail.vδT = append(δTtail.vδT, ΔTree{Rev: δZ.Tid, ΔKV: δT})
-
-			// XXX rebuild KVAtTail
-			// XXX rebuild lastRevOf
 		}
 
 		δBtail.trackSet.ApplyΔ(δtrack)
-		δTKeyCov.ByRoot[root] = δtkeycov
+		δB1.ByRoot[root] = &_ΔTUpdate1{δtkeycov1: δtkeycov, δtrack: δtrack}
 	}
 
-	return δB, δTKeyCov, nil
+	return δB1, nil
 }
 
 
 func (δBtail *ΔBtail) ForgetPast(revCut zodb.Tid) {
 	δBtail.δZtail.ForgetPast(revCut) // XXX stub
-	// XXX clean vδT
+	// TODO go through vδBroots till revcut -> find which trees to trim -> trim ΔTtails.
+	panic("TODO")
 }
 
 
-// update brings .δBtail up to date by recomputing diff XXX and taking new
-// entries in .δZtail into account.
-// func (δBtail *ΔBtail) update()
-
-
 // Get returns root[key] as of @at database state plus revision that changed it.
 //
 // if revExact=False - rev is upper estimate for the revision.
@@ -504,7 +684,7 @@ func (δBtail *ΔBtail) GetAt(ctx context.Context, root *Tree, key Key, at zodb.
 	// XXX dirty -> rebuild
 
 	// XXX -> index lastXXXOf(key) | linear scan ↓ looking for change <= at
-	δTtail := δBtail.byRoot[root.POid()]
+	δTtail := δBtail.vδTbyRoot[root.POid()]
 	if δTtail == nil {
 		panicf("δBtail: root<%s> not tracked", root.POid())
 	}
@@ -573,7 +753,8 @@ func (δBtail *ΔBtail) GetAt(ctx context.Context, root *Tree, key Key, at zodb.
 func (δBtail *ΔBtail) SliceByRootRev(root zodb.Oid, lo, hi zodb.Tid) /*readonly*/[]ΔTree {
 	xtail.AssertSlice(δBtail, lo, hi)
 	// XXX locking
-	δTtail, ok := δBtail.byRoot[root]
+	// XXX rebuild
+	δTtail, ok := δBtail.vδTbyRoot[root]
 	if !ok {
 		return []ΔTree{}
 	}

wcfs/internal/xbtree/δbtail_test.go

@@ -300,13 +300,13 @@ func (rbs RBucketSet) coverage() string {
 	return s
 }
 
-// trackSet returns what should be ΔBtree.trackSet coverage for specified tracked key set.
+// trackSet returns what should be ΔBtail.trackSet coverage for specified tracked key set.
 func (rbs RBucketSet) trackSet(tracked SetKey) PPTreeSubSet {
 	trackSet, _ := rbs.trackSetWithCov(tracked)
 	return trackSet
 }
 
-// trackSetWithCov returns what should be ΔBtree.trackSet and its key coverage for specified tracked key set.
+// trackSetWithCov returns what should be ΔBtail.trackSet and its key coverage for specified tracked key set.
 func (rbs RBucketSet) trackSetWithCov(tracked SetKey) (trackSet PPTreeSubSet, keyCover *RangedKeySet) {
 	trackSet = PPTreeSubSet{}
 	keyCover = &RangedKeySet{}
@@ -645,6 +645,11 @@ func _KAdj(t1, t2 *tTreeCommit, keysv ...SetKey) (kadj KAdjMatrix) {
 
 
 // xverifyΔBTail_Update verifies how ΔBTail handles ZODB update for a tree with changes in between t1->t2.
+//
+// Note: this test verifies only single treediff step of ΔBtail.Update.
+//       the cycling phase of update, that is responsible to recompute older
+//       entries when key coverage grows, is exercised by
+//       xverifyΔBTail_rebuild.
 func xverifyΔBTail_Update(t *testing.T, subj string, db *zodb.DB, treeRoot zodb.Oid, t1, t2 *tTreeCommit) {
 	// verify transition at1->at2 for all initial states of tracked {keys} from kv1 + kv2 + ∞
 
@@ -690,7 +695,7 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 		badv = append(badv, fmt.Sprintf(format, argv...))
 	}
 	defer func() {
-		if badv != nil {
+		if badv != nil || t.Failed() {
 			emsg := fmt.Sprintf("%s  ; tracked=%v :\n\n", subj, initialTrackedKeys)
 			emsg += fmt.Sprintf("d12:  %v\nδTok: %v\nδT:   %v\n\n", d12, δTok, δT)
 			emsg += fmt.Sprintf("δZ:               %v\n", δZset)
@@ -705,38 +710,12 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 	}()
 
 
-	txn, ctx := transaction.New(context.Background())
-	defer txn.Abort()
-
-	// zconn, δbtail @at1 with initial tracked set
-	zconn, err := db.Open(ctx, &zodb.ConnOptions{At: at1});	X(err)
-	δbtail := NewΔBtail(zconn.At(), db)
-
-	xtree, err := zgetNodeOrNil(ctx, zconn, treeRoot); X(err)
-	var ztree *Tree // = nil if treeRoot was deleted
-	if xtree != nil {
-		ztree = xtree.(*Tree)
-	}
-
-	for k := range initialTrackedKeys {
-		if ztree != nil {
-			_, ok, path, err := ZTreeGetBlkData(ctx, ztree, k);  X(err)
-			err = δbtail.Track(k, ok, path); X(err)
-		} else {
-			// if treeRoot is deleted - add it to tracked set with every key
-			// being a hole. This aligns with the following situation
-			//
-			//	T1 -> ø -> T2
-			//
-			// where after T1->ø, even though the tree becomes deleted, its root
-			// continues to be tracked and all keys migrate to holes in the
-			// tracking set. By aligning initial state to the same as after
-			// T1->ø, we test what will happen on ø->T2.
-			err = δbtail.track(k, false, []zodb.Oid{treeRoot});  X(err)
-		}
-	}
+	// δbtail @at1 with initial tracked set
+	δbtail := NewΔBtail(at1, db)
+	xtrackKeys(δbtail, treeRoot, initialTrackedKeys)
 
-	TrackedδZ = SetKey{} // Tracked ^ δZ
+	// TrackedδZ = Tracked ^ δZ  (i.e. a tracked node has changed, or its coverage was changed)
+	TrackedδZ = SetKey{}
 	for k := range initialTrackedKeys {
 		leaf1 := xkv1.Get(k)
 		oid1 := leaf1.oid
@@ -748,7 +727,7 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 		if oid2 == zodb.InvalidOid { // embedded bucket
 			oid2 = leaf2.parent.oid
 		}
-		if δZset.Has(oid1) || δZset.Has(oid2) {
+		if δZset.Has(oid1) || δZset.Has(oid2) || (KeyRange{leaf1.lo,leaf1.hi_} != KeyRange{leaf2.lo,leaf2.hi_}) {
 			TrackedδZ.Add(k)
 		}
 	}
@@ -786,35 +765,51 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 
 	ø := PPTreeSubSet{}
 
-	// verify δbtail.trackSet against @at1
 	// trackSet1 = xkv1[tracked1]
+	// trackSet2 = xkv2[tracked2]  ( = xkv2[kadj[tracked1]]
 	trackSet1, tkeyCov1 := xkv1.trackSetWithCov(initialTrackedKeys)
-	if !δbtail.trackSet.Equal(ø) {
-		badf("δbtail.trackSet1 wrong:\n\thave: %v\n\twant: %v", δbtail.trackSet, ø)
+	trackSet2, tkeyCov2 := xkv2.trackSetWithCov(initialTrackedKeys.Union(kadjTrackedδZ))
+
+	// verify δbtail.trackSet against @at1
+	δbtail.assertTrack(t, "1", ø, trackSet1)
+
+	// δB <- δZ
+	//
+	// also call _Update1 directly to verify δtkeycov return from treediff
+	// the result of Update and _Update1 should be the same since δbtail is initially empty.
+	δbtail_ := δbtail.Clone()
+	δB1, err := δbtail_._Update1(δZ);  X(err)	// XXX don't compute treediff twice
+
+	// assert tkeyCov1 ⊂ tkeyCov2
+	dkeycov12 := tkeyCov1.Difference(tkeyCov2)
+	if !dkeycov12.Empty() {
+		t.Errorf("BUG: tkeyCov1 ⊄ tkeyCov2:\n\ttkeyCov1: %s\n\ttkeyCov2: %s\n\ttkeyCov1 \\ tkeyCov2: %s", tkeyCov1, tkeyCov2, dkeycov12)
 	}
-	if !δbtail.trackNew.Equal(trackSet1) {
-		badf("δbtail.trackNew1 wrong:\n\thave: %v\n\twant: %v", δbtail.trackNew, trackSet1)
+
+	// assert δtkeycov == δ(tkeyCov1, tkeyCov2)
+	δtkeycovOK := tkeyCov2.Difference(tkeyCov1)
+	//fmt.Printf("tkeyCov1: %s\n", tkeyCov1)
+	//fmt.Printf("tkeyCov2: %s\n", tkeyCov2)
+	//fmt.Printf("δtkeycov: %s\n", δtkeycovOK)
+	δtkeycov := &RangedKeySet{}
+	if __, ok := δB1.ByRoot[treeRoot]; ok {
+		δtkeycov = __.δtkeycov1
+	}
+	if !δtkeycov.Equal(δtkeycovOK) {
+		badf("δtkeycov wrong:\nhave: %s\nwant: %s", δtkeycov, δtkeycovOK)
 	}
-//	δbtail.assertTrack(t, "1", ø, trackSet1)
 
+	δB, err  := δbtail.Update(δZ);     X(err)
+	// XXX assert δB.roots == δTKeyCov roots
+	// XXX assert δBtail[root].vδT = δBtail_[root].vδT
 
-	// δB <- δZ
-	δB, δTKeyCov, err := δbtail._Update(δZ); X(err)
 	if δB.Rev != δZ.Tid {
 		badf("δB: rev != δZ.Tid  ; rev=%s  δZ.Tid=%s", δB.Rev, δZ.Tid)
 		return
 	}
 
 	// verify δbtail.trackSet  against @at2
-	// trackSet2 = xkv2[tracked2]  ( = xkv2[kadj[tracked1]]
-	trackSet2, tkeyCov2 := xkv2.trackSetWithCov(initialTrackedKeys.Union(kadjTrackedδZ))
-	if !δbtail.trackSet.Equal(trackSet2) {
-		badf("δbtail.trackSet2 wrong:\n\thave: %v\n\twant: %v", δbtail.trackSet, trackSet2)
-	}
-	if !δbtail.trackNew.Equal(ø) {
-		badf("δbtail.trackNew2 wrong:\n\thave: %v\n\twant: %v", δbtail.trackNew, ø)
-	}
-//	δbtail.assertTrack(t, "2", trackSet2, ø)
+	δbtail.assertTrack(t, "2", trackSet2, ø)
 
 
 	// assert δB.ByRoot == {treeRoot -> ...}  if δTok != ø
@@ -824,27 +819,20 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 		rootsOK.Add(treeRoot)
 	}
 	roots := SetOid{}
-	for root := range δB.ByRoot {
+	for root := range δB.ΔByRoot {
 		roots.Add(root)
 	}
 	if !reflect.DeepEqual(roots, rootsOK) {
 		badf("δB: roots != rootsOK  ; roots=%v  rootsOK=%v", roots, rootsOK)
 	}
-	_, inδB := δB.ByRoot[treeRoot]
+	_, inδB := δB.ΔByRoot[treeRoot]
 	if !inδB {
 		return
 	}
 
-	// assert δtkeycov == δ(tkeyCov1, tkeyCov2)
-	δtkeycovOK := tkeyCov2.Difference(tkeyCov1)
-	δtkeycov := δTKeyCov.ByRoot[treeRoot]
-	if !δtkeycov.Equal(δtkeycovOK) {
-		badf("δtkeycov wrong:\nhave: %s\nwant: %s", δtkeycov, δtkeycovOK)
-	}
-
 
 	// δT <- δB
-	δToid := δB.ByRoot[treeRoot]         // {} k -> δoid
+	δToid := δB.ΔByRoot[treeRoot]        // {} k -> δoid
 	δT     = XGetδKV(db, at1,at2, δToid) // {} k -> δ(ZBlk(oid).data)
 
 	// δT must be subset of d12.
@@ -883,36 +871,64 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 
 // assertTrack verifies that trackSet == trackSetOK.
 // XXX place
-func assertTrack(t *testing.T, subj string, trackSet, trackSetOK PPTreeSubSet) {
+// XXX inline into δbtail.assertTrack?
+func _assertTrack(t *testing.T, subj string, trackSet, trackSetOK PPTreeSubSet) {
 	t.Helper()
-	eq := trackSet.Equal(trackSetOK)
-	if !eq {
+	if !trackSet.Equal(trackSetOK) {
 		t.Errorf("%s:\n\thave: %v\n\twant: %v", subj, trackSet, trackSetOK)
 	}
 }
 
-// assertTrack verifies state of .trackSet and .trackNew.
+// assertTrack verifies state of .trackSet and ΔTtail.trackNew.
+// it assumes that only one tree root is being tracked.
 // XXX place
-func (δbtail *ΔBtail) assertTrack(t *testing.T, subj string, trackSetOK, trackNewOK PPTreeSubSet) {
+func (δBtail *ΔBtail) assertTrack(t *testing.T, subj string, trackSetOK PPTreeSubSet, trackNewOK PPTreeSubSet) {
 	t.Helper()
-	assertTrack(t, subj + ": trackSet", δbtail.trackSet, trackSetOK)
-	assertTrack(t, subj + ": trackNew", δbtail.trackNew, trackNewOK)
+	_assertTrack(t, subj + ": trackSet", δBtail.trackSet, trackSetOK)
+
+	roots := SetOid{}
+	for root := range δBtail.vδTbyRoot {
+		roots.Add(root)
+	}
+
+	nrootsOK := 1
+	if trackSetOK.Empty() && trackNewOK.Empty() {
+		nrootsOK = 0
+	}
+	if len(roots) != nrootsOK {
+		t.Errorf("%s: len(vδTbyRoot) != %d   ; roots=%v", subj, nrootsOK, roots)
+		return
+	}
+	if nrootsOK == 0 {
+		return
+	}
+
+	root := roots.Elements()[0]
+
+	δTtail := δBtail.vδTbyRoot[root]
+
+	trackNewRootsOK := SetOid{}
+	if !trackNewOK.Empty() {
+		trackNewRootsOK.Add(root)
+	}
+
+	if !δBtail.trackNewRoots.Equal(trackNewRootsOK) {
+		t.Errorf("%s: trackNewRoots:\n\thave: %v\n\twant: %v", subj, δBtail.trackNewRoots, trackNewRootsOK)
+	}
+
+	_assertTrack(t, subj + ": vδT.trackNew", δTtail.trackNew, trackNewOK)
 }
 
-// xverifyΔBTail_rebuild verifies δBtail.rebuild during t0->t1->t2 transition.
+// xverifyΔBTail_rebuild verifies ΔBtail.rebuild during t0->t1->t2 transition.
 //
 // t0->t1 exercises from-scratch rebuild,
 // t1->t2 further exercises incremental rebuild.
+//
+// It also exercises rebuild phase of ΔBtail.Update.
 func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1, t2 *tTreeCommit) {
-	// XXX handle DEL
-	// XXX Update -> Track -> rebuild ...
-	// XXX can start with non-existing tree
-
 //	t1 := t2.prev
 //	t0 := t1.prev
 	t.Run(fmt.Sprintf("rebuild/%s→%s", t0.tree, t1.tree), func(t *testing.T) {
-		t.Skip("TODO") // FIXME rebuild is currently broken
-
 		tAllKeys := allTestKeys(t0, t1, t2)
 		tAllKeyv := tAllKeys.SortedElements()
 
@@ -927,7 +943,6 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 		fmt.Printf("@%s:  %v\n", xat[t1.at], t1.xkv.Flatten())
 		fmt.Printf("@%s:  %v\n", xat[t2.at], t2.xkv.Flatten())
 
-		kadj01 := KAdj(t0,t1, allTestKeys(t0,t1,t2))
 		kadj10 := KAdj(t1,t0, allTestKeys(t0,t1,t2))
 		kadj21 := KAdj(t2,t1, allTestKeys(t0,t1,t2))
 		kadj12 := KAdj(t1,t2, allTestKeys(t0,t1,t2))
@@ -958,7 +973,9 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 				// assert trackSet=ø, trackNew=ø, vδB=[]
 				δbtail.assertTrack(t, "@at0", ø, ø)  // XXX + vδB
 
-				xverifyΔBTail_rebuild_U(t, δbtail, t0, t1, xat, /*trackSet=*/ø)
+				xverifyΔBTail_rebuild_U(t, δbtail, db, treeRoot, t0, t1, xat,
+					/*trackSet=*/ø,
+					/*vδT=ø*/)
 				xverifyΔBTail_rebuild_TR(t, db, δbtail, t1, treeRoot, xat,
 					// after Track(keys1)
 					keys1,
@@ -970,10 +987,34 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 					/*vδT=*/ δkv1_1)
 
 				t.Run((" →" + t2.tree), func(t *testing.T) {
-					// tracked keys1 becomes tracked keys1_2 after Update(t1->t2)
-//					keys1_2 := kadj12.Map(keys1)
-					keys1_2 := kadj12.Map(kadj01.Map(keys1_0))
-					xverifyΔBTail_rebuild_U(t, δbtail, t1, t2, xat, /*trackSet=*/t2.xkv.trackSet(keys1_2))
+					// keys1R2 is full set of keys that should become tracked after
+					// Update()  (which includes rebuild)
+					keys1R2 := kadj12.Map(keys1)
+					for {
+						keys1R2_ := kadj10.Map(kadj21.Map(keys1R2))
+						if keys1R2.Equal(keys1R2_) {
+							break
+						}
+						keys1R2 = keys1R2_
+					}
+
+					// δkvX_k1R2 = tX.δxkv / keys1R2
+					δkv1_k1R2 := map[Key]Δstring{}
+					δkv2_k1R2 := map[Key]Δstring{}
+					for k := range keys1R2 {
+						δv1, ok := t1.δxkv[k]
+						if ok {
+							δkv1_k1R2[k] = δv1
+						}
+						δv2, ok := t2.δxkv[k]
+						if ok {
+							δkv2_k1R2[k] = δv2
+						}
+					}
+
+					xverifyΔBTail_rebuild_U(t, δbtail, db, treeRoot, t1, t2, xat,
+						/*trackSet=*/t2.xkv.trackSet(keys1R2),
+						/*vδT=*/ δkv1_k1R2, δkv2_k1R2)
 
 					// tRestKeys2 = tAllKeys - keys1
 					tRestKeys2 := tAllKeys.Difference(keys1)
@@ -984,60 +1025,69 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 							keys2.Add(tRestKeyv2[idx2])
 						}
 
-						// find out full set of keys that should become tracked after
+						// keys12R2 is full set of keys that should become tracked after
 						// Track(keys2) + rebuild
-						keys12R := keys1_2.Union(keys2)
+						keys12R2 := keys1R2.Union(keys2)
 						for {
-							keys12R_ := kadj10.Map(kadj21.Map(keys12R))
-							if keys12R.Equal(keys12R_) {
+							keys12R2_ := kadj10.Map(kadj21.Map(keys12R2))
+							if keys12R2.Equal(keys12R2_) {
 								break
 							}
-							keys12R = keys12R_
+							keys12R2 = keys12R2_
 						}
 
-						// δkv1_2 = t1.δxkv / kadj10(kadj21(kadj12(keys1) | keys2))
-						// δkv2_2 = t2.δxkv / kadj10(kadj21(kadj12(keys1) | keys2))
-//						keys12_2 := keys1_2.Union(keys2)
-//						keys12_0 := kadj10.Map(kadj21.Map(keys12_2))
+/*
 						fmt.Printf("\n\n\nKKK\nkeys1=%s  keys2=%s\n", keys1, keys2)
-						fmt.Printf("keys12R=%s\n", keys12R)
+						fmt.Printf("keys1R2:  %s\n", keys1R2)
+						fmt.Printf("keys12R2: %s\n", keys12R2)
 
 						fmt.Printf("t0.xkv: %v\n", t0.xkv)
 						fmt.Printf("t1.xkv: %v\n", t1.xkv)
 						fmt.Printf("t2.xkv: %v\n", t2.xkv)
 						fmt.Printf("kadj21: %v\n", kadj21)
 						fmt.Printf("kadj12: %v\n", kadj12)
-						fmt.Printf("t2.xkv.trackSet(%s) -> %s", keys12R, t2.xkv.trackSet(keys12R))
+						fmt.Printf("t2.xkv.trackSet(keys2)   -> %s\n", t2.xkv.trackSet(keys2))
+						fmt.Printf("t2.xkv.trackSet(keys1R2) -> %s\n", t2.xkv.trackSet(keys1R2))
+						fmt.Printf("t2.xkv.trackSet(keys2) \\ t2.xkv.trackSet(keys1R2)  -> %s\n",
+							t2.xkv.trackSet(keys2).Difference(t2.xkv.trackSet(keys1R2)))
 						fmt.Printf("\n\n\n")
+*/
 
 
-						δkv1_2 := map[Key]Δstring{}
-						δkv2_2 := map[Key]Δstring{}
-						for k := range keys12R {
+						// δkvX_k12R2 = tX.δxkv / keys12R2
+						δkv1_k12R2 := map[Key]Δstring{}
+						δkv2_k12R2 := map[Key]Δstring{}
+						for k := range keys12R2 {
 							δv1, ok := t1.δxkv[k]
 							if ok {
-								δkv1_2[k] = δv1
+								δkv1_k12R2[k] = δv1
 							}
 							δv2, ok := t2.δxkv[k]
 							if ok {
-								δkv2_2[k] = δv2
+								δkv2_k12R2[k] = δv2
 							}
 						}
 
 						// t.Run is expensive at this level of nest
 //						t.Run(" T"+keys2.String()+";R", func(t *testing.T) {
-							δbtail_ := δbtail.clone()
+							δbtail_ := δbtail.Clone()
 							xverifyΔBTail_rebuild_TR(t, db, δbtail_, t2, treeRoot, xat,
 								// after Track(keys2)
 								keys2,
-//								/*trackSet*/ t2.xkv.trackSet(keys1_0.Union(keys1_2)),
-								/*trackSet*/ t2.xkv.trackSet(keys1_2),
-								// FIXME vvv trackNew should not cover ranges that are already in trackSet
-								/*trackNew*/ t2.xkv.trackSet(keys2),
+								/*trackSet*/ t2.xkv.trackSet(keys1R2),
+								/*trackNew*/ t2.xkv.trackSet(keys2).Difference(
+									// trackNew should not cover ranges that are
+									// already in trackSet
+									t2.xkv.trackSet(keys1R2)),
 
 								// after rebuild
-								/* trackSet=*/ t2.xkv.trackSet(keys12R),
-								/*vδT=*/ δkv1_2, δkv2_2)
+								/* trackSet=*/ t2.xkv.trackSet(keys12R2),
+								/*vδT=*/ δkv1_k12R2, δkv2_k12R2)
+
+							// XXX move vvv to separate test (out of this inner loop)
+							// ΔBtail.Clone had bug that aliased klon data to orig
+							assertΔTtail(t, "BUG: after clone check", δbtail, db, t2, treeRoot, xat,
+								/*vδT=*/ δkv1_k1R2, δkv2_k1R2)
 //						})
 					}
 				})
@@ -1046,51 +1096,54 @@ func xverifyΔBTail_rebuild(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, t0, t1
 	})
 }
 
-// xverifyΔBTail_rebuild_U verifies ΔBTree state after Update(ti->tj).
-func xverifyΔBTail_rebuild_U(t *testing.T, δbtail *ΔBtail, ti, tj *tTreeCommit, xat map[zodb.Tid]string, trackSet PPTreeSubSet) {
+// xverifyΔBTail_rebuild_U verifies ΔBtail state after Update(ti->tj).
+func xverifyΔBTail_rebuild_U(t *testing.T, δbtail *ΔBtail, db *zodb.DB, treeRoot zodb.Oid, ti, tj *tTreeCommit, xat map[zodb.Tid]string, trackSet PPTreeSubSet, vδTok ...map[Key]Δstring) {
 	t.Helper()
 	X := exc.Raiseif
 	ø := PPTreeSubSet{}
 
+	subj := fmt.Sprintf("after Update(@%s→@%s)", xat[ti.at], xat[tj.at])
+
 	// Update ati -> atj
 	δB, err := δbtail.Update(tj.δZ);  X(err)
 	// XXX assert δB.Rev = tj.at; δB = δ(ti,tj)/initially tracked
-	δbtail.assertTrack(t, fmt.Sprintf("after Update(@%s→@%s)", xat[ti.at], xat[tj.at]), trackSet, ø)
+	δbtail.assertTrack(t, subj, trackSet, ø)
 	_ = δB
+	assertΔTtail(t, subj, δbtail, db, tj, treeRoot, xat, vδTok...)
 }
 
-// xverifyΔBTail_rebuild_TR verifies ΔBTree state after Track(keys) + rebuild.
-func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *tTreeCommit, treeRoot zodb.Oid, xat map[zodb.Tid]string, keys SetKey, trackSet PPTreeSubSet, trackNew, trackSetAfterRebuild PPTreeSubSet,vδTok ...map[Key]Δstring) {
+// xverifyΔBTail_rebuild_TR verifies ΔBtail state after Track(keys) + rebuild.
+func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *tTreeCommit, treeRoot zodb.Oid, xat map[zodb.Tid]string, keys SetKey, trackSet PPTreeSubSet, trackNew, trackSetAfterRebuild PPTreeSubSet, vδTok ...map[Key]Δstring) {
 	t.Helper()
-	X := exc.Raiseif
 	ø := PPTreeSubSet{}
 
 	// Track(keys)
-	txn, ctx := transaction.New(context.Background())
-	defer txn.Abort()
-	zconn, err := db.Open(ctx, &zodb.ConnOptions{At: tj.at});  X(err)
-	xtree, err := zconn.Get(ctx, treeRoot);	 X(err)
-	ztree := xtree.(*Tree)
-	for k := range keys {
-		_, ok, path, err := ZTreeGetBlkData(ctx, ztree, k);  X(err)
-		err = δbtail.Track(k, ok, path);  X(err)
-	}
+	xtrackKeys(δbtail, treeRoot, keys)
 
-	δbtail.assertTrack(t, fmt.Sprintf("@%s: after Track%v", xat[tj.at], keys), trackSet, trackNew)
+	subj := fmt.Sprintf("@%s: after Track%v", xat[tj.at], keys)
+	δbtail.assertTrack(t, subj, trackSet, trackNew)
 	// XXX vδB=[ø]
 
-	δbtail.rebuild()
+	δbtail.rebuildAll()
 
-	δbtail.assertTrack(t, fmt.Sprintf("@%s: after Track%v + rebuild", xat[tj.at], keys), trackSetAfterRebuild, ø)
+	subj += " + rebuild"
+	δbtail.assertTrack(t, subj, trackSetAfterRebuild, ø)
 	// XXX assert vδB=[δ1/T(keys)]
 
 	// XXX verify Get
 
-	// verify SliceByRootRev	XXX -> verify δbtail.byRoot[treeRoot] directly
+	// verify δbtail.vδTbyRoot[treeRoot]
+	assertΔTtail(t, subj, δbtail, db, tj, treeRoot, xat, vδTok...)
+}
+
+// assertΔTtail verifies state of ΔTtail that corresponds to treeRoot in δbtail.
+func assertΔTtail(t *testing.T, subj string, δbtail *ΔBtail, db *zodb.DB, tj *tTreeCommit, treeRoot zodb.Oid, xat map[zodb.Tid]string, vδTok ...map[Key]Δstring) {
+	t.Helper()
 	// XXX +KVAtTail, +lastRevOf
+
 	l := len(vδTok)
-	vatOK  := []zodb.Tid{}
-	vδTok_ := []map[Key]Δstring{}
+	var vatOK  []zodb.Tid
+	var vδTok_ []map[Key]Δstring
 	t0 := tj
 	for i := 0; i<l; i++ {
 		// empty vδTok entries means they should be absent in vδT
@@ -1101,16 +1154,18 @@ func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *
 		t0 = t0.prev
 	}
 	vδTok = vδTok_
-	lo := t0.at
-	hi := tj.at
-	vδToid := δbtail.SliceByRootRev(treeRoot, lo, hi)
+	δTtail, ok := δbtail.vδTbyRoot[treeRoot]
+	var vδToid []ΔTree
+	if ok {
+		vδToid = δTtail.vδT
+	}
 
 	l = len(vδToid)
-	vat := make([]zodb.Tid, l)
-	vδT := []map[Key]Δstring{}
-	atPrev := lo
-	for i, δToid := range vδToid {
-		vat[i] = δToid.Rev
+	var vat []zodb.Tid
+	var vδT []map[Key]Δstring
+	atPrev := t0.at
+	for _, δToid := range vδToid {
+		vat = append(vat, δToid.Rev)
 		δT := XGetδKV(db, atPrev, δToid.Rev, δToid.ΔKV) // {} k -> δ(ZBlk(oid).data)
 		vδT = append(vδT, δT)
 		atPrev = δToid.Rev
@@ -1125,13 +1180,49 @@ func xverifyΔBTail_rebuild_TR(t *testing.T, db *zodb.DB, δbtail *ΔBtail, tj *
 		for i := 0; i<len(vδTok); i++ {
 			want += fmt.Sprintf("\n\t@%s: %v", xat[vatOK[i]], vδTok[i])
 		}
-		t.Errorf("@%s: after Track%v + rebuild: SliceByRootRev:\nhave: %v\nwant: %v", xat[tj.at], keys, have, want)
+		t.Errorf("%s: vδT:\nhave: %v\nwant: %v", subj, have, want)
+	}
+}
+
+// xtrackKeys issues δbtail.Track requests for tree[keys].
+// XXX place
+func xtrackKeys(δbtail *ΔBtail, treeRoot zodb.Oid, keys SetKey) {
+	X := exc.Raiseif
+
+	txn, ctx := transaction.New(context.Background())
+	defer txn.Abort()
+
+	zconn, err := δbtail.db.Open(ctx, &zodb.ConnOptions{At: δbtail.Head()});  X(err)
+
+	xtree, err := zgetNodeOrNil(ctx, zconn, treeRoot); X(err)
+	var ztree *Tree // = nil if treeRoot was deleted
+	if xtree != nil {
+		ztree = xtree.(*Tree)
+	}
+
+	for k := range keys {
+		if ztree != nil {
+			_, _, path, err := ZTreeGetBlkData(ctx, ztree, k);  X(err)
+			err = δbtail.Track(k, path); X(err)
+		} else {
+			// if treeRoot is deleted - add it to tracked set with every key
+			// being a hole. This aligns with the following situation
+			//
+			//	T1 -> ø -> T2
+			//
+			// where after T1->ø, even though the tree becomes deleted, its root
+			// continues to be tracked and all keys migrate to holes in the
+			// tracking set. By aligning initial state to the same as after
+			// T1->ø, we test what will happen on ø->T2.
+			err = δbtail.track(k, []zodb.Oid{treeRoot});  X(err)
+		}
 	}
 }
 
 // xverifyΔBTail_GetAt verifies δBtail.Get on series of vt ZODB changes.
 // XXX
 // XXX kill
+/*
 func ___xverifyΔBTail_GetAt(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, vt ...*tTreeCommit) {
 	subj := vt[0].tree
 	for _, t := range vt[1:] {
@@ -1182,8 +1273,8 @@ func xverifyΔBTail_GetAt1(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, vt []*t
 	ztree := xtree.(*Tree)
 
 	for k := range keys {
-		_, ok, path, err := ZTreeGetBlkData(ctx, ztree, k);  X(err)
-		err = δbtail.Track(k, ok, path);  X(err)
+		_, _, path, err := ZTreeGetBlkData(ctx, ztree, k);  X(err)
+		err = δbtail.Track(k, path);  X(err)
 	}
 
 	// verify GetAt(k, @at) for all keys and @at
@@ -1217,6 +1308,7 @@ func xverifyΔBTail_GetAt1(t *testing.T, db *zodb.DB, treeRoot zodb.Oid, vt []*t
 		}
 	}
 }
+*/
 
 
 // ΔBTestEntry represents one entry in ΔBTail tests.
@@ -1241,7 +1333,7 @@ func ΔBTest(xtest interface{}) ΔBTestEntry {
 	return test
 }
 
-// ΔBCommit represent test commit changing a tree.
+// tTreeCommit represent test commit changing a tree.
 type tTreeCommit struct {
 	tree string            // the tree in toplogy-encoding
 	prev *tTreeCommit      // previous commit
@@ -1585,6 +1677,23 @@ func TestΔBTail(t *testing.T) {
 		"T/T/B0:a",
 		"T/B0:a",
 
+		// δtkeycov grows due to change in parent tree only
+		"T3/B1:a-B8:c",
+		"T7/B1:a-B8:c",
+		// ----//----
+		"T3/B1:a,2:b-B8:c,9:d",
+		"T7/B1:a,2:b-B8:c,9:d",
+		// ----//---- depth=2
+		"T3/T-T/B1:a,2:b-B8:c,9:d",
+		"T7/T-T/B1:a,2:b-B8:c,9:d",
+		// ----//---- found by AllStructs
+		"T1,3/B0:d-B1:a-B3:d,4:g",
+		"T1,4/B0:e-B1:a-B4:c",
+		// ----//---- found by AllStructs
+		"T2,4/T-T-T/T1-T-B4:f/T-T-B3:f/B0:h-B1:f",
+		"T4/T-T/B3:f-T/B4:a",
+
+
 
 //		"T/T/T/B1:a,2:b",
 //		"T/T/B1:a",
@@ -1671,8 +1780,8 @@ func TestΔBTailAllStructs(t *testing.T) {
 	X := exc.Raiseif
 
 	// considerations:
-	// - depth↑  better for testing (more tricky topologies)
-	// - nsplit↑ not so better for testing (leave s=1, max s=2)
+	// - maxdepth↑ better for testing (more tricky topologies)
+	// - maxsplit↑ not so better for testing (leave s=1, max s=2)
 	// - |kmin - kmax| affects N(variants) significantly
 	//   -> keep key range small  (dumb increase does not help testing)
 	// - N(keys) affects N(variants) significantly
@@ -1735,6 +1844,10 @@ func TestΔBTailAllStructs(t *testing.T) {
 		return vv[i:i+1]
 	}
 
+	// XXX rework to be effective with testing rebuild:
+	// - AllStructs accept only 1 kv
+	// - we call it 3 times for kv1 kv2 kv3
+	// - iterate via triTravelXXXway ... here instead of in treegen.py
 	testq := make(chan ΔBTestEntry)
 	go func() {
 		defer close(testq)
@@ -1744,8 +1857,8 @@ func TestΔBTailAllStructs(t *testing.T) {
 
 			kv1 := map[Key]string{}
 			kv2 := map[Key]string{}
-			for k := range keys1 { kv1[Key(k)] = randv() }
-			for k := range keys2 { kv2[Key(k)] = randv() }
+			for _, k := range keys1 { kv1[Key(k)] = randv() }
+			for _, k := range keys2 { kv2[Key(k)] = randv() }
 
 			// given (kv1, kv2) - test on automatically generated (tree1 -> tree2)
 			reqSeed := rng.Int63()
@@ -1902,39 +2015,6 @@ func allTestKeys(vt ...*tTreeCommit) SetKey {
 	return allKeys
 }
 
-/*
-// easies debugging / makes error output stable from run to run.
-func (ks SetKey) SortedElements() []Key {
-	keyv := ks.Elements()
-	sort.Slice(keyv, func(i, j int) bool {
-		return keyv[i] < keyv[j]
-	})
-	return keyv
-}
-func (ks SetKey) String() string {
-	strv := []string{}
-	for _, k := range ks.SortedElements() {
-		strv = append(strv, fmt.Sprintf("%d", k))
-	}
-	return "{" + strings.Join(strv, " ") + "}"
-}
-
-func (os SetOid) SortedElements() []zodb.Oid {
-	oidv := os.Elements()
-	sort.Slice(oidv, func(i, j int) bool {
-		return oidv[i] < oidv[j]
-	})
-	return oidv
-}
-func (os SetOid) String() string {
-	strv := []string{}
-	for _, oid := range os.SortedElements() {
-		strv = append(strv, fmt.Sprintf("%s", oid))
-	}
-	return "{" + strings.Join(strv, " ") + "}"
-}
-*/
-
 func sortedKeys(kv map[Key]Δstring) []Key {
 	keyv := []Key{}
 	for k := range kv {

wcfs/internal/zdata/δftail.go

@@ -160,7 +160,7 @@ func (δFtail *ΔFtail) Track(file *ZBigFile, blk int64, path []btree.LONode, zb
 		// XXX blk = ∞ from beginning ?
 		blk = xbtree.KeyMax
 	}
-	err := δFtail.δBtail.Track(blk, zblk != nil, path)
+	err := δFtail.δBtail.Track(blk, path)
 	if err != nil {
 		panic(err) // XXX -> error? errctx
 	}
@@ -233,7 +233,7 @@ func (δFtail *ΔFtail) Update(δZ *zodb.EventCommit, zhead *xzodb.ZConn) (_ ΔF
 	δF := ΔF{Rev: δB.Rev, ByFile: make(map[*ZBigFile]*ΔFile)}
 
 	// take btree changes into account
-	for root, δt := range δB.ByRoot {
+	for root, δt := range δB.ΔByRoot {
 		files := δFtail.fileIdx[root]
 		if len(files) == 0 {
 			panicf("BUG: ΔFtail: root<%s> -> ø files", root)